Assuming that the ATD/NTOT flies as a demonstration of technology for future DOD applications, it is most appropriate that the first astronomical goal should be a similar demonstration, or at least a test, of technological capability for astronomical applications. Based on the information at its disposal, the task group believes that many, but obviously not all, of the activities in support of this goal can be carried out during DOD’s technology demonstrations. This chapter enumerates some of the many possible demonstrations and tests of capabilities that would be of interest to astronomers.
Some of the tests suggested here are essential to validate the ATD/NTOT technology for use in future space astronomy missions. Other tests are desirable but not essential. A number of the suggested tests are directly related to the possible scientific programs that could be conducted in an extended mission, and others are not. Some of the demonstrations can be carried out entirely within the baseline mission, whereas others require one or more of the performance enhancements discussed in Chapter 2.
The task group would like to see all of its suggested enhancements to the baseline ATD/NTOT implemented. The critical ones are improving the surface of the primary mirror and adding the optical framing camera. Nevertheless, the task group recognizes that none of them may be financially feasible, and some may even be incompatible with the still-changing goals of DOD’s mission. If none can be made, the baseline mission (as understood in mid-1995) is still capable of demonstrating many important technological capabilities that are of interest to astronomers.
Clearly, the task group’s list of possible demonstrations is not intended to be exhaustive. Rather, it is an initial list from which tasks can be added or deleted as the financial constraints and programmatic goals of the ATD/NTOT mission become clearer.
The most fundamental demonstration for astronomical purposes is to show that large mirrors can be refigured on orbit. The ability to sense a wavefront and close the feedback loop to control the shape of the mirror is critical to using the ATD/NTOT’s technology in future astronomical missions.
As part of this demonstration, various approaches to closing the loop should be explored. Techniques to be
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A Scientific Assessment of a New Technology Orbital Telescope 4 Astronomical Technology Demonstrations Assuming that the ATD/NTOT flies as a demonstration of technology for future DOD applications, it is most appropriate that the first astronomical goal should be a similar demonstration, or at least a test, of technological capability for astronomical applications. Based on the information at its disposal, the task group believes that many, but obviously not all, of the activities in support of this goal can be carried out during DOD’s technology demonstrations. This chapter enumerates some of the many possible demonstrations and tests of capabilities that would be of interest to astronomers. Some of the tests suggested here are essential to validate the ATD/NTOT technology for use in future space astronomy missions. Other tests are desirable but not essential. A number of the suggested tests are directly related to the possible scientific programs that could be conducted in an extended mission, and others are not. Some of the demonstrations can be carried out entirely within the baseline mission, whereas others require one or more of the performance enhancements discussed in Chapter 2. The task group would like to see all of its suggested enhancements to the baseline ATD/NTOT implemented. The critical ones are improving the surface of the primary mirror and adding the optical framing camera. Nevertheless, the task group recognizes that none of them may be financially feasible, and some may even be incompatible with the still-changing goals of DOD’s mission. If none can be made, the baseline mission (as understood in mid-1995) is still capable of demonstrating many important technological capabilities that are of interest to astronomers. Clearly, the task group’s list of possible demonstrations is not intended to be exhaustive. Rather, it is an initial list from which tasks can be added or deleted as the financial constraints and programmatic goals of the ATD/NTOT mission become clearer. DEMONSTRATIONS DURING THE BASELINE MISSION Use of Active Optics The most fundamental demonstration for astronomical purposes is to show that large mirrors can be refigured on orbit. The ability to sense a wavefront and close the feedback loop to control the shape of the mirror is critical to using the ATD/NTOT’s technology in future astronomical missions. As part of this demonstration, various approaches to closing the loop should be explored. Techniques to be
