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Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels (1983)

Chapter: IV. Detailed Descriptions of the UVOIR Program for the 1980

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Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 136
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 137
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 138
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 139
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 140
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 141
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 142
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 143
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 144
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 145
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 146
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 147
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 148
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 149
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 150
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 151
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 152
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 153
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 154
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 155
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 156
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 157
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 158
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 159
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 160
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 161
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 162
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 163
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 164
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 165
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 166
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 167
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
×
Page 168
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 169
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 170
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 171
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 172
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Page 174
Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Suggested Citation:"IV. Detailed Descriptions of the UVOIR Program for the 1980." National Research Council. 1983. Astronomy and Astrophysics for the 1980's, Volume 2: Reports of the Panels. Washington, DC: The National Academies Press. doi: 10.17226/550.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

135 heliosphere, its variation in structure, and its effect on the modulation of cosmic rays. In situ observations with the International Solar Polar Mission and the Origin of Plasmas in the Earth's Neighborhood (OPEN) mission will provide needed information on these questions and on the solar spindown due to the solar-wind transport of angular momentum. IV. DETAILED DESCRIPTION OF THE WOIR PROGRAM FOR THE 1980 'S We now give a detailed justification of the new initi- atives in our proposed WOIR program for the 1980'S . In Section A we describe and justify our major recommen- dations; in Sections B and C we discuss other categories of W OIR projects that we believe are also vitally important to the health of astronomy. A. Major Recommendations 1. The 15-Meter New Technology Telescope and Closely Related Projects The time is now ripe for a full-scale attack on the prob- lems of the formation and evolution of galaxies and clusters of galaxies, the nature of nonluminous matter in galactic halos, the character of supermassive objects in galactic cores; the evolution of molecular clouds; and the formation and evolution of galaxies, stars, and planetary systems. Ground-based observations are a key element in all of these areas of research. The optical and infrared (JR) regions of the spectrum are unique in their ability to provide morphological studies and spectral diagnostics of velocities, compositions, and excitation that are crucial to the interpretation of astronomical phenomena. New space observatories of all types, as well as radio facilities on the ground, will increase the demand for ground-based optical and IR observations--particularly for high-resolution spectro- photometry of faint sources. Space Telescope (ST) will permit astronomers to image galaxies and stars with spatial resolution an order of magnitude greater than that routinely achievable on the ground. This capability will result in the detection of unresolved objects up to 100 times fainter than has been

136 possible in the past. This tremendous gain in spatial resolution, coupled with access to the ultraviolet (W) region of the spectrum, are the main reasons that moti- vated the astronomical community to unite behind ST as a major initiative for the 1980's. We expect ST to provide fundamental new insights into the scale and geometry of the Universe and into the formation and evolution of gal- axies. For example, ST will provide a glimpse of objects at the very limits of the observable cosmos. However, many of the sources to be discovered will be too faint for effective spectroscopic and spectrophotometric analy- sis by ST itself. Such analysis is essential for the proper understanding of the basic physical properties-- temperature, density, excitation, chemical composition, velocity--of the newly discovered objects. Rapid vari- ability is a common feature of matter near compact objects and makes high time resolution an important additional consideration. Hence, for many programs, we also require a very much larger ground-based telescope to complement, fully utilize, and understand the discoveries made by ST. As ST will do in the optical and W regions, the Shuttle Infrared Telescope Facility (SIRTF), a 0.85-m cryogenically cooled telescope in space, will vastly extend our capabilities in the IR spectral region. At wavelengths beyond 3 ~m, the low thermal background of SIRTF will permit a gain in sensitivity for low-resolution spectrophotometry between 100 and 1000 times that of the largest ground-based facilities. This telescope will greatly expand our knowledge of luminous objects in galac- tic nuclei, the early evolution of stars, and the proper- ties of star-forming regions through imaging, photoelec- tric, and moderate spectral resolution observations. SIRTF will lift the gray-body curtain of thermal radiation generated by warmer telescopes and by our own Earth's atmosphere to reveal a new thermal view of our Galaxy and Universe. However, the capabilities of SIRTF in the areas of very high spectral and spatial resolution are limited by its relatively small aperture. For obser- vations of high spectral resolution, SIRTF will generally be strongly detector-noise limited. While improvements in this area can be achieved by development and use of more sensitive detectors (both discrete and array), it is nevertheless likely that the detailed study of kinematics and chemical abundances will depend in large measure on the availability of a very large telescope capable of carrying out spectroscopic observations at a spectral resolution of about 105.

137 The SIRTF telescope is intended to be diffraction limited beyond 5 Em. A 15-m telescope located at a good site can be diffraction limited at 20 Em during periods of excellent seeing and will provide more than 10 times the spatial resolution of SIRTF in the near- and mid-infrared regions. Thus, a large ground-based telescope can also provide an essential, high-resolution imaging capability as a complement to the SIRTF observing program. High-energy observations, especially those from the Einstein x-ray observatory, have demonstrated that the full utilization of x-ray data demands extensive optical work. Samples of quasars and galaxy clusters selected by x-ray criteria require extensive optical analysis for determinations of magnitudes, spectra, and red shifts. While this work is within the reach of ground-based telescopes of moderate size, not enough telescope time is available at present to carry it out effectively. Finally, we note that the Very Large Array (VLA)--a centerpiece of the astronomy program for the 1970's--is now in nearly full operation. It has begun to map the radio sky at spatial resolutions once thought to be the province of optical astronomy alone. Critical problems such as the origin and evolution of energetic galactic nuclei are receiving major impetus from VLA observations. Again, appropriate complementary studies at optical and IR wavelengths require spatial and spectral resolutions difficult to achieve with current ground-based tele- scopes. Already half of the requests for use of the 4-m telescope at Kitt Peak National Laboratory (KPNO) are for programs directly related to, or stimulated by, space- craft and radio observations. The reason for this is clear--observations at optical and IR wavelengths are necessary for a broad understanding of the physical mech- anisms responsible for the peculiar behavior of these unusual and newly discovered objects. a. The Scientific Impact of the New Technology Telescope A 15-m New Technology Telescope (NTT) will have capabilities unique in the long history of astronomy. It will increase by an order of magnitude the photon- gathering power of our largest telescopes; with the use of interferometric techniques it will furthermore achieve angular resolution of 0.03 arcsec at the shorter IR wavelengths and resolution of about 0.3 arcsec near 20 Em. NTT will provide the spectroscopic capability that

138 will be absolutely essential to follow observations and will provide high angular resolution at IR wavelengths less than 20 Em. For spectroscopy, a 15-m telescope is superior to ST at all wavelengths acces- sible from the ground except for observations requiring high angular resolution (e.g., galactic nuclei). Spectra of faint stars are necessary for an under- standing of the chemical evolution of our Galaxy, the dynamics of the Galactic halo, and the ages of dwarf spheroidal galaxies. With a 15-m telescope, halo stars down to magnitude 25 can be found by broadband photo- graphy and studied using abundance-sensitive intermediate- band spectrophotometry. White dwarfs in the nearest glob- ular clusters can be studied, and the main sequence can be reached in nearby dwarf spheroidal galaxies. All of these problems are threshold problems in the sense that there are no nearby bright objects suitable for study. Without the light-gathering power of a 15-m NTT there will be insufficient photons to mount these decisive spectroscopic programs. Distant galaxies and dim quasars provide their own challenge to image and to study spectroscopically. When did clusters collapse? Inhomogeneities in the surface density of galaxies can be measured out to a red shift of unity. Spectrophotometry of high-red-shift galaxies will allow us to see the 2000-3000-l region in the spectra of distant galaxies and search for spectral evidence of young stars. Study of QSO absorption lines will allow us to measure directly the chemical and isotopic composition of gas at large red shifts. Present-day spectra do not achieve a high enough signal-to-noise ratio at high dis- persion to determine the strengths of faint lines needed to obtain accurate chemical composition; for example, neither H2 nor D has been detected with certainty in quasar spectra. m e combination of high spatial resolution with suf- ficient photon-collecting power to achieve spectral resolution on the order of 105 will permit definitive IR studies of molecular clouds and imbedded objects. For studies at wavelengths less than 20 Am, the 15-m NTT will provide an important programmatic and scientific link between SIRTF and a Large Deployable Reflector in space. and SIRTF

139 b. Technical Considerations for a 15-Meter New Technology Telescope The New Technology Telescope will almost certainly take a form rather different from that of the traditional telescope, which has a long primary focal length and thick, monolithic primary mirror. A short-focal-length telescope based on the MMT concept, or a segmented pri- mary mirror, or perhaps even a thin monolith are examples of new approaches that can substantially reduce the weight, dome size, and, hence, cost of very large tele- scopes. There are, furthermore, compelling reasons for constructing a very large collecting aperture in the form of a single telescope, rather than simply combining data obtained by all the existing ground-based telescopes. To begin with, we anticipate that one important class of applications of such a facility will center around its power for infrared observations. The IR scientific impact of a 15-m-class ground-based telescope is based primarily on two aspects of its performance: photon-collecting power and spatial resolution capability. Photon-collecting power is particularly crucial for IR spectroscopy. At wavelengths less than 2.5 ~m, the thermal emission of the telescope itself is very small, and the performance of present-day large telescopes and of the proposed SIRTF facility is detector-noise limited at these shorter wavelengths. A 15-m telescope will provide the aperture for at least an order-of-magnitude increase in sensitivity for a wide range of spectral observations, ranging from moderate-resolution studies of continuum and emission lines from atoms and molecules to very high spectral-resolution studies of atomic and molecular emission and absorption systems. In the 3-25-pm wavelength range, thermal emission by the telescope will generally limit performance at low and moderate spectral-resolutions, but at a resolution of 105 or greater, similar order-of-magnitude gains are possible. In addition, as mentioned earlier, the spatial- resolution capabilities of a 15-m telescope, utilizing interferometric techniques, will permit observations at spatial resolutions of about 0.03 arcsec or less at the shorter IR wavelengths, while resolutions of about 0.3 arcsec will be possible near 20 ~m. In order to realize the scientific potential of such a New Technology Telescope, it appears necessary that the collecting area be placed on a single mount rather than distributed among individual telescopes, for the following reasons:

140 1. Present IR detectors and near-future prospects for the 1-25-pm range are such that moderate- and high- resolution spectroscopy will be detector-noise limited. Thus, an efficient, convenient system for focusing photons from the entire collecting area onto a single detector element is required to take full advantage of the larger collecting area. - ~ The gain in signal-to-noise ratio (S/N) tor a collecting area focused on a single detector, compared with n separate elements/detectors of the same total collecting area, is equal to the square root of n. The most straightforward configuration for this arrangement would place the entire collecting area on a single mount. 2. m e capability for high-spatial-resolution obser- vations at the shorter IR wavelengths requires phased beam combining for interferometric purposes at wavelengths of about 1 Am or greater. _ This can be most easily achieved with a single mount. The gain in spatial resolution is approximately the maximum separation of phased elements divided by the diameter of a coherent array element, which is also equal to the square root of n if the n elements are combined into a filled phaseable aperture. 3. m e potential of improved seeing or seeing correc- tions at the longer IR wavelengths, particularly at 10 and 20 Em, can lead to smaller beam sizes for photom- etry, imaging, and spectroscopy. In order to realize this, coherent beam combining over a significant field of view is required, which again leads to a single mount if not a single dish. The potential gain in S/N is again equal to the square root of n if the n elements are combined into a filled aperture. In addition, at optical wavelengths there will also be some spectroscopic applications that will be detector- noise limited, and for such problems arguments similar to those made for the IR program apply. But of perhaps greater importance is the fact that the emerging field of speckle interferometry promises outstanding capabilities for large single-mount telescopes. Speckle techniques for faint objects have so far been limited to the derivation of simple quantities such as angular size or the separation of two or three components of a multiple stellar system. For example, studies employing two telescopes have already resolved Pluto. Full image reconstruction is available and appears to have been achieved for a star both by simple crude

141 techniques and by detailed computer processing. The limiting magnitude for speckle studies is independent of telescope size but depends strongly on seeing. The limiting angular resolution of a 15-m equivalent aperture telescope will depend on how the aperture is arranged, but it may well be about 4 milliarcsec at 5000 A. Note that at 10 Em the corresponding resolu- tion will be 0.08 arcsec, or almost as good as the performance of ST at visual wavelengths. The visual angular resolution is comparable with that which can be achieved by radio interferometry using intercontinental baselines. A study of possible goals for these new techniques reveals that they are so advanced that they will cer- tainly open up entirely new areas of research. The use of the highest possible resolution at all wavelengths would be helpful in trying to understand the process of star formation and the origin of protoplanetary conden- sations. At a distance of 200 pa, the range of angular resolution available would cover the linear range from 0.75 AU in the visible to 30 AU at 20 Am. The fundamental requirement that the NTT be optimized for both IR and optical observations places a number of constraints on its design and location: Field of View. A large field of view for NTT is desirable both to complement the relatively small field (2.7 aramin) of ST and to take advantage of instruments that can obtain simultaneously the spectra of many objects in the same field. Coherence. NTT should be diffraction limited at 20 Am. A diffraction limit of 1 arcsec at 20 Am requires that the diameter of an individual coherent element be at least 5 m; it would be preferable for the entire collect- ing area to be coherent, since coherence over larger sizes achieved in excellent seeing or by interferometric methods will allow even better spatial resolution. Geometry. If NTT is not of a single-dish design, then for spectroscopy in both the optical and IR regions the light from the program object(s) must be brought with high efficiency to a common instrument. This is required to realize fully the low-noise potential of silicon detec- tors and to provide an optical configuration that will allow detector-noise-limited performance from existing near-IA detectors and projected arrays. Emissivity. For telescope-background-limited observa- tions in the thermal IR region, a reduction of the emis-

142 sivity by a given factor is equivalent to an increase in collecting area by the same factor. The effective emis- sivity of NTT should be as low as possible--5 percent is a difficult but realizable goal. In addition, the tele- scope emission must be steady to avoid introduction of excess noise. Image Quality. Seeing is of extreme importance in determining the limiting magnitude of a ground-based telescope. A reduction of a factor of 2 in the diameter of the seeing disk reduces the threshold brightness by a factor of 2; for sky-limited observations this is equiv- alent to doubling the telescope aperture. Good seeing is also important for spectroscopy, where the efficiency of a spectrograph is controlled principally by the size of the slit that is required to pass most of the light. Since for a well-designed telescope the site sets the limit to seeing, it is absolutely critical that the 15-m-class NTT be located at a site of known excellent seeing. At some sites already in use, the optical seeing is occasionally observed to be 0.5 arcsec or better. In the IR the seeing might be even better. Experience with the Multiple-Mirror Telescope (MMT) has shown that, if care is taken to minimize the effects of local dome and mirror seeing and to make the optics of very high quality, images of this size will be achieved for a useful frac- tion of the time. The effect of mirror seeing (convec- tion from the mirror surface) can be minimized through the use of thin mirrors, which reach thermal equilibrium in 2-3 h. Site Selection. Choice of a site is critical in obtaining optimum performance from the 15-m-class NTT-- or, indeed, from any telescope. In addition to seeing factors (such as low water vapor), absence of light pol- lution, accessibility, and reasonable cost of development are important. We note that the National Science Founda- tion (NSF) has undertaken a program of identifying and protecting excellent astronomical sites. This is an important effort because the majority of sites now in use are either overcrowded and/or threatened with light pollution. We therefore strongly endorse the NSF efforts in this area and recommend that the program proceed vigorously enough to allow selection of a site for the 15-m-class NTT by the mid-1980's. Instrument Changes and Scheduling. The 15-m-class NTT will be capable of doing more in an hour than present large telescopes can do in a night, and in some cases more than can be done in an observing run of several

143 nights. To take the best advantage of the varying conditions that arise at ground-based observatories-- changes in seeing, in cloud cover and photometric con- ditions, and in sky brightness from moon and twilight-- provision for rapid interchange of key instruments should be an integral part of the telescope design. (For example, sky-limited imaging or spectroscopy should be carried out in the best seeing. Again, it would be advan- tageous to change instruments at moonrise or moonset and again at dawn and dusk.) It follows that NTT should be queue-scheduled for at least a large fraction of its operation, with several programs interleaved. The pattern of use will probably be rather different from what is now customary; one can anticipate scheduled programs that require as little as an hour of integration, given the enormous power of the telescope, and in many cases the observer will not need to come to the telescope. Construction. The experience gained from the building of this decade's 4-m-class telescopes allows the design and construction of much larger telescopes that will be substantially less expensive than the extrapolated costs of the traditionally designed facilities constructed in the past. Indeed, there is now general agreement that, through the use of thin mirrors (10 cm) and altazimuth mounts, the weight and dome size of 10- to 15-m telescopes can be kept comparable with those of 4-m telescopes built in the last decade. A number of groups are already considering the prob- lems associated with the construction of 7- to 15-m-class instruments. The concepts that now appear most viable are a multiple-mirror telescope with 5- to 6-m monolithic elements or a single-mirror telescope of short focal ratio, probably synthesized from separately constructed off-axis segments. During the early 1980's, support should be provided for a coordinated program of technology development leading to the selection of a specific tele- scope design. With these studies in hand, it should be possible to select an appropriate design for a 15-m NTT in the mid-1980's and to begin construction with a goal of First light" by the end of the decade. The New Technology Telescope: Summary Broad support exists within the astronomical com- munity for construction of larger telescopes. Evidence for this support comes from the initiatives already taken by the Universities of California, Texas, and Arizona and

144 by KPNO to carry out design studies and tests of various concepts for building very large telescopes. Indeed, it is precisely because of these initiatives that we have a clearer picture of the performance standards for a 15-m NTT and confidence that these standards can be met. If the steps outlined in this recommendation are taken, we will enter the 1990's with the first dramatic increase in telescope aperture since the completion of the 5-m Hale telescope 35 years ago. d. Support Telescope Program for the 1980's Most of the NTT observing time will of necessity be dedicated to those frontier and threshold programs that require the enormous photon-gathering power and high spa- tial resolution of this magnificent instrument. However, each of these scientific programs encompasses a continuum of sources; for each quasar, star, and galaxy of the 20th magnitude, there are objects in the magnitude range 17-19 that are just accessible to the largest telescopes cur- rently available. Observations of these still-faint objects will teach us the astrophysics of quasars at a variety of lookback times. Other observations will determine the shapes of color-magnitude relations of stars in external galaxies and hence delineate the his- tory and evolution of stellar populations external to our own. Still other observations of halo stars in our own Galaxy in the magnitude range 17-20 will yield insights into the early history of galactic formation. It is reasonable to expect NTT to work only at the limits of these populations. The combination of the 15-m threshold observations with those from the ~smaller" telescopes will be crucial in solving many of the frontier problems of the 1980's. These "smaller" telescopes might range in sizes from about 2.5 m to larger than 7 m, but they must be state-of- the-art facilities with the capability of producing results competitive with the largest telescopes in use today. For the National Astronomy Centers, dedicated telescopes of the 4- to 5-m class seem more appropriate than a versatile, fully instrumented telescope. Such new telescopes should be dedicated to one or two primary programs. This will result in decreased construction, instrumentation, and operating costs. Scientific results obtained with the 2.5-m telescope at Las Campanas offer convincing evidence that a well- constructed, imaginatively instrumented telescope of this size at a superb site can attack and solve problems at

