A Scientific Assessment of a New Technology Orbital Telescope (1995)

The end of the Cold War and a decline in the fortunes for space research have gone hand in hand. If the space sciences are to continue advancing and not slip into a decline matching their dwindling budgets, new and innovative ways will have to be found to perform space missions. This is particularly true for space astronomy, given that certain features of spacecraft design (e.g., telescope apertures and detector sizes) are constrained by the laws of physics and cannot be miniaturized and still carry out their scientific tasks.

Not all of the events of recent years have been detrimental for space science. The decline in superpower rivalries has opened new avenues for international cooperation. Similarly, once-secret military technology has become available for civilian applications. Indeed, declining defense and space budgets have given rise to hybrid projects with both military and scientific goals. Prime among these was the recent Clementine lunar orbiter. This report assesses another such project, a large space telescope. In addition to demonstrating technology of interest to the Department of Defense (DOD), this mission has significant scientific capabilities, both in enabling direct astronomical observations and in demonstrating technology that may drastically alter the cost/performance ratio of future NASA missions.

At the height of the Cold War, the DOD’s Strategic Defense Initiative Organization (SDIO) actively sponsored development of the technology needed to make space-based laser weapons feasible. As part of this program, SDIO developed many components of an agile, ultra-lightweight, 4-meter space telescope, equipped with an advanced active-optics system. Budgetary shortfalls and program redirection led to the cancellation of any tests of laser weapons in space. However, many components of the system had other applications of interest to both the DOD and the scientific community, and so development of the space-based telescope continued under the so-called Advanced Technology Demonstrator (ATD) program of SDIO’s successor, the Ballistic Missile Defense Organization (BMDO).

To assist in the evaluation of the ATD’s scientific potential, BMDO asked the Space Studies Board to provide advice on instrumentation, data management, and science operations to optimize the scientific value of a 4-meter mission. Following the initiation of the study by the Task Group on BMDO New Technology Orbital Observatory, however, a combination of budgetary pressures and redirected defense priorities forced BMDO to defer the ATD mission. Nevertheless, BMDO reaffirmed that “planning advice and recommendations [about the scientific aspects of the 4-meter mission] would still be valuable in formulating future joint experiments should this program or a similar one be funded in a subsequent Defense Plan.”¹

Despite the uncertain future of a flight test of the 4-meter telescope and the currently unknown national

security goals of such a mission, the task group proceeded to analyze the astronomical potential of the deferred mission. Given the potential scientific aspects of the 4-meter telescope, this project is referred to as the New Technology Orbital Telescope (NTOT), or as the ATD/NTOT, to emphasize its dual-use character. The task group emphasizes that it was specifically charged to assess the astronomical capability of the ATD/NTOT and therefore included only people with competence for that specific assessment.

The ATD/NTOT mission was conceived as a low-cost demonstration of technology, intended for use in future national security spacecraft, but having implications for astronomy. As such it is:

• Designed to cost (~$350 million, including launch);

• Uses existing technology and/or designs wherever possible;

• Has a 3-year development schedule and a nominal orbital lifetime of 1 year; and

• Is not driven by specific astronomical requirements.

Given these characteristics, the task group adopted the basic philosophy that any potential involvement of the astronomical community in the ATD/NTOT should, at least initially, be predicated on the assumption that the ATD/NTOT is primarily a test of new technology for astronomy and is not a mission driven by any particular astronomical requirements. In this light, the ATD/NTOT’s greatest benefit to the astronomical community will be to show whether or not it is possible to break the Hubble paradigm—that is, to demonstrate that it is possible to obtain large space optics at low cost. While doing this, it could carry out major astronomical studies not possible with the Hubble Space Telescope (HST) even with its currently planned improvements in instrumentation.

The basic features of the ATD/NTOT are the following:

• A 4-meter-aperture, 17-mm-thick, primary mirror equipped with some 260 actuators for on-orbit refiguring;

• Afocal optics with an image-stabilization mirror located at an image of the entrance pupil to adjust the pointing anywhere within a ± 5.7-arc-minute region without moving the spacecraft;

• Graphite polycyanate (graphite epoxy) structures for the entire telescope assembly;

• Use of an on-board inertial reference to maintain pointing stability over a bandwidth from 1 to 300 Hz;

• The ability to track stars, as faint as 19th magnitude, through the full aperture of the telescope to maintain pointing stability against disturbances at frequencies of less than about 10 Hz;

• A design optimized for agility and rapid slewing from one part of the sky to another; and

• A highly eccentric orbit with a 12-hour period allowing continuous viewing of targets over much of the sky for periods up to about 8 hours.

