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

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. This task group’s basic conclusion is that the ATD/NTOT mission does have the potential for contributing in a major way to astronomical goals. This is true both in the area of evaluating the ATD/NTOT’ s new technology for subsequent use in space astronomy missions, and in the area of performing selected, significant astronomical observations. There is a very high probability that the ATD/NTOT will be able to carry out astronomical programs that cannot be done with any other facility in existence or in 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.

   The task group’s basic philosophy is that the ATD/NTOT mission is primarily a demonstration of technology for future national security missions, with a nominal lifetime of only 1 year. Furthermore, the mission is designed to cost, using existing technology, and it is not responsive to specific astronomical requirements. The astronomical applications are a bonus, a point of view that NASA and the astronomical community must share. Consequently, the task group recommends the following:

2. If the ATD/NTOT mission flies, a suite of tests of the suitability of its technology for astronomical applications—tests that may be different from the tests and demonstrations requested by DOD—should be carried out. Some of these tests can be conducted concurrently with the tests of defense technology, but others require an astronomical phase of operation after the conclusion of DOD’s 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. In this sense the ATD/NTOT will be like possible future missions under the aegis of NASA’s proposed New Millennium program.

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.

   If an extended mission occurs, the task group offers several additional recommendations regarding the way in which it should be managed. These recommendations reflect the philosophy that development costs for astronomical research programs should be kept to an absolute minimum because of the ATD/NTOT mission’s nature as a demonstration.

5. An extended ATD/NTOT mission devoted to astronomical research should be carried out by a principal investigator and a science team, with rotating membership to involve people with different types of expertise during different phases of the mission. No provision should be made for a traditional guest observer program.

6. An extended mission should concentrate on extensive surveys that repeatedly use the ATD/NTOT in a single mode. Pointed observations of specific targets suggested by guest observers should be carried out only if they can be done simply by adding fields to the survey program and provided that suitable guide stars are available in those fields.

7. All scientific data collected during the ATD/NTOT mission should be delivered promptly to an existing public archive that is independent of and expected to outlive the mission. Such an archive might be that for the Hubble Space Telescope, the Planetary Data System, or the Infrared Processing and Analysis Center, with a specific choice to be made at a later date.

   Because DOD sponsorship of the ATD/NTOT is uncertain, the mission ’s exact specifications are not yet clear. As noted in this report, the task group has assumed a baseline performance predicated on the requirements necessary to perform a mission that BMDO has now deferred. Several of the potential enhancements outlined by the task group would have a significant impact on the ATD/NTOT’s astronomical capability relative to that baseline. 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. All of the suggested improvements must be evaluated for their cost-effectiveness during the system engineering phases of the mission. The task group has discussed its suggested enhancements with representatives of Lockheed and Itek. In the case of improvements to the figure of the primary mirror, the costs are well defined and the performance benefits reasonably well determined. In other cases, neither the costs nor the actual improvement in performance are very well determined, but it is the sense of this task group that the 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 two to reduce its surface error to ~17 nm, and, thus, the total system’s wavefront error to roughly 50 nm rms. This enhancement would allow both a better evaluation of the ATD/NTOT’s technology and significantly enhance its astronomical potential. The cost of this improvement is reasonably well determined at $100,000 according to representatives of Itek.

9. A large-format, framing, optical CCD of astronomical quality should be included in the ATD/NTOT’s focal-plane package. This CCD should be optimized for performance in the far red (say, ~0.8 micron) and should be of normal astronomical quality with respect to dark current, readout noise, and other relevant parameters. Its inclusion would enhance astronomers’ ability to evaluate the technology and would lead to dramatic increases in the astronomical return from the mission. 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 InSb infrared array should be optimized for sensitivity by, for example, further cooling of the array itself and by minimizing the number of emitting surfaces in the optical path. The astronomical return would be significantly enhanced if the system ultimately reaches the sensitivity limit set by the zodiacal light.

Some additional enhancements have been considered by the task group but were not discussed extensively with representatives from Lockheed and Itek. Furthermore, no quantitative details of these enhancements have been considered by the task group itself. It is likely that these enhancements would have a much greater cost impact on the mission than those discussed above. In addition, it is not clear if their implementation would be consistent with the goals of a possible national security mission. Therefore they are included here only for completeness and as suggestions for more detailed consideration at a later date. These improvements include:

• Modifying the ATD/NTOT’s orbit to minimize and/or stabilize the thermal load on the spacecraft. The thermal load in the Molniya orbit varies significantly with orbital phase. Several possible alternatives exist, and their practicality should be explored.

• Optimizing the design to enhance the passive-cooling characteristics of the telescope and focal-plane instruments. Cooling could have a significant impact on the astronomical return and could enhance understanding of the technological aspects of, for example, stabilizing an active mirror at low and variable temperatures.

• Possibly adding dedicated instruments for scientific research either instead of or in addition to modifying the ATD/NTOT’s instrument suite. Such an instrument might be an infrared camera with far fewer optical components in the path than are in the baseline design.

Even without all the possible enhancements, it is clear that the ATD/NTOT mission, at least if flown with the task group’s suggested, critical enhancements (improved figure of the primary mirror, optical framing camera, and thermal control), can contribute dramatically to astronomy, both in the development of future missions and in the return of key astronomical data. The ATD/NTOT’s advertised ability to make these contributions at relatively low cost, by using off-the-shelf components of previously unavailable technology, offers the possibility of dramatic breakthroughs for the future of astronomical missions in space.