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A Scientific Assessment of a New Technology Orbital Telescope explored should range from autonomously closing the loop on the spacecraft to deliberately varying the focus in order to attempt phase retrieval on the ground and uploading of new transfer matrices for the feedback loop. The interferograms from the wavefront sensor must be recorded and returned to the ground to enable thorough analysis. Image Quality A top priority is to evaluate the quality of the images. With the baseline equipment, evaluation will be primarily indirect, based on the output from the optical wavefront sensor. The quality of the images must be evaluated under a variety of conditions, including various positions in the field of view that can be reached through the fast steering mirror and a variety of thermal conditions. To explore the effects of thermal environment, the image quality should be studied under a variety of orbital configurations. If the DOD phase of the mission is carried out in low Earth orbit, then these tests should first be done in the low Earth orbit and then repeated in the Molniya orbit. In addition, the image quality should be assessed after the DOD mission during a period in which the telescope is pointed so as to let it cool to the minimum possible operating temperature. In addition to evaluating the images with data from the wavefront sensor, it is also important to evaluate actual images from both the InSb array and the fine-tracking array. Although neither of these detectors is ideal for analyzing the images, they do provide important information about the images to supplement that available from the wavefront sensor, and they also provide an end-to-end test of the system. Thermal Background The next critical test is to determine the thermal emission characteristics of the telescope and other components of the system. Like the test for image quality, this demonstration should be done both in the initial orbit and in the Molniya orbit to understand the changes in thermal background as a function of the orbital characteristics. The task group’s initial estimates are that, when in low Earth orbit, the telescope’s thermal emission will be too high for deep-infrared observations. However, a Molniya orbit, in which substantial passive cooling can occur, should allow such observations. The ATD/NTOT’s design has not been optimized for thermal control. In addition, it is very different from traditional infrared-optimized telescopes on the ground. Thus, there are no detailed analyses available that can be used even as a guideline for estimating its performance. If designed from the outset for low thermal background, it would likely be quite different from the baseline system described in Chapter 2. Thus determining the thermal emission characteristics is critical for understanding the astronomical capability of the ATD/NTOT. Low thermal background is essential for carrying out the near-infrared cosmological survey discussed in Chapter 5. Evaluation of the baseline telescope’s thermal environment would be carried out using images obtained with the InSb array. It is not clear to the task group whether this detector will have sufficient sensitivity to fully characterize the telescope’s emission under all possible circumstances. It will certainly be adequate for characterizing the emission whenever the system is, in some relative sense, warm. Furthermore, the thermal background seen at the focal plane is likely to vary as the fast steering mirror follows the motion of the target with the field of the telescope. These variations must be evaluated in order to assess the limits that they place on detecting faint sources at infrared wavelengths. Field Distortion Stability Since the ATD/NTOT is fundamentally different from most ground-based telescopes, the stability of its field distortion must be evaluated. This factor is important because it directly affects the ability to make long exposures. Optical stability also affects the ability to detect moving targets and to register successive images of the same field to remove cosmic-ray hits (something which may be essential because the ATD/NTOT’s eccentric orbit will take it well outside the shielding of Earth’s magnetosphere). A particular concern is that the field distortion is very likely to change as the fast steering mirror moves the
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A Scientific Assessment of a New Technology Orbital Telescope field of view to counter the spacecraft’s motion. Furthermore, the distortion may change as the actuators respond to remove fluctuations in the shape of the primary mirror. The distortion of the field has not been assessed, and so it is impossible at this point to evaluate the changes in distortion. Much of the evaluation of field stability can take place in prelaunch computer simulations. The ultimate test will, however, be done in space. Distortions should be evaluated during the DOD phase of the mission in low Earth orbit as well as during the astronomical phase in a Molniya orbit. Orbital Maneuvering The DOD’s technology demonstrations require that the ATD/NTOT be highly maneuverable and be able to slew rapidly from one point on the sky to another. This ability combined with the substantial reserves of hydrazine (i.e., beyond