Estimates of the performance of the ATD/NTOT suggest that it approaches the diffraction limit at near-infrared wavelengths. In the optical, its full width at half maximum (FWHM) is better than that for any current or planned facility, while the diameter for 50% encircled energy is comparable to the HST’s. The reason for this is the relative roughness of the primary mirror. In fact, the primary mirror dominates all other sources of wavefront errors in the ATD/NTOT’s error budget. This suggests an obvious enhancement: improving the figure of the primary mirror by a factor of two so that its contribution to the telescope-level error budget is comparable to that of the other components. Not only could this improvement be achieved at relatively low cost, but it would also have a dramatic impact on the ability of the ATD/NTOT to do both the technology demonstrations and the observing projects outlined in this report.

The baseline instrument package for the ATD/NTOT consists of a variety of optical- and infrared-array detectors. The one of most interest is a 1024 × 1024 indium antimonide (InSb) infrared array that would have state-of-the-art astronomical capabilities if operated at a cold enough temperature. The two passive, visible, fine-tracking arrays would have some astronomical applications. These would, however, be limited because their charge-coupled devices (CCDs) are line-transfer devices, and the arrays and their amplifiers are not optimized for low readout noise. The obvious deficiency in the instrument package is the absence of an optical framing camera of astronomical quality. The addition of such an instrument would have a very significant impact on the astronomical capabilities of the ATD/NTOT.

Understanding the areas in which the ATD/NTOT might have significant advantages over existing and

planned facilities is critical to deciding which scientific projects and technological demonstrations to emphasize. To do this the task group considered two aspects of the ATD/NTOT: its optical and near-infrared performance, and the operational modes in which it can be used most cost effectively. Consideration of performance factors led to the conclusion that the ATD/NTOT has major advantages:

• In the near infrared (2 to 4 microns), where the sky background is reduced by several orders of magnitude;

• In the far red (>0.7 micron), where the sky background is reduced by one order of magnitude;

• For programs that depend on high contrast between a point source and its neighborhood, or those that require subarc-second spatial resolution; and

• For programs that are photon-starved, that is, receive little attention with the HST.

Consideration of operational factors led to the conclusion that the ATD/NTOT is best suited to large surveys because repeated use of the telescope in a single mode, by a small team of scientists, is the most cost-effective operating procedure. In addition, large surveys make less than optimum use of complex, multiuser facilities such as the HST.

In determining what scientific and technological projects the ATD/NTOT is most suited to perform, the task group’s overriding priority has been to minimize cost while still ensuring the capability to do exciting science. The first astronomical goal of an ATD/NTOT flight should be to test the applicability of its technology for use in future space science missions. Prime among these tests are (in no particular order):

• Demonstration by actual astronomical application of the ability to adequately refigure a large mirror in orbit to obtain astronomical-quality images, both with and without ground-based intervention;

• Evaluation of image quality and its consistency both across the field of view as the fast steering mirror stabilizes the telescope’s line of sight and in a variety of thermal environments;

• Characterization of the likely degree of passive cooling by establishing the thermal emission from the optical and other components of the system in both the initial low Earth orbit and the eventual highly elliptical (or Molniya) orbit;

• Investigation of the stability of field distortions, particularly as the figure of the primary mirror responds to its control actuators and, also, as the fast-steering mirror stabilizes the telescope’ s line of sight;

• Exploitation of the ATD/NTOT’s agility and large fuel reserves to actively maneuver the spacecraft so that it is in the right place, at the right time, to observe ephemeral events such as occultations;

• Exploration of the possibilities presented by the Molniya orbit to conduct very long integrations in a cost-effective manner; and

• Utilization of the facility as an experimental testbed for various modes of ground operations that may be needed for future space science missions.