145 the forefront of astronomy. Telescopes of this class are attractive to university groups and offer the opportunity for a wholesome diversity of styles and approaches to observational astronomy. They could accommodate the needs of specialized programs and would also encourage technical innovation. One telescope might concentrate on long-term, large-scale programs or on astrometry or on observations coordinated with those from space vehicles. Still another might arrange its scheduling to emphasize speed of response to sudden events such as the appear- ances of comets or supernovae or maximum coverage of periodic phenomena in predictable systems. Instrumental innovations such as the development of the pulse-counting Reticon system often grow out of such environments, where technically adventurous projects can be attempted. As emphasized at the beginning of the section on scientific opportunities for the 1980's, the problem of access to telescopes with sufficient aperture to attack the important problems perceived by the astronomical community as scientifically exciting and fruitful has reached a critical state. The need is so severe that university groups have begun on their own to search for nonfederal funding for large telescopes to serve their needs. The federal government can play an important role here by providing matching funds and encouragement through the National Centers. Other large instruments should be part of our national facilities. The selection of new telescopes for construction during the 1980's will depend on a variety of considerations, including the impact on major scientific objectives, state of the relevant technology, cost, and timeliness. Particularly important initiatives are as follows: A 2-Meter-Class Telescope Dedicated to High-Resolution Spectroscopy for Solar-Stellar Studies. The application of advanced spectroscopic-diagnostic techniques makes it possible to extract, from subtle features of spectral-line contours, information concerning such properties as the distribution of temperature and density in the atmosphere, the amplitude and structure of velocity and magnetic fields, the abundances of the elements, and the rotation rate of the star. The reliable inference of some of these properties from stellar spectral lines requires very-high-resolution, high-S/N data of the kind routinely obtained in solar studies but hitherto unobtainable with stellar instrumentation. The spectroscopic resolution should be high enough (greater than 2 X 105) to over-

146 sample adequately the Doppler width of the line profiles; the S/N ratio should exceed 200 to allow reliable detec- tion of subtle diagnostic indicators of important prop- erties such as magnetic fields, velocity fields, and rotational rates. Moreover, because a central concern in solar-stellar physics is the study of time-dependent phenomena on scales ranging from seconds (flares) to years (stellar activity cycles), the stellar instrumentation should be operated in a mode permitting both highly effi- cient use of observing time ("automated") and stability of equipment, observing methods, and research objectives over one or two decades ("dedicated"). Equipment appropriate to achieve these scientific goals consists of a 2-m telescope with advanced pointing control, a very-high-resolution spectrometer in cross- dispersion echelle format, and a large two-dimensional detector (2000 X 1000 pixels) with high quantum effi- ciency. The echelle format is desirable both because of its intrinsic efficiency and because it permits the simul taneous measurement of a large spectral range. This is important for several reasons, including the facts that certain critical lines (Ca II H and K, H-alpha, He 1.083 ~m) occur in widely separated parts of the spectrum and that the uncertainty in determination of quantities such as magnetic fields and rotation rates can be reduced by measuring a large number of lines. Spectrophotometric Telescopes. One can identify optical and IR spectrophotometry as basic ground-based observations for which there will be unrelenting need, a need that cannot be fulfilled adequately by the NNT or existing telescopes. For these observations it is most cost effective to build a telescope of sizable aperture but little versatility. We recommend the construction of two new national 4-m-class instruments equipped only with these basic spectrophotometric instruments for use in both hemispheres. Through the use of innovative approaches, such as lightweight mirror and support, an altazimuth mount and small dome, and fiber-optics feeds or per- manently mounted instruments at Nasmyth foci, the costs of both construction and operation should be much less than present telescopes of the same aperture. Telescopes to Support the Space Program. The inter- play between space and ground-based observations operates at many levels. On the one hand, our understanding of many of the most active and exciting objects requires observations across the entire electromagnetic spectrum.

147 Those in the optical and IR regions will inevitably form a substantial part of the program of most major ground- based telescopes. On the other hand, space observatories also place demands on large qround-based telescopes for direct support of specific missions. We have in mind such things as obtaining wide-field images in excellent seeing to pinpoint the most productive fields for ST imaging; location of precise position for guiding and acquisition for ST; search for optical counterparts of new sources found by x-ray, IR, or radio telescopes; observations of variable stars or galactic nuclei scheduled simultaneously with those from a particular spacecraft; and imaging support of space missions within our solar system. The strong demand for such direct- support activities merits the construction and research support of one or more dedicated telescopes in the 4-m class, operated and scheduled along with ST and other space observatories. 2. A Large Deployable Reflector in Space m e resolution of a variety of astronomical problems requires observations from 30 to 1000 Am at high spatial or high spectral resolution or, in many cases, with a com- bination of both. Since the Earth's atmosphere is opaque from 30 to 300 ~m, these observations require a large telescope above the Earth's atmosphere: a Large Deploy- able Reflector (LDR) of the 10-m class in space, diffrac- tion limited out to 30 Am. This facility will provide a tenfold improvement in spatial resolution by comparison with the Kuiper Airborne Observatory (KAO), balloon tele- scopes, and SIRTF--our existing and planned facilities for exploiting this spectral region. LDR should be a free-flyer with an extended (10-year) lifetime, launched from a single Shuttle flight and deployable without the aid of astronauts in extravehicular activity. The tele- scope should be passively (not cryogenically) cooled, with access provided for instrument cryogen resupply and instrument replacement or refurbishment. Scientific Impact - The large collecting areas, high angular resolution, and total freedom from atmospheric interference will give LDR unique capabilities to carry out classes of observa- tions not otherwise possible. While it is already pos-

148 sible to define areas where it would have a major impact, the capabilities of LDR and the potential of the 30-300-pm region are such that much of its impact is likely to lie in areas not yet known. The advantages of spectroscopy in the far IR region are twofold. First, since visually opaque dust clouds become nearly transparent at long wavelengths, we will be able to probe objects of great astrophysical interest that would otherwise be obscured. Second, many partic- ularly important atomic and molecular transitions lie in this spectral region. For instance, the S(0) 28-pm rc~tat tonal 1 ine of He and the R ( O ) 112-u m, R ( 1 ) 56- m, R(2) 38-pm, and R(3) 29- m lines of HO are of great scientific interest for abundance studies and determination of the D/H ratio in molecular regions, where there is little W starlight to dissociate HO or H2. The true cosmic D/H ratio has an enormous impor- tance for cosmology, being determined by the details of the early expansion of the Universe and reflecting high sensitivity to the total cosmic mass density. In addi- tion to providing constraints on cosmology, these obser- vations will yield a better understanding of cloud chemistry. Numerous fine-structure lines of heavy- element species (e.g., O I, O III, N II, Si II, C I, C II) lie in the far IR region. These transitions are important for studying abundances and excitation con- ditions in H II regions and behind fast shock fronts, and they can be the dominant cooling mechanisms for such regions. The O I line at 63 Am and the C II line at 157 Em are particularly good probes of transition regions where H is not ionized. Finally, higher level rotational transitions in molecules such as OH, CO, and H2O occur at longer IR wavelengths. These transitions are important to the study of the hotter and denser por- tions of molecular clouds. h. Galactic Nucle hi and Galactic Structure - The angular resolution of LDR in the far-IR spectral region is some 3 arcsec or less. This resolution cor- responds, at the distance of nearby galaxies, to a linear scale of 100 pa, which is comparable to the angular extent of giant H II region/molecular cloud complexes in our own Galaxy. ~ ~~ Such a telescope in space would also detect a far-IR source equivalent in luminosity to a very luminous single star within a distance of 10 Mpc. These capabilities can be brought to bear on a number of prob-

149 lems in galactic studies. For example, the central regions of a nearby galaxy like M82, which has a lumi- nosity peaking at 100 m, can be mapped on a spatial scale of about 25 pc, which would show whether the emitting material is clumped or smoothly distributed. mese two possibilities have differing implications concerning the source of the luminosity and the triggering of the high-luminosity phase. If the mapping at higher angular resolution uncovers regions that appear to be sites of enhanced star formation, high-angular-resolution spectroscopy of the associated atomic and molecular gas would yield crucial diagnostic information about velocities, composition, and excitation conditions. The recently discovered IR and submillimeter lines of H2 (2.2 vary), and O I (63 nary), and C I (600 ~m) illustrate that LDR would have access not only to the ionized and cold molecular components of the gas but also to a hot, neutral phase suggestive of shock phenomena, which could play a major role in energetic galactic nuclei. Infrared studies at 10 ~m, and, more recently, at far-IR wavelengths, indicate that powerful IR excesses (predominantly at wavelengths between 30 and 300 Em) are relatively common features of the emission from spiral galaxies. Much of this activity may be associated with extremely energetic bursts of star formation in dusty regions as large as 500-2000 pc. Determining the structure and extent of such regions and distinguishing these processes from activity peculiar to galactic nuclei (at scale sizes of 100 pc or less) will require the high- est possible angular resolution. A thorough sampling of the brightest galaxies would require a telescope aperture ranging up to 10 m. Infrared fine-structure lines, such as those of O II (55 Am), N III (57 ~m), and S III (33 ~m), have now been seen in several galactic sources. Since these are less sensitive to density and extinction effects than are optical lines, they can also be used to search for abun- dance gradients across the disks of spiral galaxies. High-angular-resolution atomic and molecular spectro- scopy can also be used to study the problems of galactic dynamics, particularly in the cases of dust-embedded, high-lumino~ity galactic nuclei (e.g., MB3 and NGC 253), where optical determinations are impossible and where gas-dynamical information may be crucial to a complete understanding of the mass distribution and energetics. Complementary high-angular-resolution studies of the spatial distribution of the stars may also be possible.