that needed for station keeping and dumping angular momentum) carried by the baseline mission raises an interesting possibility. It should be possible to maneuver the spacecraft so that it is in the correct position to observe a spatially localized event such as a stellar occultation (e.g. by an asteroid or by a planetary atmosphere). Like solar eclipses, the tracks of these events are generally visible in regions considerably smaller in size than the Earth; thus mobility is key to observing them. Moreover, the much larger geometric cross section presented by the Molniya orbit will allow the ATD/NTOT to potentially intercept more such events than are possible from Earth-based facilities or telescopes in low Earth orbit. The ability to maneuver a space telescope so that it is in the right place at the right time to rendezvous with an occultation path would be as revolutionary as the advance realized in the 1970s and 1980s by exploiting the mobility of NASA’s Kuiper Airborne Observatory—a capability that led to the discovery of the rings of Uranus and Pluto’s atmosphere. A 4-meter space telescope in a Molniya orbit could observe occultations inaccessible to ground-based observatories or telescopes in low Earth orbit, greatly increasing the potential application of the occultation technique. Successful demonstration that an orbiting telescope could be maneuvered to record occultations unobservable from Earth would open the way for comprehensive investigations of the occultations of virtually any chosen body in the solar system (including the possibility of measuring diameters of Kuiper Disk objects). Since most occultation tracks are roughly parallel to Earth’s equator, each such track would cross a highly inclined orbit, such as the ATD/NTOT’s Molniya orbit, somewhere and thus could be intercepted by changing the phase of the telescope in its orbit. If major changes in orbital energy are discounted, few additional events would be reached by adjusting the eccentricity or inclination of the ATD/NTOT ’s orbit. If a truly spectacular occultation could be reached by more radical orbital maneuvers, then the impact of such changes on other operations would have to be carefully assessed. The orbital variations needed to adjust the ATD/NTOT’s orbital phase so that it would arrive in line with the body to be studied and a bright star might typically require velocity changes of a few meters per second (the amount of fuel required for a velocity change of 10 m/s is 0.5% of the mass of the spacecraft, that is, about 30 kg). Although time critical, a sequence of maneuvers for a given event could be planned well in advance, with the motions specified by Earth-based or space-based astrometric measurements. The largest maneuver required would be an advance (or retardation) in orbital longitude of the telescope by 180 degrees over a period of 2 months. For an orbit with a period of 12 hours, this maneuver would require a velocity change of 5.4 m/s. If the accuracy of this maneuver were only 10%, then one would “home in” on the correct orbit by two more maneuvers of the same magnitude—the first, 6 days prior to the occultation and the final one, 14 hours before. The final accuracy of the ATD/NTOT’s location, in this hypothetical case, would be some 85 km. This would be sufficient to position the ATD/NTOT close enough to the center of the shadow of, for example, Triton or Pluto to probe into their atmospheres as deeply as is possible during an occultation. Imaging arrays are now the preferred tool for observing occultations, because the data they provide enables the construction of sophisticated models to remove the effects of background light. Contrary to the case for lunar occultations, no additional information is gained in planetary occultations by the use of kilohertz recording rates, because Fresnel diffraction produces a blur on a time scale of
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A Scientific Assessment of a New Technology Orbital Telescope For a typical asteroidal occultation observed at 0.75 micron, the distance D = 1.5 AU and the velocity v = 20 km/s imply a time scale of 0.02 s. In addition, rather than reading out the full array, subarrays are read as needed. Thus, occultation data recording rates are typically 10 to 50 Hz, and the recording should be synchronized with absolute time and spacecraft position. Since the high-speed array detectors planned for the military mission would provide simultaneous occultation light curves at visible and infrared wavelengths at these rates, no special changes to the baseline plan appear to be necessary for this astronomical technology demonstration other than the on-orbit maneuverability. Successful demonstration of this maneuvering capability would, for example, permit the program to investigate global change on Pluto and Triton as described in Chapter 5. Very Long Exposures During a Single Orbit Being able to reach faint limiting magnitudes is a major advantage of large telescopes. Observing the faintest objects requires long integration times and correspondingly