The implementation of some of the task group’s suggested enhancements to the baseline ATD/NTOT would significantly improve the evaluation of both the technology and the astronomical significance of a program of technology demonstrations. Two of the most important enhancements are enhancing the figure of the primary mirror and adding an optical framing camera. These improvements would allow far more rigorous tests of, for example, the image quality that can be realized with the ATD/NTOT technology. Addition of just the optical framing camera would permit studies of the system’s photometric stability. Also of great importance is enhancing the cooling of the InSb array, since this would allow a better evaluation of the ATD/NTOT ’s infrared performance.

If the ATD/NTOT’s technology passes its key tests and can exceed its 1-year design lifetime, then it will have a significant capability for astronomical research. An area in which the ATD/NTOT should excel is in studies of origins. The creation and evolution of the universe and its component galaxies, stars, and planets is a topic of great scientific and popular interest and one in which large-scale surveys play major roles. To highlight the ATD/NTOT’s potential in these areas, the task group discusses four possible observing programs:

• A series of deep surveys of the early universe at near-infrared wavelengths to study the evolution of galaxies, define the magnitude/number-count relation, and search for new “standard candles” at high redshifts. All

of these projects are consistent with the baseline mission but would benefit significantly from the addition of an optical framing camera and enhancement of the primary mirror.

• A survey of the outer solar system to define the size- and radial-distributions of the primitive bodies constituting the Kuiper Disk (down to ~1-km bodies at ~40 AU). This project requires an optical framing camera and would benefit significantly from the use of an enhanced primary mirror.

• High-resolution optical studies of the disks, jets, and winds associated with young stellar objects during the embedded, accretion-dominated, and post-accretion phases of their evolution. The success of this project depends critically on enhancement of the primary mirror and the addition of an optical framing camera.

• Synoptic occultation observations of Pluto and Triton to monitor global atmospheric change due to seasonal variations in insolation. Although compatible with the baseline mission, this project may be expensive in terms of operations and use of spacecraft resources because it would involve extensive spacecraft maneuvering and orbital changes.

In the course of the task group’s deliberations, a number of items arose that raise questions about the astronomical utility of the ATD/NTOT. In particular:

• Little or no systems analysis has been performed to verify that the ATD/NTOT’s individual components can be combined to form a working astronomical telescope.

• There is some doubt about the ability of the baseline ATD/NTOT to track guide stars as faint as would be needed for certain observations. Although there are solutions to this problem, the problem may be moot if the telescope is devoted to surveys.

• Aspects of the ATD/NTOT’s design, particularly that of the tripod supporting the secondary mirror, may scatter stray light into the focal plane.

• The rate at which the figure of the primary mirror will need correction as it deforms due to thermal and other drivers, and the impact corrections may have on observing overhead, are not clear.

• Software problems of the type that ultimately doomed the Clementine mission must be avoided.

• The effect of cosmic-ray events on the ATD/NTOT’s imaging arrays when the spacecraft is operating in the highly eccentric Molniya orbit is a significant factor.

• Although building to cost is becoming a key feature of NASA’s present and future missions, neither NASA nor the space science community has much experience with this mode of operation.

While the resolution of these issues is beyond the scope of this study, they must nevertheless be resolved as the ATD/NTOT mission is further defined.

The task group’s analysis shows that the ATD/NTOT mission uses advanced technology that has important potential applications for future space astronomy missions. Furthermore, its advertised cost-effectiveness is crucial to NASA’s ability to carry out significant space astronomy missions in an era of tightly constrained budgets. Both of these factors have not escaped the notice of other groups. Thus, both the High-Z and Polar Stratospheric Telescope concepts draw heavily on the capabilities of the ATD/NTOT’s technology. The task group’s basic conclusion is that the ATD/NTOT mission does have the potential for contributing in a major way to astronomical goals. It is equally clear that if the ATD/NTOT performs as advertised, it could undertake astronomical observations that could not be matched by any other facility now in existence or under development. Thus the task group ’s first and foremost recommendation is as follows:

1. To optimize the return to astronomy from the ATD/NTOT, the astronomical community should be directly involved in the continued study and development of this mission, including system engineering and complete mission analysis. These community representatives should be selected by NASA, and their role should be to advise NASA on the continuing value of this mission for astronomy. The group should include not only astronomers proposing specific observing programs, but also individuals with particular expertise in the design of large telescopes and space missions.

   Since any scientific applications of the ATD/NTOT are a bonus, the task group further recommends the following:

2. If the ATD/NTOT mission flies, a suite of tests of the suitability of its technology for astronomical applications should be carried out. Some of these tests can be conducted concurrently with DOD’ s demonstration mission, but others require an astronomical phase of the mission.