150 c. Star Formation and Evolution The high angular resolution of a large reflector in space will greatly enhance our understanding of star formation by permitting direct measurements of the sizes of nearby protostellar objects at many wavelengths. For example, calculations for a protostar of 20 solar masses at an intermediate evolutionary phase, corresponding to a luminosity of 3600 times the solar value (as might be appropriate for the BN object), show that its "effective size" will be 6 X 1015 cm at 30 Em and 3 X 1016 cm at 100 ~m. For an object at a distance of 500 pc, the corresponding angular sizes are 0.8 and 4 arcsec, respec- tively, comparable with or greater than the angular reso- lution of a 10-m reflector in space. If stars begin their lives as fragments of a size on the order of a Jeans length, then the fragment size should be about 2 X 1017 cm for a solar-mass object--a value consistent with typical values for molecular-cloud complexes. At a distance of 500 pc, the angular size of such a protostellar fragment is about 20 arcsec. Thus, a 10-m LDR in space would be capable of studying the detailed structure of such frag- ments, e.g., the variation of temperature and density within such an object. Protostars are often found in compact groups, and thus high spectral resolution is essential to a study of these regions. In the case of W3, for example, there are at least four compact 20-pm sources, which appear to be young stars in a range of early evolutionary stages, located within a 20-arcsec-diameter region that cannot be resolved in the far IR at present. Far-IR spectroscopic and photometric studies of this region with angular reso- lution comparable with existing near-IA and radio observa- tions will be possible with a 10-m LDR in space. Such data are necessary to determine the properties of each individual object and to study their interactions with each other, as well as the evolution of the entire cluster. m e preceding remarks about star formation reflect the current state of our knowledge in that they pertain chiefly to massive, luminous stars forming in nearby regions of the Galaxy. It is important to remember, however, that LDR in space will permit us to extend our studies of star formation to regions that are currently inaccessible because of limitations on sensitivity and/or angular resolution. Two obvious examples are studies of the formation of low-mass stars in nearby Galactic regions, like the Taurus and Ophiuchus dark clouds, and

151 studies of individual regions of star formation in the spiral arms of nearby galaxies. d. Mass Loss in Stars Mass loss in stars is a commonly observed phenomenon, particularly in stars in the later stages of evolution. However, the processes are still poorly understood. A few stars show evidence of circumstellar dust shells with an angular extent of about 1 arcmin; for example, IRC +10216, a carbon star, has been found to have a diameter of about 1 arcmin in the 50-100-p range. The detailed study of the shape and structure of such shells with a 10-m LDR in space would yield basic information on the mass-loss process in cool supergiants and the history of mass-loss rates over extended periods of time. Status of Technology A variety of past and current studies and demonstra- tion projects indicate that the technology is ripe for designing and beginning construction of LDR in space in the 1980's. Many of the issues were addressed, and the technologies identified, in a NASA-sponsored feasibility study completed in 1975 for a space-based, 30-m laser transmitter (NAS 3-19400). A number of active military programs are pursuing technologies relevant to a large space telescope. These programs are studying the tech- niques required for fabrication, segmenting, deployment, active alignment, passive cooling, and operation. There is at present a NASA-sponsored feasibility study specifi- cally for a 10- to 30-m deployable IR astronomy telescope directed jointly by NASA Ames Research Center and the Jet Propulsion Laboratory. This study will identify the most promising overall design appropriate for 1985 technology, identify areas that need further investigation, and deter- mine breakpoints with respect to aperture, diffraction- limited wavelength, and cost. Instrumentation for LDR in space will be an outgrowth of instruments now being used or developed for balloons and for the KAO and can take advantage of further devel- opments for SIRTF. The current programs include high- spatial-resolution mapping and high-resolution spectros- copy. Fourier-transform spectrometers now operate with a potential resolution of O.Ol/cm, providing a resolving power of 5 X 105 at 2 Em and 3 X 104 at 30 ~m. Extending the mirror travel and hence the resolution of these instruments by a factor of 3 or 4 would be straight- forward. A far-IA, cryogenically cooled, dispersive

152 spectrometer is being designed for the KAO with a resolu- tion approaching 104 at 30 Am. Fabry-Perot interfer- ometers provide a means for examining individual spectral lines. Obtaining a resolution of 10 or greater at the longer wavelengths requires further development. Hetero- dyne techniques show considerable promise for wavelengths immediately shortward of 1 mm. Discrete detector/cavity arrays for the far IR are both in use and under develop- ment. The prospect of LDR in space provides additional impetus to pursue these technological advances through the 1980's. 3. Far-Ultraviolet Spectrograph in Space Between the interstellar hydrogen absorption edge at 912 ~ and the 1200-A cutoff of conventional optical/ detector systems, such as those being built for the ST spectrographs, lie many spectral features of extraordi- nary interest for studies of interstellar gas, high- temperature plasmas in stellar coronas, supernova rem- nants, galactic nuclei, and abundances in stellar photo- spheres. For instance, the discovery of the O VI lines at 1032 and 1037 ~ in stellar winds and in interstellar space is now interpreted as indicating, in both cases, the presence of even hotter gas at a million degrees. This conclusion has led to revolutionary changes in our understanding of these important, low-density regions. It is now clear that studies of galactic-halo gas must include the O VI lines before astronomers can reliably measure temperatures, pressures, and halo extent. Atomic and molecular lines not available in the ST spectral region include the only strong lines of D I, H2, and HO. The former are required for detailed determinations of the Galactic D/H ratio, variations in which must be studied if we are to confirm the hypothesis that most deuterium is a relic of the primordial fireball. H2 provides information on densities, radiation fields, and temperatures in dense interstellar clouds. The density of H2, together with densities of HO and the many other molecules found in the clouds, provide information on the local cosmic-ray density as a function of position. Cosmic-ray gradients or local cosmic-ray enhancements in our own Galaxy and the galaxies of the Local Group can thus be studied in detail for the first time. For many other important species (e.g., 12CO, 13co, N I, Fe II), complementary studies with the proposed instrument and

153 the high-resolution spectrograph on ST are required, neither being sufficient alone. We therefore recommend the early initiation of planning leading to launch as a free-flyer in this decade of a telescope and spectrograph with a resolving power of about 3 X 104, concentrating on the 900-1200-! spectral region. The technology for building a high-resolution 900-1200-! instrument now exists, but large improvements in sensi- tivity may be possible and should be pursued. The chief design features are use of windowless detectors and a minimum number of optical elements. Interesting scien- tz~c goats that should set the scope and cost of the project include (1) the detection at 3 X 104 resolution of a 12th-magnitude, unreddened BO star in a few hours with a S/N ratio of 20/1 to allow detailed studies of Galactic gas up to 8 kpc from the Sun, at all Galactic latitudes and in a few lines of sight to the Large and Small Magellanic Clouds; (2) similar detections with longer integration of 14th-magnitude objects to allow study of a number of Seyfert galaxy nuclei, as well as of our Galactic halo and halos of other galaxies; and (3) the detection of 17th-magnitude objects with lower resolu- tion to allow study of many Seyfert galaxies and several quasars--the latter provide probes of halos of a few other (foreground) galaxies. At the 17th-magnitude limit des- cribed above, chromospheric and coronal emission lines of many cool stars will be detectable. Active stars (RS CVn, T Tauri, dMe stars, young F-K dwarfs) could be observed down to visual magnitude 11 or fainter. The 17th- magnitude unreddened BO star limit is probably within the reach of a 1-m telescope and spectrograph of modern design with a minimum number of optical elements, high reflectiv- ity, and two-dimensional photon counters. The proposed far- W spectrograph can be designed to accommodate other spectral regions as well. The higher orders of the grating will provide high-resolution spectra at least down to 300 A (and perhaps even to 100 A) if the technology for aluminum overcoating of optical elements in oxygen-free space becomes feasible. Such a capability would allow studies of interstellar He and of hot plasmas associated with nearby stars and solar-sYstem objects. The unexpected discoveries recorded by the Einstein, Copernicus and IUE spacecraft of coronas and hot plasmas in stars of nearly all spectral types, and by Voyager in the Jovian system, could be followed up by detailed line-profile studies at resolutions high enough to yield a virtually complete picture of these plasmas. ,

154 For example, temperatures can be derived from a number of key lines of N V, O VI, Ne VIII, Mg X, Si XII, Fe XVI, Fe XVII, and Fe XVIII. Emission measures, combined with densities derived from ratios of lines in the isoelec- tronic sequences of He, Be, and B (abundant in the 100-1200 ~ spectral region), can be used to derive volumes of the hot plasmas. In some lines of sight it might be possible to detect Seyfert galaxies and quasars in the extreme W if the sensitivity of the telescope extends down to 100 A. Observations from the Einstein x-ray observatory have shown that such objects are power- ful soft x-ray emitters. If possible, the 1200-3100 ~ spectral region might be incorporated, giving astronomers an important capability to follow up ST discoveries of both Galactic and extragalactic objects. As evidenced by the oversubscription of International Ultraviolet Explorer (IUE) satellite observing time, there is intense interest in the W spectral region. 4. Advanced Solar Observatory The Solar Optical Telescope (SOT), now an approved NASA program for Space Shuttle flights in the late 1980's, will revolutionize the study of stellar atmospheric structure and dynamics as the result of its tenfold increase in angular resolution compared with previous observations in the wavelength range 0.1-10 Am. SOT is designed to provide better than 0.1-arcsec resolution observations of physical parameters at all layers in the solar atmosphere up to the low corona; the corresponding spatial scale on the Sun (70 km) is estimated to be suf- ficient to resolve high-field magnetic-flux tubes, which probably define the dominant scale of atmospheric inhomo- geneity. With these data we anticipate a definitive study of atmospheric density and temperature structure and of solar magnetic fields and dynamics. Complementary spectroscopic observations at X W and E W wavelengths and chronograph observations, both carried out on the Space Shuttle, will provide the connection with the higher solar atmosphere and the interplanetary medium. The duration of early Space Shuttle flights, however, will be too short to permit SOT and its cluster of other solar-imaging devices to study effectively the challeng- ing problems of solar activity. Solar flares, for example, require flights of long duration (6 months) to study both a representative sample of flares as well as

155 the critical magnetic energy buildup process that appears to occur for several weeks prior to the flare event. Similarly, SOT and its cluster of instruments cannot study some other important aspects of solar activity, such as the development of active regions, the study of eruptive prominences, and other transient solar events, without long-duration flights. Yet the SOT cluster still has capabilities unique for the study of solar activity, exemplified perhaps most dramatically by its design capa- bility of resolving the high-energy (10 K) flare plasma to at least 0.1 arcsec and perhaps eventually down to 0.03 arcsec (20 km on the Sun) by means of imaging in the Fe XXI line at 1354 L. These capabilities can be com- plemented beautifully by a group of high-energy detectors developed to study flares during the next solar maximum. Such devices should include a gamma-ray spectrometer, a hard x-ray imaging array, and a pinhole camera for ex- tremely high spatial resolution of the flaring plasma in hard x rays. The long-duration flights of these separate instruments developed for the Space Shuttle, followed by their combination to form a free-flying Advanced Solar Observatory (ASO) in space, will result in a vastly improved study of solar activity with spatial resolution exceeding that of the Solar Maximum Mission (SMM) space- craft by two orders of magnitude. The proposed modular plan for the development of ASO is a cost-effective method of obtaining the full ASO capability in the necessary time. We recommend that the ASO, consisting of SOT and its ancillary spectrographs and imaging instruments, be flown on a free-flyer (such as a proposed NASA space platform) during the 1991 solar maximum. 5. Requirements for Improved Detectors and Instrumen- tation in the 1980's a. Introduction For many applications the information-gathering capa- bility of astronomical telescopes and instruments can be improved as much by increased detector capabilities as by a costly increase in telescope aperture. Although there have been major advances in detector technology in recent years, resulting in increased capability of existing ground-based and space telescopes, there is still much room for improvement. In addition, new and improved instrumentation techniques may make possible new types of measurements previously unachievable, regardless of telescope aperture.

