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



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