3. Although scientific goals must be kept in mind and accommodated insofar as possible during the planning of the ATD/NTOT mission, these goals should not impose requirements that would have a major impact on development or operations costs.

4. The ATD/NTOT’s astronomical promise is sufficient, even at this preliminary stage, that it is appropriate to plan for a mission phase devoted to astronomical observations. The resources devoted to planning an astronomical mission should be kept to a minimum until such time as the ATD/NTOT’s scientific and technological capabilities are better defined.

A mission phase dedicated to astronomical observations, while highly desirable, could be extremely expensive if not managed appropriately. Given the philosophy of designing to cost, the development costs for astronomical research programs must be kept to an absolute minimum. In order to minimize operational costs, the task group recommends the following:

5. The astronomical phase of the ATD/NTOT mission should be carried out by a principal investigator and a science team, with rotating membership to accommodate a range of scientific expertise. No provision should be made for a traditional guest observer program.

6. An extended ATD/NTOT mission should concentrate on extensive surveys that repeatedly use the ATD/NTOT in a single mode.

7. Astronomical data collected in the ATD/NTOT mission should be delivered promptly to an existing public archive that is independent of and expected to outlive the mission.

Because DOD sponsorship of the ATD/NTOT is uncertain, the mission ’s exact specifications are unclear. The task group has assumed a baseline performance predicated on the requirements necessary to perform the mission that BMDO has now deferred. This analysis revealed several areas in which enhancements beyond the baseline specifications would have a significant impact on the ATD/NTOT’s astronomical capabilities. Some of these enhancements may ultimately be required by, or at least be consistent with, a DOD mission if and when it is finally defined. Although the task group has not evaluated the cost-effectiveness of all of these enhancements (something that must be done during the system engineering phases of the mission), it has discussed their potential impact with representatives of Lockheed and Itek. In one case the costs are well defined and the performance benefits reasonably determined. In other cases, neither the costs nor the actual improvements in performance are very well determined. It is, however, the sense of this task group that these enhancements are likely to be very cost-effective and important for the astronomical aspects of the mission. In particular:

8. The figure of the ATD/NTOT’s primary mirror should be improved by roughly a factor of two to reduce its surface error to ~17 nm, and, thus, the total system’ s wavefront error to roughly 50 nm rms. Itek estimates that this enhancement would cost ~$100,000.

9. A large-format, framing, optical CCD of astronomical quality should be included in the ATD/NTOT’s focal-plane package. A less expensive but clearly less desirable option would be to replace the baseline line-transfer CCDs in the fine-tracking sensors with frame-transfer CCDs.

10. The ATD/NTOT’s baseline infrared detector, an InSb array, should be optimized for sensitivity by, for example, additional cooling and by minimizing the number of emitting surfaces in the optical path.

The task group briefly discussed other enhancements that could significantly improve the scientific return from an extended mission dedicated to astronomical observations. Unlike those discussed above, however, all of these modifications would add significantly to the cost of the mission and/or perhaps be incompatible with the national security objectives of the mission. These enhancements include:

• Modifying the ATD/NTOT’s orbit to minimize and/or stabilize the thermal load on the spacecraft;

• Optimizing the design to enhance the passive-cooling characteristics of the telescope and focal-plane instruments; and

• Adding additional instruments (e.g., a dedicated infrared focal plane) for scientific research.

Even if the ATD/NTOT mission is not eventually funded by BMDO and does not find another sponsor in the national security community, the task group believes that the time devoted to this study has been of use. While the complete package of technologies embodied in the ATD/NTOT proposal promises exciting advances in astronomical capabilities, it should be remembered also that much of its hardware already exists. A complete adaptive-optics system and examples of thin primary mirrors, for instance, currently sit gathering dust in testing chambers. Many of these subsystems are themselves interesting additions to the tools at the disposal of astronomers and may find scientific applications very different from those for which they were designed. The ATD/NTOT may never fly, but if this report does nothing more than illuminate some of the capabilities lurking in the shadows of the Cold War, it will have achieved something worthwhile.

1. Payton, Gary E., Deputy for Technology Readiness, BMDO, memorandum to the Space Studies Board, October 24, 1994.