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6 Observational Techniques and Facilities INTRODUCTION Many of the recent findings in planetary astronomy have been the result of applying new techniques to planetary problems. This chapter provides a brief description of the more important techniques, since they are likely to be un- familiar to many. It also surveys the major optical, radio, and radar facilities with capabilities for planetary research. RADIO AND RADAR TARGET MAPPING TECHNIQUES The relatively long wavelength of radio and radar observing systems as com- pared with optical telescopes makes it difficult to obtain a high degree of angular resolution of the target by the straightforward use of large reflectors alone. For example, for the low-frequency radiation from Jupiter, resolution of 1 sec of arc requires an aperture of roughly 3000 km; and at the short wave- length end of the radio spectrum near 1 mm, an aperture of about 200 m is required for this resolution. To achieve angular resolutions comparable with those available optically, therefore, radio and radar astronomers have been forced to develop other methods that can synthesize the large effective aper- tures demanded. A conventional "filled" receiving aperture, such as a parabolic reflector, 46
OBSERVATIONAL TECHNIQUES AND FACILITIES 47 contains highly redundant statistical information on most of the spatial Fourier components of the incoming signal, since a number of spacings of less than the full diameter can be found, all of which have the same separation and angular orientation. Thus, it is possible to devise an array of small antennas, each of manageable size, whose elements may be paired off with each other to produce substantially all the needed spatial Fourier components. In fact, the complete set of components need not be obtained simultaneously, provided the angular distribution of the received power in the sky varies only in a predictable way with time. Several installations based on these principles that should be able to yield angular resolutions of the order of 1 sec of arc at radio wavelengths have been proposed. The signal-to-noise ratio obtained from these "unfilled" apertures will be considerably lower than that produced by a filled aperture in the same observation time, but for many targets useful results can still be obtained. For radar observations of a rigid body, there is even greater redundancy in the returning signal, since the echoes represent the scattering of a coherent transmission by a collection of scattering elements maintaining a fixed relation- ship to one another. Given a priori information concerning the motion of the target, the phase history of the echo from each element of the surface is com- pletely predictable. It becomes possible, therefore, to analyze the returned echo power for its frequency Fourier components and to relate the components to the scattering from known locations on the target. The resolution thus ob- tained varies with the motion of the target, the radar carrier frequency, and the duration of the observation; but it frequently permits surface localization of the scattering that compares favorably with optical results. Most of the celestial objects studied by radar have dimensions such that echoes from different regions on their surface will differ measurably from each other in time of flight. For nearly spherical targets like the Moon or planets, equidistant regions of the surface will lie on small circles concentric with the visible disk. Since the target is assumed to be a rigid body, all points at the same projected distance from the instantaneous apparent axis of rotation must have the same component of motion in the direction of the radar. Again, for a spherical target, these loci of constant Doppler shift become a set of small circles "edge-on" to the radar and, therefore, at right angles to the loci of con- stant delay. The geometry of this "delay-Doppler" coordinate system is shown in Figure 3. Since the returned echo power for many targets (including the Moon, Mercury, and Venus) may be analyzed simultaneously at high resolu- tion in both coordinates, a basis exists for mapping the distribution of radar reflectivity. The chief drawback in applying delay-Doppler mapping techniques lies in the need to resolve a basic ambiguity in the coordinate system as shown in
48 PLANETARY ASTRONOMY FIGURE 3 Geometry of the lunar surface with respect to the radar in the delay-Doppler mapping technique. The apparent lunar rotation differs from the intrinsic lunar rotation about its north pole because of the motion of the Moon in its orbit around the Earth and because of the motion of the radar with respect to the Moon due to the Earth's diurnal rotation. Note the two conjugate surface areas, P and P1, which have the same values of range (delay) and Doppler frequency. A, North lunar pole; B, pole of ap- parent rotation; D, Doppler contour for relative frequency, /; R, range contour for relative range, t. Figure 3. This ambiguity arises because only two of the three surface Cartesian coordinates are measured, and, while the surface of a sphere is constrained by x2+y*+z2 = r2, there are two roots to the solution for the third coordinate. It is interesting to note that an equivalent ambiguity would also exist for direct angular mapping if the target were transparent so that the back and front sides were equally visible. In the case of the Moon, where a disk of relatively sub- stantial angular size is presented to the radar, the points of ambiguity can sometimes be isolated by using the angular resolution afforded by the antenna beamwidth. An example of the application to the Moon of this type of mapping is shown in the Frontispiece. The extension of this technique to the planets, where the target disk is very much smaller and where the antenna