156 The basic detector characteristics of importance are (1) detector quantum efficiency and its variation with wavelength, (2) equivalent readout noise (photoelectrons per pixel), and (3) total number of pixels in the detec- tor (available for readout in a single exposure). Other characteristics--such as linearity, dynamic range, com- pactness, and low power requirement--are also of impor- tance but need not be discussed here. The detector situation in the IR is particularly urgent; for example, SIRTF will be detector-limited in many IR bands. ~~ ~ ~~ is Producing impressive results, and there is a clear However. the military IR detector Program ~ ~ _ _ need to declassify the relevant technology so that high- performance detectors can be used for scientific purposes. Not only will it be important to push for the development of higher sensitivity, but it is also important to develop and deploy two-dimensional IR detectors, so that images of objects such as protostars can be obtained with high efficiency. Prototype optical detectors currently being tested (but not yet widely available to astronomers) include solid-state array detectors--charge-coupled devices (CCD) and charge-injection devices (CID), both in photon-input mode and (with auxiliary photocathode) in photoelectron- input mode [e.g., imaging charge-coupled devices (ICCD)]. They also include devices based on microchannel plates, as well as various types of medium- to large-format elec- trouraphic detectors. ~ , ~ _ Many of these are being developed primarily for space applications, such as instrumentation for ST. NASA and NSF should coordinate development and produc- tion to guarantee that, whenever possible, detectors suit- able for ground-based as well as space observations (for example, CCD's) be produced in sufficient quantities to satisfy the needs of the astronomical community. Where necessary, support should be provided for modifications in the architecture of specific devices to render them more useful for astronomy. Large formats are essential if we are to realize the full potential of the large telescopes that received the highest endorsement of the W OIR Panel. Both spectroscopy and deep surveys with large telescopes depend on the successful enlargement of the CCD area. This may be accomplished by finding tech- niques for enlarging the chip size or by developing a chip architecture that permits the construction of a mosaic of CCD chips. Since many detectors developed for other purposes can be used for low-light-level astronomi-

157 cal observations only after extensive testing or modifi- cation, groups involved in these activities should be supported. In some cases a group may wish to negotiate with manufacturers for special modifications of devices. In return for support, such groups should freely provide information on complete systems to observatories and astronomers, who have limited facilities for developing them. In the detailed program described below we have divided the discussion of detectors and some instrumentation by wavelength region, since the technological methods and prospects are wavelength-dependent. b. Infrared Spectral Region Detectors. All aspects of IR astronomy depend criti- IR astronomy to date has cally on detector technology. depended solely on discrete detectors. Imagery and spectroscopy both from the ground and from space would be enormously advanced by the introduction of array detec- tors. Full utilization of the potential of existing and future facilities also depends crucially on these devices. At present, array detectors in the 1-30-pm range are being developed for other purposes but have not become astronomical tools. Therefore, an urgent requirement for IR astronomy is the evaluation and application of current IR arrays and the further development of discrete and array detectors in order to use more effectively existing and future IR astronomy facilities. Because of the very wide range of wavelength covered by the IR, several different detector approaches are required. Intrinsic photoconductors are available or potentially available for the 1-15-pm range. Extrinsic silicon photoconductors are available out to 30 Am with extrinsic germanium photoconductors sensitive to about 200 Am. Beyond 200 Am, halometers or other detection techniques are required. Heterodyne techniques offer potentially very powerful spectroscopic capability, par ticularly for wavelengths beyond 100 ~m. Improvements in detectors and associated circuitry are required over this entire wavelength range, in addition to the incor- poration of IR photoconductive materials into array detectors. Instrumentation. The achievement of high spectral resolution over the whole IR wavelength range is an impor- tant next step. Spectrometers for this wavelength region will generally require cryogenic cooling and would benefit greatly from the use of array detectors. Mapping instru- -

158 meets based on array detectors will also be important, particularly for SIRTF. Visual and Near-Infrared Spectral Region Detectors Conventional Systems The image pulse-counting systems developed during the last several years in nearly all cases require the use of image intensifiers, which are increasingly difficult to obtain. Manufacturers should be encouraged to continue the fabrication of high-quality intensifiers for these and other astronomical purposes. Further development of microchannel-plate detectors (utilizing W- and visual-sensitive photocathodes) and of medium- and large-format electrographic detectors should be given high priority. The microchannel detectors may offer a less expensive alternative to the ICCD for some photon-counting applications. Electrographic detectors are the only currently available electronic imaging devices other than image tubes with photosphor-screen recording on photographic plates offering large-format capabilities (10,000 X 10,000 pixels) comparable with those of photographic plates, along with the compact data storage advantages of the latter. Application of opaque III-V photocathodes, having high near-IA quantum effi- ciency, to ICCD's and other photon-counting detectors and to electrographic detectors, should be actively pursued. New Technology At present CCD's offer the greatest advance in detector technology. This technology must be developed as rapidly as possible and the devices made available to observatories and astronomers who can use them. Between 0.55 and 1 ~m, they provide gains in quantum efficiency of 5 to 100 times that of present-day photocathodes. Their low readout noise permits these gains even at very low light levels. For some purposes they may outperform conventional photocathode devices even in the blue and violet spectral range. When used as ICCD's, they provide an excellent noise-free readout device for conventional photocathodes in the blue and W in either an analog or pulse-counting mode. m e largest CCD devices now made have areas of about 1.5 cm2 and have pixels of 10-30 ~m, although for some devices it is possible to combine pixels electronically before readout so that read amplifier noise is held to a

159 minimum. CCD detectors have very high quantum efficiency from the green through wavelengths exceeding 1 Em, and a recently measured 60 percent quantum efficiency at 0.4 Am in a wellpassivated, backilluminated RCA device shows that near-unity quantum efficiency is not restricted to the red part of the spectrum. As CCD's become more generally used, astronomers will want to measure the entire spectrum, from the vacuum W to 1.1 Am, with near-unity quantum efficiency. For ground-based observa- tions, unmodified devices will be used; in the vacuum W it may be necessary to coat the CCD with high-efficiency phosphors. CCD detectors currently available are too small to match efficiently the large telescope proposed in this report. Thus, development of larger detectors is an important area that must proceed apace with the construc- tion of a NNT. Multiple-Object Spectroscopy. For spectroscopic problems involving objects of comparable brightness lying close together in the sky, a large increase in efficiency can be achieved by optical multiplexing. One method for achieving this is the mapping of images scattered over two dimensions along a single spectrograph slit by means of single, fused-silica fibers. For example, the veloci- ties of a great many galaxies in a cluster, needed for mass determinations, can be obtained in the same time as a single spectrum using this approach. Another approach is the continued development of slitless spectrographs; these have already proven useful in surveying for faint quasars with contemporary 4-m telescopes. These and similar techniques should continue to be developed. High-Precision Radial Velocities. It should now be possible to build instruments for stellar telescopes capable of measuring radial velocities to an accuracy of 10 m/sec. Since this is one of the means by which other planetary systems and very low mass companions of stars can be detected, these techniques should be pursued. Studies of solar surface mass flows use very similar techniques but can reach accuracies better than 1 m/sec. Image-Restoration Techniques. As in the case of detector development, there is a major development program outside the astronomical community concerned with the restoration of images to compensate for atmospheric- seeing degradation. Ground-based solar and stellar astronomy would benefit from use of these restoration techniques to increase the spatial resolution of existing

160 ground-based telescopes. Lack of spatial resolution is now the most important limitation of ground-based solar astronomy, so that the capability of removing atmospheric- seeing effects will represent a major breakthrough. Astrometric Techniques. The major advances in astrom- etry during the past decade have been due to the introduc- tion of new instrumentation and the upgrading of existing instruments. Advances in measuring engines have permitted a tenfold gain in the precision of measurement of both new and archival astrometric photographic plates. Existing plate-measuring machines are already fully committed, and a demonstrated need exists for at least three higher- precision machines. Focal-plane measuring devices offer the potential for obtaining extremely precise relative positions of an accuracy limited primarily by photon statistics; adequate support for the design, fabrication, and operation of the proposed instruments is crucial for reaching our scientific goals. Also required is instru- mentation for measuring large angles, including support for ground-based telescopes and research designed to identify and design new approaches to large-angle mea- surements on the ground and in space. Finally, we stress that space astrometry offers enormous advances in pre- cision, together with freedom from some of the most seri- ous systematic errors that limit ground-based astrometry; the continued development of new approaches to astrometric observations from space deserves a high priority. d. Ultraviolet Spectral Region Detectors. In the W spectral range, there is a need for the development of detectors that take advantage of the higher photoelectric quantum yield of opaque photo- cathodes, as opposed to the conventional, semitransparent variety. In addition, in the wavelength range below 1200 A, operation of detectors in the windowless mode is required to eliminate the effects of absorption by the window materials available. These detectors could be of the ICCD, microchannel plate, or electrographic variety, depending on the specific application. Also, COD detec- tors with W phosphors appear to be attractive possibili- ties. Further development of all these types, and pos- sibly others, should be supported. Instrumentation. Requirements include a need for . high-transmission, narrow-band filters for the far W below 1300 #, as well as improved, all-reflective optical systems providing simultaneously high-resolution and large-field coverage for telescopes and spectrographs.