long times on-source. Performing long integrations is a very inefficient use of telescopes in low Earth orbit (e.g., the HST) since half of each orbit is occupied with Earth occultation. Moreover, the target must be reacquired during each orbit. In practice, only 30 to 40 minutes of each of the HST ’s 90-minute orbits is actual observing time for a single, very long exposure. Thus it would take 15 to 16 orbits, or 24 hours, to achieve an 8-hour integration. In contrast, this same observation could be performed in a single, 12-hour Molniya orbit. Even with ground-based telescopes, 8 continuous hours of observation is rare, unless the observation is done at large air masses. Only the International Ultraviolet Explorer in its geostationary orbit can match or exceed the efficiency of the ATD/NTOT’s baseline Molniya orbit. Long integration times have other uses besides just pushing to faint limiting magnitudes. They are preferred when looking for rotational or pulsational periodicities in objects because aliasing problems are reduced if interruptions in the data stream are minimized. Typical observations from the ground suffer severe aliasing problems since an object is observed only at night. For ground-based observations, this problem can be overcome only by highly coordinated observing programs involving many telescopes around the world. With a Molniya orbit and on-board data storage for times when there was no direct communication with a ground station, it would be possible to achieve almost continuous coverage of an object, eliminating much of the aliasing. The removal of aliasing is especially critical in studies of pulsation modes in stars. Testing Modes for Operations and Scheduling Operations and scheduling of orbiting telescopes can be and have been accomplished in many different modes. The different approaches to key observational issues discussed in this section include the following: Observational programs defined by science teams and individual observers; Block scheduling and queue scheduling; Operations from a dedicated operations center and from a private institution (such as a university); and Response to targets of opportunity. Although it has urged that routine operations of the ATD/NTOT be conducted in the first of these modes (that is, by a science team), the task group notes the possibility of experimenting with other modes that might be important for certain types of observations on future missions. The choice among these options should be considered in terms of cost, “scientific efficiency,” and the degree of autonomy granted to users (within the bounds of safely operating the facility). However, quantitative comparison of these factors, especially the latter two, is difficult, if not impossible. For example, scientific efficiency could be gauged in terms of quantifiable measures, such as on-target time or the number of research publications produced by users of the facility. High marks with respect to these measures, however, could ignore missed opportunities for discoveries that might have resulted from programs not carried out by the facility because they did not fit into a more rigid program. As discussed in Chapter 3, the task group anticipates that the operations of this telescope will generally be
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A Scientific Assessment of a New Technology Orbital Telescope under the control of a science team rather than a support group charged with meeting everyone’s needs. It also expects that in an extended mission this science team might evolve. Although the task group does not anticipate that the ATD/NTOT will ever be operated primarily as a facility for guest observers, some experimental changes could be made in the mode of operation to enable important, direct comparison of different operational philosophies. One key point of comparison is the way in which the telescope responds to targets of opportunity (TOs). Responses might range from never deviating from a planned sequence (almost certainly the least expensive approach) all the way up to attempting to respond to a variety of TOs within hours of discovery. Typical targets of this type might include novae and supernovae as well as comets and an occasional, particularly interesting asteroid, such as one passing unusually close to Earth. These might be called externally discovered TOs. There are also what might be termed internally discovered TOs, for example, the detection of an anomalous event during monitoring of data being collected in survey projects. The possibility of such events suggests the need to investigate the efficiency of several different preplanned responses. In other words, a survey for supernovae, for example, might have a preplanned response to the discovery of a supernova that would stop the survey and monitor the light curve of the supernova. Past space missions have traded the operating efficiency of planned programs to respond to TOs in a variety of ways. These range from immediate hands-on control, such as the ability to reprogram the International Ultraviolet Explorer in