OBSERVATIONAL TECHNIQUES AND FACILITIES 49 beamwidth of any existing or proposed radar system is too large to resolve the ambiguity, will require interferometric measurements. The interferometer is the basic tool of the high-resolution array. For base- lines up to a few hundred kilometers, the individual elements of an inter- ferometer or array can be connected by cables or radio links. The high-resolu- tion arrays planned for the Owens Valley Radio Observatory and the National Radio Astronomy Observatory are designed to give resolution to a few seconds of arc at wavelengths over the 3- to 21-cm range using a number of moderate- sized reflectors interconnected by cables. These large facilities will satisfy many of the requirements for high-resolution planetary observations over this wavelength range. A similar array is needed to extend the wavelength coverage from 3 on down to millimeter wavelengths. For element separations greater than a few hundred kilometers, as in the case of high-resolution observations of Jupiter in the 15-m wavelength range, systems have been developed that record on magnetic tape the radio waves measured at each element location for subsequent correlation. In one system, the stored electrical signals from which the radio-wave phase information has been removed are combined in a video correlator. This system is relatively insensitive to scintillations. In the system, which retains the full phase informa- tion of the wavefront and thus has far greater sensitivity, atomic frequency standards are used at the separated locations to provide phase-stable local oscillators. HIGH-RESOLUTION FOURIER SPECTROSCOPY Fourier spectroscopy has recently been very successfully applied to the planets. The technique makes use of all the light reflected from a planet, collected by a large telescope, to derive high-resolution spectra. It is particularly effective for measurements in the 2-^ window in the infrared, where, because of practical considerations, the interferometer yields a very substantial gain in resolution over the spectrometer. The recently developed Connes interferometer, responsible for the success- ful measurements, has a long-term mechanical stability that can be used in conjunction with a large telescope. The motion of the interferometric mirror can be measured to an accuracy of a fraction of a wavelength of visible light. Precise absolute wavelengths are obtained by this system, since the position of the moving mirror is monitored by observing the interference fringes produced by an accurately known visible wavelength. The spectrum of Venus was ob- tained to a precision of 0.002 to 0.003 cnr1 at 1.6 ^ closely approaching that of the best measurements available from laboratory sources. The resolution
50 PLANETARY ASTRONOMY obtained is 0.08 cm-1, which is approximately the theoretical resolution of a 4-in.-wide diffraction grating used at 60° angle of incidence. In a favorable case in the laboratory, a resolution equivalent to the theoretical resolution of a grating of 1-m width has been obtained. Several hours are required to scan the 1.6-p atmospheric window; thus several runs can be made at a given atmo- spheric window in a single observation period. The instrument has the same limitations as conventional spectrographs, for spectra can only be obtained in the atmospheric window regions. However, because of the great detail obtained and the high precision of wavelength measurement, the data provide a unique and nearly complete picture, both qualitatively and quantitatively, of the planetary atmospheric constituents outside of the telluric bands. It is true, of course, that only gases having infrared-active vibrations or a sufficiently large quadrupole moment can be observed. The spectra obtained by this interferometric method have been superior both in resolution and wavelength accuracy at least by an order of magnitude to those obtainable by conventional methods. This instrument is best used in conjunction with the largest telescopes, since the spectral resolution attainable is directly proportional to the flux available. With such telescopes the interferometer seems to be able to achieve a spectral resolution limited only by the widths of the lines being observed and an accuracy of wavelength measurement that is a small fraction of the line-width. Results of this order have already been obtained on the spectra of Venus, Mars, and Jupiter. Since a smaller amount of flux is available for other planets, the resolution of their spectra will be somewhat less. Nevertheless, the inter- ferometer could be used to search for an atmosphere of Mercury, and useful information could be obtained concerning any atmosphere associated with Jupiter's moons or Saturn's rings. USE OF AIRPLANES, BALLOONS, ROCKETS, AND EARTH SATELLITES Recent technical advances in the use of upper-air vehicles have opened up wider spectral regions for planetary investigations. Although this approach is much more expensive than ground-based observations, it is far cheaper than the use of Earth-orbiting satellites. Observations in the near infrared, free from the heavy absorption of water vapor, are being made from airplanes just above the tropopause. A plot of the solar spectrum obtained at different latitudes in the spectral region 1-15 /x is shown in Figure 4.