161 e. Computer Support Electronic detectors must be accompanied by computer systems adequate to permit efficient and effective acqui- sition and analysis of the data. m erefore, the W OIR Panel strongly endorses the recommendations of the Pane' on Data Processing and Computational Facilities (see Chap- ter 5) and urges the wide dissemination of computing systems capable of carrying out adequate image analysis, first at primary acquisition sites and then at other major university centers. f. Optical Coatings Although the fabrication of optical components for telescopes, spectrographs, and other instrumentation has reached technical maturity, the overall efficiency of the telescope-plus-instrument combination depends critically on the use of suitable coatings. In the difficult E W region below 1000 A, where even the best current coat- ings have relatively low reflectivities, improvements are badly needed. In the visual region, the use of available high-reflectivity and good antireflection coatings can improve telescope-instrument combinations by factors of 4 or more. There is good evidence that it will also be possible to improve the efficiency of mirror coatings in the IR region; this will allow construction of sophisti- cated multireflection instruments and will reduce substan- tially the IR emissivity of warm telescope mirrors. The cost of lowering reflection and grating losses on existing auxiliaries to optical telescopes is modest; support of these efforts is needed. B. Endorsement of Continuing NASA Programs The long lead times required for major satellite programs has resulted in a number of very-high-priority 1970 ini- tiatives becoming operational only during the 1980's. The programs listed below continue to receive a very strong endorsement from the W OIR Panel. We have separated them from our major recommendations only because they are approved or continuing programs. 1. The Space Telescope Space Telescope (ST) is scheduled to be launched in 1985. It will be the chief instrument for UV space

162 astronomy during the 1980's, and we also expect it to revolutionize optical astrometry. It is imperative that this expensive and powerful facility be operated with the highest efficiency and with a deep commitment to achieving the maximum scientific yield possible. We believe that two aspects of its operation are central to ensuring maxi- mum scientific effectiveness. They are the following. a. Space Telescope Science Institute (STScI) There have been numerous studies of the management structure appropriate for the STScI. Foremost among these is the study, Institutional Arrangements for Space Telescope (National Academy of Sciences, Washington, D.C., 1976)--the Hornig report. NASA has accepted most of the recommendations of the Hornig report and has now selected a university consortium to operate the Institute. We endorse the recommendations of the Hornig report and commend NASA's effort to implement those recommendations. Not only must the Institute ensure the proper operation of the ST, but it must also handle the data flow effec- tively and draw the wider astronomical community into full participation and use of ST research results. b. Refurbishment of ST Instrumentation The ST instrumentation package is designed for refur- bishment and updating, and we urge NASA to ensure that timely updating does in fact occur. Particularly urgent will be the improvement of ST spectrographs, made possible by steadily advancing detector technology. Two of the present ST spectrographs employ linear-array detectors. While these detectors will doubtless be efficient, the power and usefulness of the ST spectrographs can clearly be improved through use of two-dimensional detectors. In an echelle format, these would permit recording of the entire spectrum at high resolution with a single exposure. A two-dimensional detector would also permit spatial reso- lution in one coordinate in addition to spectral resolu- tion; higher quantum yield for the detectors may also be possible. Development of suitable detectors should have high priority. The incorporation of both improvements into a single, new high-resolution spectrograph would strengthen the scientific program of ST. 2. The NASA Infrared Astronomy Program During the 1980's, IR astronomy will record an improve- ment in sensitivity by orders of magnitude when approved,

163 cooled IR telescopes of moderate size are launched by NASA. These instruments will be detector-limited, so that actual sensitivity gain will depend critically on the gains that can be achieved in state-of-the-art detectors. The fifth of the Recommendations for Major Initiatives of the W OIR Panel addressed this problem (see Section I.A). m roughout the whole of the IR spectrum, absorption by atmospheric constituents--primarily H2O, but also CO2, O3, and other minor species--impedes observations from the ground. From 1 to 30 Am, atmospheric windows do permit observations with ground-based telescopes. While in fact most IR observations to date have been made from the ground, the potential exists for even greater utiliza- tion of ground-based telescopes in the areas of high- resolution spectroscopy, imaging, and high spatial resolu- tion. On the other hand, because the sensitivity of low spectral resolution observations is limited by fluctua- tions in the thermal background radiation from the tele- scope and atmosphere, an enormous gain in sensitivity for these observations can be made with cryogenically cooled telescopes in space. For wavelengths between 30 and 300 ~m, observations must be made from the stratosphere or above because of absorption by water vapor in the atmosphere. The poten- tial in these areas has only begun to be realized; much more can be accomplished in both photometry and spectros- copy with present aircraft and balloon facilities. Cryo- genically cooled telescopes in space are extremely sensi- tive in the 30-100-pm range, and they even provide a substantial gain in sensitivity at wavelengths beyond 100 Am, but the relatively small size of their optical sys- tems, imposed by cooling requirements, limits the achiev- able spatial resolution. High spatial and spectral reso- lution and high sensitivity at the longer wavelengths require a large (passively cooled) telescope above the atmosphere, as described in the second of our Recommen- dations for Major Initiatives (see Section I.A). Two planned programs soon to be implemented are par- ticularly important first steps in this program. mese are the Shuttle Infrared Telescope Facility (SIRTF) and the Cosmic Background Explorer (COBE). a. Shuttle Infrared Telescope Facility SIRTF has received the strongest possible support from the IR community as the first major IR telescope in space. It will contain a cryogenically cooled mirror of

164 0.85-m diameter and will accommodate multiple, changeable instruments in the focal plane. The first SIRTF flight is scheduled for the end of the decade, with about one flight per year thereafter. SIRTF will have by far the greatest broadband and low- resolution spectroscopic sensitivity of any existing or planned IR facility for wavelengths beyond 3 ~m. It will be the most effective instrument for carrying out a large number of detailed photometric and spectrophoto- metric studies of a wide range of objects, including extragalactic sources, survey sources, stars, molecular clouds and embedded objects, and faint solar-system objects. Therefore, we strongly endorse the construction and operation of SIRTF as an extremely powerful tool for infrared astronomy. However, construction and flight of SIRTF Will not, in itself, assure its effective use. If SIRTF is to be a central pillar of IR astronomy in the late 1980's and beyond, it is of utmost importance that advanced spec- troscopic and imaging instruments be developed for SIRTF and that sufficient observing time be available to take advantage of its potential. The latter requires extended Spacelab missions of two weeks or more and consideration of eventual use of SIRTF as a free-flyer. b. Cosmic Background Explorer COBE is designed for measurements of fundamental importance to cosmology--observations of the total back- ground radiation at millimeter and submillimeter wave- lengths and of the spectrum and isotropy of the 3 K cosmic background radiation. Therefore, we endorse COBE because of its broadly based scientific value to physics and astronomy. 3. Solar Optical Telescope Many astrophysical phenomena, from stellar activity to quasars, exhibit evidence of dramatic interaction between magnetic fields and hot plasmas. The Sun provides a unique laboratory where this interaction can be spatially resolved and studied in detail. The Solar Optical Telescope (SOT) is an approved NASA Spacelab facility scheduled for its first flight on Shuttle in 1987-1988. SOT will have a 1.25-m-diameter primary mirror designed to provide a resolution of 0.1 arcsec at 5000 A (some 70 km on the solar surface).

165 It will operate from 1100 A into the IR region and will observe solar phenomena from the photosphere up through the chromosphere and transition zone to the base of the corona. Focal-plane instruments will range from high- resolution spectrographs to imaging experiments with interference filters, capable of measuring magnetic and velocity fields with unprecedented precision. Operating from low-Earth orbit, SOT will provide images with uniformly high spatial resolution, unaffected by the terrestrial atmosphere. Major scientific objectives of the SOT program are to study and analyze the physical processes occurring on and above the surface of the nearest star. Included among the phenomena to be studied are the following. a. Heating and Energy Balance in the Solar Atmosphere Theoretical work through the 1970's pointed to short- period acoustic waves (periods less than 100 see) as the source of heating in the low chromosphere. While it is believed that these waves are generated by the turbulent motions observed in the low photosphere, long time sequences of velocity and brightness measurements at 0.1 arcsec resolution are needed to resolve the structure in which the heating probably occurs, in order to test this theory of chromospheric heating. The situation in the upper chromosphere and transition region is more chaotic: the transition region appears to be the site of major losses of energy from the corona, as heat is conducted and advec ted downward and then radiated away. The details of the temperature structure of the transition region appear to control the mass flux between the upper levels of the chromosphere and the corona. The relative importance of various energy transport mechanisms (e.g., acoustic or Alfven waves) depends sensitively on the fine-scale geometry of the emitting features, which are probably determined by the magnetic field. Knowledge of that geometry can be obtained only from space observations. b. Plasma/Magnetic-Field Interaction in Subarcsecond Structures The relation of magnetic fields to mass flow, wave motion, and mechanical-energy transfer in the solar atmo- sphere is one of the major problems in astrophysics. Models of nearly every phase of behavior of magnetic fields incorporate an interaction with plasma motion on a small scale. High-resolution observations, obtainable

166 only from space, are required for progress. A major scientific objective of SOT is to provide better defi- nition of the convective input in the photosphere, the magnetic field geometry as a function of weight, velocity fields, mass motions, and pressure gradients from the photosphere through the transition zone. c. Sunspots Sunspots are especially fascinating because they are among the most accessible of astrophysical phenomena in which the interaction of plasma and magnetic fields can be studied. Because of the strong magnetic fields present in sunspots, the energy- and mass-transport processes are very different from those in the quiet, nonmagnetic regions of the solar atmosphere. While recent observa- tions support the idea that convection is strongly sup- pressed in sunspots, they also show that the kinetic- energy density in small-scale motions in sunspots equals that present in the quiet sun. The presence of intense magnetohydrodynamic (MUD) waves is almost certainly the cause of observed large kinetic-energy density in spots and has recently been invoked to explain the energy deficit in sunspots. Theoretical considerations place their scale between 0.02 and 0.2 arcsec, well below the resolution limit of ground- based telescopes, but within the reach of SOT. SOT may be able to study not only the structure of these MAD waves at photospheric levels but also their propagation upward into the corona and their reflection in the upper chromosphere and transition region. C. Recommendations for Other Outstanding Programs and Projects for the 1980's The programs listed under the Recommendations for Major Initiatives (Section I.A) will play a central role in the astronomical discoveries of the 1980's and beyond. These are the programs likely to provide the most dramatic discoveries and to capture public attention. Neverthe- less, astronomy cannot remain healthy if only a few major instruments and groups are supported. The arguments, collaborations, brilliant discoveries, and long tedious hours devoted to survey programs are characteristic of a healthy science in which diverse groups, using a multitude of instrumental approaches, sift through the rain of cos- mic photons in search of some better understanding of our