real time, through a more typical response involving a lag of several hours to days, on down to delays of several weeks, as when the HST is used to respond to less urgent TOs. A variety of operational modes can be achieved with the telescope under the control of the science team that is responsible for implementing all observations, including those of TOs. Such a team can determine the necessary trade-offs on the basis of the expected scientific return, provided, of course, that the team (or at least the team in charge of operations for a particular time period) has an interest in TOs. The key point is to put the workload for implementing a particular capability on the users, that is, the science team, for implementing observations of TOs. A key test of the capability to respond to externally discovered TOs would be to carry out follow-up observations on Earth-approaching objects discovered by the Spaceguard1 or equivalent surveys of near-Earth objects. It should be kept in mind that other telescopes will be available for follow-up observations of TOs, so that observations of such targets with the ATD/NTOT should in any case be limited to tests of responsiveness and to observations that cannot be done with other telescopes such as the HST. ASTRONOMICALLY ENHANCED DEMONSTRATIONS Although the astronomical community can learn much from those demonstrations that can be performed with the baseline ATD/NTOT, certain mission enhancements offer improved capabilities for astronomical technology demonstrations. The task group notes that, as always, a cost/benefit analysis must be performed prior to enhancing the baseline mission. The following are among the technology demonstrations that could be performed with enhancements to the baseline ATD/NTOT. Enhanced Image Quality The single most valuable enhancement for testing the applicability of the ATD/NTOT’s technology to astronomy is improving the figure of the primary mirror. Figuring the primary to an accuracy of better than 20 nm rms would reduce to roughly 50% its contribution to the total error budget. Although still significant, this level would allow investigations of the effects of other error sources more relevant to the ATD/NTOT’s unique technology. These additional sources include the tracking system, the fluctuating thermal load, the varying field distortion as the fast steering mirror moves the field, and so on. The addition of an optical framing camera would lead to significant additional information about the performance of the telescope. As noted in Chapter 2, the fine-tracking camera (if equipped with line-transfer CCDs) does not completely sample the point-spread function (PSF) and does not allow the long exposures typically used in astronomical observations. The InSb array, on the other hand, will fully sample the PSF but only at a relatively long wavelength such that the larger diffraction limit is a significant factor in the PSF. Thus the optical framing camera would lead to a major improvement in the ability to evaluate the PSF under realistic astronomical conditions.
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A Scientific Assessment of a New Technology Orbital Telescope Long exposures at the shortest practical wavelength (approximately 0.5 micron) would provide the best, complete, end-to-end test of the ATD/NTOT’s technology. Such exposures would also serve as a direct test of the telescope’s utility for astronomical imaging. Long exposures would allow a complete characterization of all aspects of the system at all spatial frequencies, including effects at spatial frequencies higher than those sampled by the wavefront sensor. While image quality must be tested and calibrated by observing point sources, the real demonstration of quality suited to astronomical purposes will be in the ability to separate a point source from a close, fainter neighborhood. Ideal as a test for this task are attempts to resolve the galaxies surrounding quasars and cometary nuclei from their comae. Thermal Background In the baseline mission, the thermal background would be evaluated using the InSb array. It is not clear to the task group whether this detector would be limited internally by its own noise or externally by the emissivity of the optics in the infrared beam. Enhanced cooling of the InSb array, required for sky-limited astronomical performance, might be an important addition for evaluating the thermal performance of the system if the telescope and its optics are allowed to cool to a sufficiently low temperature. Field Distortion Stability The addition of an astronomical framing camera would allow several additional measurements that are related directly to the astronomical projects discussed in Chapter 5 and that also can demonstrate stability of the field, or at least the ability to calibrate the distortions. The distortion of the field is naturally a factor in all astrometric observations, but it is also a very important factor in co-adding images to remove artifacts caused by cosmic rays. There should certainly be a series of astrometric tests on rich