OBSERVATIONAL TECHNIQUES AND FACILITIES FIGURE 4 Solar spectra observed by Strong in the 1- to 13-/* region at different altitudes. Insert: atmospheric window from 16 to 24 /j, observed by Adel at Flagstaff, Arizona. The total precipitable water was about 2 mm, and the cutoff of the KBr prism was near 24 /*. Because the atmospheric water vapor is somewhat colder than the Earth's surface, the transmission indicated may be somewhat high. More recent measurements from the sur- face under comparable water-vapor conditions by Farmer and Key have shown that this window (with lower transmission) extends to 38 /*. They also found a window with 5 percent transmission centered at 345 ft. Atmospheric transmission at longer wavelengths has been observed at 7,000 ft when about 2 mm of precipitable water was present. Observations of the solar spectrum made under similar water-vapor conditions indicate a continuation of this window, but with significantly lower transmission, to 38 ft,. Solar energy has also been detected in a region centered at 345 /t, where the maximum estimated transmission is 5 percent. The 20-38-^ window is particularly appropriate for work on Jupiter, Saturn, Uranus, and Neptune, because they radiate most of their energy in this spectral region. Observations can be extended toward the ultraviolet through the use of unmanned balloons. Above 110,000 ft, measurements are possible from 1950
52 PLANETARY ASTRONOMY to 2350 A and at wavelengths longer than 2750 A. There are still ozone absorptions of the order of one magnitude, and since they vary with the float altitude, it appears that absolute photometry should not be attempted. Some spectroscopy, photography, and polarimetry can be carried out from balloons. Direct photography is attractive, especially for Venus, where detail on the disk has been photographed at 3300 A; the cloud features may be even more pronounced at shorter wavelengths. Determination of optical thickness of a molecular atmosphere, such as for Mars, also appears promising. Polar- imetry may be the best technique, owing to the inherent precision of polar- imetric differential measurements. The attraction of a balloon-borne telescope is that it can be flown during one night, and then, if desired, the experiment can be modified or changed and reflown. Rocket flights offer considerable potential for exploratory observations of the ultraviolet spectra of the brighter planets. The region longward of about 1800 A is likely to be observable, in the few minutes an Aerobee sounding rocket is above the atmosphere, with spectral resolutions of a few angstroms for Venus, Mars, Jupiter, and possibly Saturn. Not only absorption bands and continua may be expected, but fluorescent scattering of sunlight may be seen, depending on the atmospheric composition. Further, any resonance scattering of sunlight by atomic hydrogen or oxygen in the outermost atmospheres of these planets (analogous to the Earth's hydrogen "corona"), if detected in the 1200-1300 A region, would be most informative and valuable. These emis- sions would not only indicate the presence of these constituents but would also give some indications of the temperature governing the rate of escape of the atmosphere. Raman scattering of the solar Lyman-alpha line ofH (1215 A) by H2 and by He on Jupiter or Saturn, intense enough to be detected by a rocket, is also a distinct possibility. IMAGING TECHNIQUES Earth-based photography will continue to be a principal method of exploring planetary surfaces and atmospheres. It provides the greatest opportunity for the observation and recognition of totally unexpected features, conditions, and processes and the best method of monitoring changes of a planetary surface and atmosphere. Since photography gives a permanent record of constant, transient, and unexpected features, it constitutes a most important background for other investigation. Finally, comparison of close-up photography from spacecraft with Earth-based photographs made simultaneously should provide the basis for interpreting earlier photographs obtained with the same Earth- based telescopes.