167 Universe and the objects in it. We give here a brief dis- cussion and endorsement without implication of priority of some of the other programs and initiatives that we believe are vital to the health of astronomy in the 1980's. 1. Solar-Physics Program In addition to launching the SOT and ASO, it is necessary to initiate two other programs that address important problems that ASO and SOT cannot study. a. Solar Coronal Explorer Satellite The basic physics of solar-wind acceleration and the time-evolution of solar coronal structures in response to changing magnetic-field configurations are two fundamental problems that can be addressed by a solar coronal mission of the Explorer class. Measurements provided by this mis- sion will provide unique insight into the more general astrophysical problems of mass loss from stars and the energy and momentum balance of stellar coronas, topics that are becoming increasingly important as a result of recent Einstein x-ray observatory and IUE discoveries. The Solar Coronal Explorer (SCE) should contain at least five complementary instruments: 1. A resonance-line coronagraph operating at Lyman- alpha and the O VI resonance line at 1032 ~ is a new type of instrument recently flown successfully on a rocket but not yet orbited. This instrument provides a new capabil- ity for directly measuring expansion velocities low in the corona. It also yields information on hydrogen column densities and measures coronal kinetic temperatures from line widths. 2. A white-light coronagraph on SCE will provide elec- tron column densities. 3. A soft x-ray telescope will delineate the basic coronal structures--coronal holes in which the magnetic fields are open, loops in which the fields are closed, and x-ray bright points where new fields are emerging--and the temperatures and emission measured in these structures. 4. A coronagraph operating at the He+ 304-A resonance line will, in combination with the hydrogen Lyman-alpha coronagraph, give the observations necessary to determine the probably variable He/H abundance ratio in the solar corona and inner solar wind. The mechanisms leading to

168 the fractionization of atomic species in the solar wind should be elucidated by these coronal observations. 5. Finally, a magnetograph aboard SCE will measure the input of magnetic flux to the corona. Together, these five instruments will measure a com- plete set of coronal plasma parameters (temperatures, densities, magnetic fields, abundance differentiation, and expansion velocities) as a function of radial dis- tance from 1 to 5 solar radii with which to test in detail our present theory of the solar wind. While the SCE is a unique facility in its own right, which ideally should fly during the next solar minimum (1986) when the wind structures are relatively simple, it takes on added importance if flown simultaneously with solar polar passages of the spacecraft of the Inter- national Solar Polar Mission (ISPM) and the operation of the Interplanetary Plasma Laboratory (IPL) spacecraft at the Sun-Earth libration point. Simultaneous observations by the coronagraphs and x-ray telescopes on SCE and ISPM would give stereo views of the solar corona, which is absolutely fundamental for understanding the three- dim!ensional structure and evolution of coronal structures and transients. b. Solar Interior Dynamics Program The discovery of an unexpectedly low neutrino flux from the Sun has stimulated a broad re-evaluation of our ideas about the structure and dynamics of the solar inte- rior. Such questions as whether the Sun has a rapidly rotating core or whether the interior undergoes episodes of mixing have an important bearing on our understanding of solar and stellar activity cycles. Solar seismology, the measurement of the frequencies and amplitudes of various solar modes of oscillation, is an important new tool for the study of the structure and dynamics of the solar interior. The Solar Interior Dynamics Program has the following four major goals: 1. m e determination of the interior dynamics and structure of the Sun (e.g., rotation versus depth, large-scale flows, pulsations, properties of the radiative core); 2. The combination of the resulting models with dynamo theories to illucidate the global magnetic behavior of the Sun and specifically to explain the

169 observations of the solar activity cycle; 3. The development of a more physically self-con- sistent theory of convection; and 4. The application of these results to other stars on which activity cycles, surface activity, and rotation have been observed as the result of high-resolution spectroscopy and x-ray observations by satellites. Components of the Solar Interior Dynamics Program are as follows: 1. Theoretic modeling, including computer simulations of solar interior dynamics with dynamo processes. 2. High-precision observation of solar surface behav- ior, specifically solar-surface motions by means of Doppler shifts with accuracies of about 1 m/sec. The development of a tachometer capable of 1 m/see precision on an absolute scale is crucial. We also require mea- surements of large-scale magnetic patterns, of large- scale brightness patterns related to interior circula- tions with relative accuracies of about 0.01 percent, and of solar-diameter measurements with accuracies of 1 milli- arcsec. The accurate measurement and identification of the frequencies of oscillation necessary for the solar internal temperature stratification will require continu- ous observing sequences of a week or more. Such obser- vations can best be obtained from experiments in Sun- synchronous orbits. The program should begin with long-duration observations from the South Pole, followed by a two-week Shuttle experiment, followed later by a longer-duration experiment (6 months or more) flown on a free-flyer (Solar Interior Dynamics Mission) or in conjunction with the ASO. 3. An understanding of other causes of spectral line shifts, which, if misinterpreted, may lead to systematic errors in the surface-motion observations. SOT will make major contribution toward this understanding and will, in addition, provide important information on the relation- ship between plasma motions and magnetic fields at small spatial scales and the variation of surface convection across the solar surface in both space and time. 2. Sky Surveys Needed to Support Major Missions ST, SIRTF, and the 15-m ground-based telescope are all expensive facilities that require the support of surveys

170 directed to discovering important rare objects that will be studied in greater detail by these more powerful instruments. A number of important survey programs have been proposed. The W OIR Panel notes and endorses the following. a. Infrared Surveys from Space Deep, all-sky surveys covering the IR from 5 to 130 Am at various spatial resolutions are essential as a foundation for the orderly progress of IR astronomy in the 1980's. Present survey experiments include the Infrared Astronomy Satellite (IRAS) to be launched in 1982, the Spacelab Small Infrared Telescope to follow in 1984, and military programs. By virtue of their unbiased sky cover- age and high sensitivity, these surveys will vastly increase our knowledge of the IR sky and will identify significant classes of IR emitters in the Universe, some of which may represent the discovery of new types of objects. It is of the utmost importance that deep-IA surveys from space be successfully completed. b. Moderate and Wide-Field Imaging in the 1200-10,000-' Wavelength Region The ST Wide-Field/Planetary Camera and Faint Object Camera will provide a capability for imagery in the visible and W wavelength ranges with 10 times better than ground-based resolution and with correspondingly improved point-source detection sensitivity. However, the Wide-Field Camera (WFC) has a maximum field of view of 2.7 arcmin square, and there is a demonstrated need for imagery over larger fields in the 1200-10,000-[ range from space-based telescopes, supplemental to the imagery to be provided by ST. This imagery would tenta- tively have two characteristic combinations of field and resolution: (1) high-resolution, moderate-field (nominal 0.5° field of view, 0.3 arcsec resolution) and (2) wide- field, moderate-resolution (roughly 5° field of view, 1 to 2 arcsec resolution). In the following discussion, we present the scientific rationale and justification for these two types of supplemental imagery. High-Resolution, Moderate-Field Imagery. There are a number of important astrophysical problems requiring photometric-quality imagery at considerably better than ground-based resolution but with a much wider field of view than provided by ST. Additional advantages of such imagery, in comparison with ground-based imagery, include

171 accessibility to the W and the darker sky background. The particular advantages of a wide field of view (0.5°, or 10 times that of the ST WFC) are for programs requir- ing the observation of objects of large angular extent (e.g., star clusters, nearby external galaxies, cluster of galaxies); programs requiring large statistical sam- ptes; study of positional dependencies; and searches for rare objects. Such programs would require inordinately large amounts of observing time with the ST cameras and also would represent inefficient and perhaps inappro- priate use of such observing time in comparison with the programs likely to be of highest priority for ST. The use of a "grism" with the imaging camera on the wide-field telescope would allow spectrographic surveys over large fields. This capability permits more accurate determinations of spectral type and effective tempera- ture, correction for interstellar extinction, and easier identification of QSO's and other peculiar objects than would be possible with imaging photometry alone. Inclu- ~ _ g . . · · . . Sian of the W (1200-3000 A) is especially important for grism surveys. Wide-Field, Moderate-Resolution Imagery. The scien- tific problems addressed by a space-based telescope with a wide field (5°) and ground-based resolution (1-2 arcsec) are primarily (1) deep surveys in the ground-inaccessible W region and (2) studies of extended, low-surface- brightness objects at all optical wavelengths, for which a fast focal ratio (f/3 or faster) and observations above the terrestrial airglow are especially important. Deep- W surveys provide a much more sensitive means for mapping the spatial distributions of high-temperature objects than is possible in ground-based surveys. Also, for specific problems, far- W observations can provide quantitative photometric data that are much more useful than those obtained from the ground. This is particu- larly true in the cases of hot, subluminous objects near the end points of their evolutionary cycles, in the central bulges or disks of external galaxies, and in crowded regions of the Milky Way, where the far more numerous late-type stars dominate ground-based measure- ments. In addition, a wide-field telescope would be appli- cable to many of the problems discussed in connection with the high-resolution, moderate-field telescope; although the lower resolution would make it less useful in crowded fields, the wider field of view would allow collection of a larger statistical sample of a given type of object in the observing time.