stellar fields, but there are other possible tests that would also represent a real advance in astronomical capability, for example, determination of the geocentric parallax of objects in the Kuiper Disk. Once a suspected Kuiper Disk object is discovered, its orbit must be determined to see if it is really a member of the cometary reservoir located beyond Neptune’s orbit. This is a laborious task for ground-based telescopes because it entails many months of astrometric observations to define an orbit accurately enough to determine its distance to better than 10%. Furthermore, the follow-up observations must be made soon enough (within weeks) to ensure that the object is not lost. There is a faster way to measure the distances to suspected Kuiper Disk objects. An object at 40 AU has a parallax of about 0.5 arc sec (about 5 pixels) when viewed from one side of the HST’s orbit to the other. This is a difficult observation since it requires being able to produce a good secondary reference frame for each image. In a Molniya orbit, the baseline between apogee and perigee is much longer than that for the HST and the corresponding parallax is 1.8 arc sec. Coupled with the higher spatial resolution of the ATD/NTOT, this displacement translates to 60 pixels, and the determination of geocentric parallax becomes straightforward. Thus the combination of a larger orbit and smaller PSF leads to an order-of-magnitude improvement in the ability to measure parallax. These factors, in turn, lead to an improvement by two orders of magnitude in the time needed (days instead of a year) to determine an orbit for an object in the Kuiper Disk. Of course, many other more typical astronomical applications of astrometry could also be used as a test of the field stability and astrometric performance of the telescope. A particularly interesting possibility is to directly measure motions of stars and emission-line sources in star-forming regions using the infrared detector. Photometric Stability Although the ATD/NTOT’s instruments are imagers, they may also be used to make accurate photometric measurements of objects. One important test of the ATD/NTOT ’s capabilities will be the precision and accuracy of such measurements, both on short time scales and over long time periods. However, it should be noted that
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A Scientific Assessment of a New Technology Orbital Telescope many photometric studies do not require absolute accuracy but can be accomplished in a differential sense with high precision. Unless the point-spread function is highly oversampled, photometry does require a camera without “gaps,” such as the masked alternate rows in the fine-tracking camera’s line-transfer CCD, in order to collect all the light from a source. The inclusion of an optical framing camera would circumvent this problem. An excellent example of the necessary level of precision is seen in the field of stellar seismology. In these observations, astronomers search for radial and nonradial pulsational modes in stars by looking at the variation of the light with time. The amplitude of the variations and the relevant time scales are functions of the properties of the star. The Sun, for example, has oscillations with periods between 3 minutes and 27 days, but relatively small amplitudes. Observations of much more distant solar-type stars will require rapid sampling at high precision, an approach that probably will not be possible to high enough precision with an imaging detector. However, if a star is mostly degenerate and has a large effective gravity (e.g., white dwarfs and neutron stars), the amplitude of the pulsations will be as large as a few hundredths to tenths of a magnitude. CCDs are capable of accuracy of this order even sampling at the 20- to 60-Hz rate at which white dwarfs and neutron stars pulse (though this requirement may compromise the low-read-noise performance of the CCD system, and so some trade-offs may be needed). Observations of this type are typically made in a differential sense, that is, by comparing the target star with nearby field stars so that drifts in instrumental sensitivity with time are not important. A critical problem with ground-based studies of white-dwarf pulsations is the aliasing introduced by observing only at night. With sufficient control of the photometry, the ATD/NTOT’s Molniya orbit could minimize or eliminate much of the aliasing, allowing determination of pulsation modes more accurately and more quickly than would be possible from the ground. A program to test the photometric stability over time scales from milliseconds to months is an excellent test of the ATD/NTOT’s technological capability for astronomical applications. If the photometry is found to be precise and stable, surveys of oscillating stars may become a feasible science project to consider for an extended mission. REFERENCE 1. Morrison, David (ed.), The Spaceguard Survey: Report of the NASA International Near-Earth-Object Detection Workshop , NASA, Office of Space Science and Applications, Washington, D.C., 1992.