OBSERVATIONAL TECHNIQUES AND FACILITIES S3 Since photography is so important in planetary astronomy, any technique that increases the information obtainable from a planetary image is important. Improvements in planetary imagery can be made at three stages in the process: before the image is recorded, during the recording, and after the image is recorded. The location of the telescope is a prime factor in determining the quality of the image that arrives at the detector, since resolution is usually limited by turbulence in the atmosphere rather than by the instrument. Recent studies have shown that sites exist, especially in northern Chile, where star images may average no more than half the size of images observed elsewhere. Tele- scopes at such sites would be capable of four times the areal resolution pres- ently available and would constitute a significant advance in planetary image recording. For this purpose telescopes of aperture larger than 60 in. provide no additional advantages and may be inferior because of problems associated with mirror figure. It has been shown that image quality may be improved by careful visual or photoelectric selection of the time at which a photograph is made. The typical exposure time of a planetary photograph is in the range from 1/100 sec to a few seconds; yet one image of exceptional quality per hour may be a satis- factory data rate. Many images have been recorded and sorted at a later time and the better quality ones superimposed to form composites. For several planets, exposures obtained several minutes apart cannot be used in this way because of the planets' rapid rates of rotation. Image tubes can be used in such a way that high-speed corrections are electronically applied to the position of the centroid of a planetary image. The stabilized image on the output phosphor of the tube is then projected and photographed. This method is particularly effective when the planetary image is sharp but jumps about in the focal plane of the telescope. Still more sophisticated image-correction methods might be further investi- gated. They would involve high-speed correction of irregularities in the wave- front arriving at the telescope. It is possible that planetary images can be enhanced after they have been registered on photographic plates, perhaps after digitization or directly by an analogue technique. After-the-fact enhancement of detail in the Mariner IV television pictures of Mars and of the Surveyor pictures of lunar soil, and progress in restoring atmospherically degraded pictures of Earth satellites, indicate that postdetection processing may offer a promising means to improve ground-based photography in the next decade. Lunar and Martian image data in digital form have been treated by computer filtering; ground-based photo- graphs of Earth satellites have been restored by direct digitization of film images; Ranger photographs have been treated by the optical analogue of
54 PLANETARY ASTRONOMY computer filtering. The considerable experience with the filtering technique from radio-frequency applications can also be applied to ground-based plane- tary photography. Further improvements in postdetection digital processing are also likely to result from making use of known characteristics of the images being recon- structed. For example, much a priori information about a planetary image already exists; it is circular, of finite size, has a known range of albedos, and so on. In principle, it should be possible to combine this information with knowledge of image degradation effects to produce a processed image of maximum resolution. An additional advantage of digital processing is that the integration of information from numerous individual images can be carried out in either intensity or frequency and with a variety of weighting factors. Analogue filtering, which is accomplished by apodizing the aperture of a coherent optical system with a transparency of the image to be processed, and then filtering at the image plane of the special coherent system, appears to be particularly applicable to the great body of existing high-quality planetary photographs. Since digitization is avoided, the method is inexpensive and rapid. SCANNING TECHNIQUES Spectral or area scanning can be used advantageously in a number of ways to make quantitative measures of planetary features. These include