172 Ground-Based Survey. There is a need to improve the Palomar Sky Survey by repeating it on fine-grained emul- sions. There have been several proposals to construct a large Schmidt telescope to undertake such a survey. The W OIR Panel suggests, however, that the Palomar Schmidt, finished in 1948, be upgraded and used instead. To permit the use of the fine-grained emulsions, the Palomar Schmidt should be upgraded with an achromatic corrector that will reduce the image size at all optical wavelengths to less than 20 ~m; a IIIaJ sky survey should be then carried out. Such a survey would be invaluable for ST pointing and for astrometric purposes. Also, an objective prism can be mounted without creating balance and weight- distribution problems. This would permit low-resolution spectroscopy over a wide field, a valuable capability in the search for distant, high-red-shift quasars. 3. Planetary Observations a. Dedicated Orbital Telescope for Solar-System Studies The power of remote sensing from Earth-orbiting telescopes is now receiving wide recognition in planetary science as a result of the successful application of OAO-2, Copernicus, and IUE to a selection of planetary and cometary problems. Consequently, there is great anticipation for solar-system studies with ST and SIRTF. However, in spite of the capability of these facilities to attack many solar-system problems, there are also many drawbacks and mismatches. Many solar-system observations require telescopes to be pointed in the close vicinity of the Sun or the Earth or require special orientations of a spectrograph slit or require complex attitude maneuvers. There are solar-system observations that need to be made in spectral regions (such as shortward of 912 A), or with combinations of spectral and spatial resolutions, that are not necessarily appropriate for astrophysical problems. In addition, there is clearly a problem with the restricted time available. Many solar-system obser- vations have either special timing requirements or need extended periods of observations, particularly if the observations are associated with an interplanetary mission in progress. Our assessment is that special consideration should be given in the future to the concept of a dedicated orbital telescope for solar-system studies. However, we believe

173 that it is essential that this concept be developed by NASA in the context of the overall priorities for an integrated program of solar-system exploration. b. Extrasolar Planetary Detection The detection and accumulation of statistics on other planetary systems would be an important development for planetary astronomy; given the likelihood of reaching the required accuracy by means of astrometric techniques, we support the start of a cautious program in this field. The development that we envisage should include at least two independent research programs in order to ensure adequate cross-checking of apparently positive results. At present, it is generally assumed that the formation of planetary systems is common during star formation, but, in fact, no proof exists. The statistics of planetary formation will reflect directly on the physical status and the relative peculiarity of our own solar system. They are also involved in the estimation of the prob- ability of other life in the Galaxy. While attempts have been made in the past to detect planetary systems by astrometric means, these efforts have not attained the required accuracy. A precision of 10 4 to 10 5 arcsec--one to two orders of magnitude better than what is possible now--is required, and this precision must be maintained over a time scale of 10 years. Recent studies in NASA-sponsored workshops have indi- cated that such precision may now be technically possible through use of a dedicated astrometric facility. Other methods include the detection of small periodicities in radial velocity, spacecraft interferometry, and direct detection by means of specially apodized telescopes of larger aperture. At present, it appears that the two most likely candidate instruments are a dedicated astro- metric telescope or a high-spatial-resolution IR tele- scope that might detect fragmentation in collapsing proto- stars. While it is not now possible to identify a specif- ic instrument concept, it is likely that there will be a convergence of opinion on instrumentation for this prob- lem in the next two or three years. 4. Observatory Support Optical astronomy is in a transitional period. The optical facilities approved and proposed for spaceflight in the 1980's will provide a capability in the decade

174 that far exceeds the capabilities of the 1970's. mere is good reason for this evolution--astronomy from space is free from the deleterious effects of the Earth's atmo- sphere. Nevertheless, ground-based astronomy will con- tinue to play a pivotal role in the 1980's. On an abso- lute scale, space astronomy will continue to be very expensive; there are many astronomical problems that can be solved using ground-based techniques, and many space programs will require ground-based support observations. For a telescope to be useful, it must be properly instrumented and used. Ground-based astronomy is now suffering from a lack of adequate support funds. Both at the National Astronomy Centers and in the universities, the corrosive effects of inflation have seriously reduced the capabilities of observatory staffs to maintain instru- mentation at a high level of efficiency. Observatories that derive a major portion of the core support from pri- vate endowments or state funds are particularly hard hit, as is the Cerro Tololo Inter-American Observatory, which has suffered from the effects of isolation from the United States, Chilean inflation, and changes in the U.S. tax laws. Observatory groups are responding to the finan- cial pressures by carefully examining their budgets and operations to find areas of cost savings. Nevertheless, it is clear that the current support of observatories is not adequate. Therefore, the W OIR Panel urges NSF and NASA to increase substantially their support of observa- tory operations. We recognize that, traditionally, NASA has been required to link this support to a space mission, but we see no difficulty in doing so, particularly since SIRTF, ST, and x-ray observations will require even more ground-based backup observations. The KAO has been extremely productive scientifically and provides a unique and flexible access to the IR region beyond 30 Em. Until the space IR telescopes are fully operational, the KAO provides one of the major facilities for observation in the 30-300 Am wavelength band. It also provides a means of rapidly testing new instruments. Flight durations have recently been cur- tailed owing to lack of funds for jet fuel. This has severely restricted the amount of observing time avail- able on this important national facility. Augmentation of the KAO flight program would increase its productivity and should be undertaken.

175 5. 2.5-5-Meter Telescope Program There is a demonstrated need for a number of small special-purpose instruments. Perhaps most of these would be operated by university groups that are likely to be successful in attracting state and private funding, particularly if some federal matching funds could be obtained. We have noted and particularly endorse the following: (a) A dedicated 4-m-class IR telescope in the southern hemisphere. The cost of such a telescope need not be large if advanced construction techniques are used. This telescope should be viewed in the perspective that Amer- ica, and indeed the world, has no large IR telescope in the southern hemisphere, the hemisphere that is often said to be the more important hemisphere for modern astrophysics. (b) A 2-m-class astrometric telescope in the southern hemisphere. There has long been felt a need for a tele- scope optimized for astrometry in the southern hemisphere. In fact, such an instrument was recommended in the 1972 report of the Greenstein Committee. This instrument would complement the U.S. Naval Observatory instrument at Flagstaff, Arizona, and rectify a major imbalance in positional astronomy in the southern hemisphere. This program is particularly important in view of the need for accurate astrometric data to support space missions that view both hemispheres. (c) Interferometric telescopes. For some specialized applications, substantial increases in our ground-based capability can be obtained with relatively modest invest- ments. Spatial interferometry in the IR region using speckle, Michelson, and heterodyne techniques has produced highly interesting results. A specialized facility for this purpose, capable of resolving structure in the 0.1-0.01-arcsec range over a wide range of wavelengths, would be extremely exciting for the detailed study of the structure of late-type stars, circumstellar shells, and embedded objects. Atmospheric windows exist from 300 Em to 1 mm, which can be effectively exploited with innovative far-IR and submillimeter telescopes. (d) University telescopes. A number of universities are seeking funds for the contraction of general-purpose, intermediate-sized (2-m-class) telescopes at good sites to support faculty research and graduate-student instruc- tion. It has been shown that such instruments can be

176 constructed economically with today's technology. m e scientific utility of such instruments is high because they have sufficient aperture to attack frontier problems in astronomical research. Among other uses, they will permit university groups to carry out observing programs in support of space observations by staff scientists. Since major support funds for these instruments will flow from private or state sources, they are, from the federal point of view, a very cost-effective way of advancing astronomical research. (e) Balloon facilities. At present, IR experiments account for a substantial portion of the astronomical use of the National Scientific Balloon Facility. Balloon altitudes offer excellent sensitivity and atmospheric transparency at the longer wavelengths. Balloon tech- niques are especially suited to specialized experiments, the testing of new ideas, and new-technology development. Balloonborne IR astronomy warrants continued and expanded support. 6. Moderate Cost Space Missions a. Astronomy Payloads on Space Shuttle Spacelab II is intended to demonstrate the effective- ness of the Space Shuttle for carrying a variety of astro- nomical instruments on short missions. Instruments on Spacelab II include a small cryogenically cooled IR tele- scope, a W telescope, solar telescopes, and others. The Shuttle promises to be an important means for orbiting small experiments with far longer observing times than rockets provide and at a cost less than that of free- flying satellites. Unfortunately, despite the outstanding promise of the Space Shuttle for astronomical research, funding for Spacelab experiments has still not reached substantial levels; moreover, funding for a number of experiments that had already been approved was recently reduced, and the selection of additional experiments in the Principal Investigator class has been deferred. These programmatic constraints have affected Shuttle flight opportunities for a wide range of programs. However, they have had a particularly serious impact on prospects for a coherent program of W astronomy during the 1980's, which should be based to a significant extent on small, special-purpose instruments designed for Shuttle flights to obtain data in areas not well covered by IUE, ST, or the far- W spectrograph in space recommended in this report.

177 We therefore urge an augmentation of the Shuttle small- payload program at NASA as a minimum requirement to allow currently foreseen, high-quality investigations to be carried out through reasonably frequent flight opportu- nities during the 1980's. In addition, we urge NASA to give high priority to implementation of methods for in- creasing the flight duration of individual missions, so that the observing time available can be increased without a proportionate increase in launch costs. Such methods could include extension of the orbital-stay times of Shut- tle missions or transfer of the instruments to long- duration space platforms or free-flyers that could be revisited by the Shuttle at intervals of 3 to 6 months. b. Explorer Program The NASA Explorer program provides for a class of exploratory science missions involving small, free-flying satellites, generally not recoverable, dedicated to spe- cific new types of investigations (e.g., IUE, IRAS). Such satellites are most suitable for missions requiring very long observing times with relatively simple and routine measurement techniques and not requiring instrument changes or film recovery. The only currently approved Explorer mission in the area of W astronomy is the Extreme Ultraviolet Explorer. This mission is intended to survey the sky for sources of radiation in the loo-9oo-A (E W) wavelength range and provide relatively broadband photometric measurements of the sources detected. It is expected that all Explorer-class satellites (after the IRAS launch) will use the Shuttle as a launch vehicle, along with an Inertial Upper Stage (IUS) if even higher orbits are required. Although we can foresee sev- eral areas in which Explorer-class satellites would be extremely valuable to WOIR astronomy, many of these will be more expensive than the historical cost of Explorer missions. We therefore urge that both the total Explorer funding level and the cost limit for individual Explorer missions be substantially increased. Also, we urge that NASA look into means for providing the high-capability pointing systems required for astronomical observations with tele- scopes in the 1-m-aperture class and strive to minimize the cost of such pointing systems. Despite attempts at economy, it is clear that inflation and the increased sophistication required of exploratory science missions are seriously compromising the effectiveness of a highly

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