spectro- photometry, polarimetry, and colorimetry. A new method of scanning spectra has recently been used successfully: in a few seconds, the energy distribution over ranges of hundreds of angstroms is repeatedly measured and recorded by rocking the grating in such a way that its spectrum moves perpendicularly across the exit slit of the spectrograph. One of the advantages of the method is that it can be used when the atmo- spheric transparency is far less than what is generally regarded as "photo- metric." This is of particular importance in planetary observations involving transient phenomena and has the added advantage of effectively increasing telescope time for precise photometric photometry. When an aperture is moved across the image in the focal plane of a telescope it is called area scanning. Area scanning has been demonstrated to improve substantially the effective photometric resolution obtainable in the polarimetry and colorimetry of planets. Quantitative measures of double stars have shown a four- or fivefold advantage over conventional photoelectric techniques used in work on visual binaries with separations of less than 5 sec of arc. Area scanning has also been used successfully by periodically moving a small focal-plane
OBSERVATIONAL TECHNIQUES AND FACILITIES 55 aperture along the entrance slit, while the exit slit of a spectrograph remains at a position corresponding to a preselected wavelength. In the above applications, a quantitative and integrated picture of the physical conditions along the line of scan has recently become easily obtainable through the availability of multichannel analyzers which can add the pulse counts obtained during successive scans and accurately store them in a suc- cession of channels. For observations of the Moon and planets the apertures used in these scan- ning techniques can be substantially smaller than the seeing image of a point source at the focal plane of a telescope. Examples of Martian scans showing changes of intensity and polarization at different wavelengths are shown in Figure 5. RADAR, RADIO, AND OPTICAL FACILITIES Radar Since the first radar contact with a planet in 1961, six working groups in three countries (the United States, the Soviet Union, and the United Kingdom) have reported research in planetary radar astronomy. For the present, the United States is pre-eminent in radar astronomy, with work proceeding at three major installations (see Table 2). The groups currently involved are centered at the Jet Propulsion Laboratory (using the Goldstone Deep Space Communi- cation Complex), Cornell University (using the Arecibo Ionospheric Observa- tory), and the MIT Lincoln Laboratory (using primarily the Haystack Microwave Facility). The fourth U.S. working group, at the National Bureau of Standards, has been concerned almost exclusively with the ionosphere and is no longer active in planetary radar studies. Relative sensitivities at these, and several older, facilities are plotted in Figure 6. Observational opportunities are shown in Figure 7. From the standpoint of personnel, the current U.S. effort in planetary radar astronomy is very small: a total of perhaps 12 professional scientists and six graduate students is actively involved. The total dollar investment in facilities is relatively larger; it must be remembered, however, that all the facilities were conceived and constructed primarily for other purposes, and the bulk of their activities relates to other fields. Prorating to the percentage of planetary radar usage is difficult and perhaps misleading; the average annual figure is prob- ably between 5 and 30 percent of the total budgets of these installations. As has been noted, the facilities listed in Table 2 were developed largely for purposes other than planetary radar astronomy, for example, ionospheric
PLANETARY ASTRONOMY ClOUD ON â MOININO UMl A344S APR 17, 1967 SCAN SLIT- ITS EXIT SLIT-20A APR 17, 1967 SCAN SLIT- 1"5 EXIT SLIT- 20A JUN 9, 1967 AP- 0:2 [« 10 IK OF AIC >j FIGURE 5 Scans of Mars near its 1967 opposition. The upper two curves are area scans made by Boyce in the E-W direction through Elysium. A spectral scanner was used to limit the wavelength band under observation. The solid curve at the bottom is one of eight intensity scans made by Hall in a N-S direction with a polarimeter. The resulting polarization is shown by the open circles.
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PLANETARY ASTRONOMY «tuu '' I97£? ,. / MOONS OF JUPITER / ⢠s A in *"MF T3 MERCURY (SUP CONJ.) Al° V 0350 ~VENUS(SURCONJ.) DSCC», - O CO ' MERCURYdNFfCONJ.) Vln^**MH ⢠JnUJ* Ul xx QC / »JB H VENUStlNf-CONJ.) . co / " 0300 $/ - X 1â ⢠0. <£/ " ⢠X ' ,^-SICNAL CORPS 250 MOON . rfXll ,1.1 1940 1950 I960 1970 YEAR FIGURE 6 Growth in sensitivity of radar astronomy systems since the first detection of the Moon in 1946. The ordinate is a logarithmic scale and represents the path loss which the plotted facility is powerful enough to overcome. Several representative experimental landmarks are shown next to their associated path loss. JB, Jodrell Bank, University of Manchester, England; JRO, Jicamarca Radar Observatory, Environmental Science Ser- vices Administration and Institute Geofisico del Peru, Jicamarca, Peru; MH, Millstone Hill, MIT Lincoln Laboratory, Westford, Mass.; USSR, Institute of Radio Engineering and Electronics, Crimea, USSR; DSCC, Deep Space Communication Complex, Jet Pro- pulsion Laboratory, Goldstone Lake, Calif.; AIO, Arecibo Ionospheric Observatory, Cornell University, Arecibo, P.R.; HMF, Haystack Microwave Facility, MIT Lincoln Laboratory, Tyngsboro, Mass. research, radio astronomy, and tracking and communicating with deep-space probes. Because of their high cost, future radar facilities will likely also be shared with other types of research that require large antenna systems. At the present time, no firm plans exist for the construction of facilities that would maintain the momentum of radar growth as shown in Figure 6. Table 3 lists several possibilities currently under consideration and the dates by which they
OBSERVATIONAL TECHNIQUES AND FACILITIES 59 might be realized if funding and authorization were provided in the near future. The antenna is the element of a large radar system that normally costs the most and presses hardest at the frontiers of technology. Since it is involved twice in the radar process, first in transmission and later in reception, the sensi- tivity of the system is critically dependent on antenna size. In fact, at a given 290 300 310 320 330 §340 CO CO 3 350 CL 360 370 380 390 400 ©[SUN] f. ICARUS ®(JUN'68) EROS (MAY '68) ' % [SATURN] V [URANUS] [NEPTUNE] \TITAN RHEA J<RHEA . i ⢠3tr~5fr 10 DELAY FIGURE 7 The estimated radar detectability of a number of possible targets in the solar system shown in terms of their logarithmic path loss and plotted against the round-trip radar echo delay. The variation in path loss and delay for a given target reflects its vary- ing distance from Earth. Targets shown with dotted lines have deep atmospheres and consequently have radar scattering properties which are difficult to predict. They are shown for reference under the (unlikely) assumption that they have reflectivity as low as 0.1.
roT SI . «) ^"^ Q s .§- s 3 r \ ? J 2 e 3|ril| mil N 00 o\ 1 ^ « â s ** â¢n en O O S o o â¢Â£ H (2 =^ «* _r"2 g 1 S - 1-i "° â¢Â« â¢o 1 1 "S l.a i* rt 0 Q, p b 9 Q M 6 f A â¢S â¢^ UO O_ â¢3 N 00 00 | o o y S 53 VN ^ h â â¢* M -^ I Id o 1 III O in i T-t a c\ 1 1 8 2 'S » ff JH 2 a IS SP a C/3 11? a, 2 IM <U u fr 3* 1 *J > ^ -a a £^^ t> i â . a M S a Q «r o O ire Configurate Location Goldstone Lak Calif. Arecibo, Puert 8 Not definite 3 a rt a JD *9 1 1 H jy Communi' plex ^o J o 1 *C u o 1 oration I fr Deep Space tion Com Arecibo Ion servatory Northeast R tory Corp 1 60
OBSERVATIONAL TECHNIQUES AND FACILITIES 61 wavelength, sensitivity varies as the fourth power of the antenna diameter. Thus the antenna is identified as the key element in any radar configuration and should receive the major emphasis. Since many years' lead time is involved in realizing the next generation of facilities, essential decisions on funding responsibility should be made promptly. Radar astronomy has come of age in the shadow of large installations built primarily for other purposes, and its source of support has tended to remain invisible. Both the value and the cost of radar research call for a more forthright recognition of its potential in planetary astronomy. Radio Radio observations have been used to study planetary surfaces either at low resolution, referring to the entire surface, or at high resolution to make more detailed studies. More than a dozen radio telescopes have been used to mea- sure radiation from the entire disk of planets. Only two, however, at wave- lengths greater than 3 cm, have sufficiently high resolution to study limited portions of planetary surfaces. They are described in Table 4. In addition, high angular resolution has been achieved with a very-long-baseline interferometer to locate the sources of decametric emission from Jupiter. Using delay-Doppler techniques, the resolution currently attainable by radar for the Moon and Venus is about 2 sec of arc. The resolution of the existing connected radio interferometers having the highest resolution is about 10 sec of arc. A resolution of 1 sec of arc has been achieved with a very-long- baseline interferometer at 15-m wavelength. There is now no plan or proposal to fill the need for greater collecting area or for higher resolution at wavelengths shorter than 3 cm. This spectral region is important in studies of planetary atmospheres and for surface studies of TABLE 4 Major U.S. Facilities Used for High-Resolution Planetary Radio Astronomy Institution Location Number and Size of Reflectors Wavelength (cm) California Institute of Tech- nology National Radio Astronomy Observatory Big Pine, Calif. (2) 90 ft (3) 83 ft >3 >3 Green Bank, W. Va.
62 PLANETARY ASTRONOMY TABLE 5 Possible Future Major Facilities That Could Be Used for Planetary Radio Astronomy X Institution Location (cm) Facility California Institute of Tech- nology Cornell University Big Pine, Calif. >3 >10 >3 >3 High-resolution array of (8) 130-ft reflectors Upgrade surface of 1,000-ft reflector High-resolution array of (36) 82-ft reflectors Fully steerable 440-ft reflector National Radio Astronomy Observatory Northeast Radio Observa- tory Corporation Arecibo, Puerto Rico Southwest U.S.A. Massachusetts Mercury and Mars similar to those made of the Moon. Such radio telescopes would also contribute significantly to galactic and extragalactic research. Four possible future radio facilities which will make possible improved planetary studies are listed in Table 5. Optical The percentage of time currently allocated to the observation of the Moon and planets with large optical telescopes is shown in the final column of Table 6. Although these percentages seem very low, two points should be made with TABLE 6 Recent Use of Optical Telescopes for Lunar and Planetary Research Institution (Telescopes) Aperture (in.) Years Included Use (Percent of Time) Mt. Wilson and Palomar 200 1966-1967 3 100 1966-1967 4 60 1966-1967 10 Lick 120 1965-1967 5 KittPeak 84 1965-1967 5 60 1965-1967 14 McDonald 82 1965-1967 23 Perkins' 72 1966-1967 13 University of Arizona Lunar & Planetary Laboratory 61 1966-1967 28 60 1966-1967 28 â¢Perkins Telescope of Ohio Wesleyan and Ohio State Universities at the Lowell Observatory.
OBSERVATIONAL TECHNIQUES AND FACILITIES 63 TABLE 7 Optical Telescopes under Construction for Lunar and Planetary Research Institution (Telescope) Aperture (in.) Estimated Completion University of Texas (McDonald) University of Hawaii (Mauna Kea) 105 84 1968 1968 regard to their interpretation. Those responsible for the assignment of tele- scope time have made it clear that each request is judged solely on its merits and not on the branch of astronomy to which it pertains. Also, it is not pos- sible to assign large blocks of time on a major instrument to any single project. Except for the occasional need for large blocks of time (either for use of special instruments for very favorable planetary oppositions or for unusual events such as the appearance of a bright comet or unusually favorable weather conditions), ground-based observational requirements in the Northern Hemi- sphere appear to be met by existing telescopes. Additional needs in the near future could be filled at least in part by the two major instruments now under construction (Table 7) and by an inexpensive large telescope recommended in Chapter 8. There is, however, only one moderately large (60-in.) American telescope in the Southern Hemisphere. Its use is shared by American and Chilean astronomers. Another instrument placed at a similar site of superb seeing is badly needed to permit observations of all planets where maximum resolution is of utmost importance and for observations of Mars at its most favorable oppositions which occur when the planet is at large southern declinations.
