A Scientific Assessment of a New Technology Orbital Telescope (1995) / Chapter Skim
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2 ANTICIPATED TECHNICAL PERFORMANCE OF THE ATD/NTOT
2 ANTICIPATED TECHNICAL PERFORMANCE OF THE ATD/NTOT
Pages 14-29
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From page 14... ...
These features include: . refiguring; A 4-meter-aperture, thin-facesheet, primary mirror equipped with some 260 position actuators for on-orbit · 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)
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From page 15... ...
These separate beams are subsequently directed to a number of different focal planes by additional beam dividers and reimaging optics. The visible branch contains a wavefront sensor used to assess the shape of the primary mirror and generate correcting signals for the primary's figure control actuators.
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From page 16... ...
Upper panel: This view of the ATD/NTOT packed inside the shroud of a Proton booster shows the layout of the principal components, including Lockheed's spacecraft bus the F-SAT and the thin, 4-meter primary mirror derived from Itek's Large Optical Segment (LOS) project.
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From page 17... ...
These separate beams are subsequently directed to a number of different focal planes by additional beam dividers and reimaging OphCS. The visible branch contains a wavefront sensor (not shown)
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From page 18... ...
Since scattered light scales with the square of the figure error, even modest improvements in figure can result in significant performance gains. These gains and their consequences are discussed in Box 2.3, "Enhanced Telescope Design." Point-Spread Functions To model the PSFs the task group generated a synthetic power spectrum of the errors in the surface figure to represent the figure error of the optics as filtered through the actuator system.
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From page 19... ...
Bezel Subtotal Other Metering structure AFT metering structure Instrument support structure Shroud Electronics Fast steering mirror assembly Wavefront sensor Miscellaneous Subtotal DOD Instruments Scientific Instruments F-SAT F-SAT Fuel Contingency TOTAL 440 291 130 27 22 100 1 ,01 0 32 3 3 4 1 43 2 9 90 210 168 41 73 9 35 37 663 680 (up to)
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From page 20... ...
At optical wavelengths, however, the image quality is dominated by errors in the primary mirror in the baseline, so that improving that single specification makes a large difference in both the Strehl ratio and the diameter of the circle with 50% encircled energy. (In the task group's crude modeling, the FWHM of the central peak is already very good and does not change significantly with the improvement in figure)
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From page 21... ...
1 ~ . FILTERED PSD UNFILTERED PSD \ \ \ \ \ \ 1 / , 1 1.0 O.s 0.8 z O 0.7 2 0.6 0.5 0.4 0.3 0.2 0.1 0.0 0 1 0 20 30 40 50 SPATIAL FREQUENCY IN UNITS OF CYCLES PER APERTURE 0 1 0 20 30 40 50 SPATIAL FREQUENCY IN UNITS OF CYCLES PER APERTURE FIGURE 2.3 The synthetic power spectrum of surface figure errors generated by the task group to represent the figure error of the optics both before and after filtering through the actuator system.
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From page 22... ...
and encircled energy curves are shown for an enhanced design of the ATD/NTOT (i.e., one with a primary mirror having a wavefront error of 35 nm and assuming a pointing jitter of 6 milliarc see)
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From page 23... ...
The baseline design includes a pixel size of 0.05 arc sec. The task group assumes that the final scale of the imaging optics will be easily adjustable during the detailed design phase if it is decided to optimize the sampling of the PSF.
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From page 24... ...
The absence of a CCD of normal astronomical quality (i.e., one having a shutter, low dark current, slow readout, low-noise amplifiers, and so on) is a significant deficiency of the baseline focal plane.
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From page 25... ...
The beam from the fast steering mirror is reflected by a folding mirror and then passes through a dichroic beam splitter to spatially separate infrared and optical beams. These in turn pass through neutral beam splitters and reimaging optics to bring them to focal points on various array detectors (Courtesy of Lockheed.)
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From page 26... ...
While a 2048chip is adequate for the technology demonstrations, the astronomical return from the mission is dramatically improved by increasing the size of the array as much as possible. The task group notes that there is ample space available for this camera in the focal plane as laid out in Lockheed's preliminary designs.
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From page 27... ...
It appears that it should be possible to pick off part of the telescope beam that is nearly on axis, namely the part that would strike the back of the fast steering mirror, and use it with suitable optics to feed a focal plane with a minimum of intermediate surfaces. The task group has done no studies of the optics required nor determined how best to deal with tracking in this configuration, although using an internal steering mirror driven by the feedback signals from the fine-tracking array seems a possibility.
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From page 28... ...
The baseline mission briefed to the task group would use the additional propulsion modules to boost the spacecraft at some time, either shortly after launch or after completing demonstrations of defense technology, from the parking orbit to a Molniya orbit (named after the class of Russian communications satellites that use this orbit)
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From page 29... ...
The orbits to be considered might include IRAS-like polar orbits orthogonal to the solar direction, lUE-like geosynchronous or even geostationary orbits, and SlRTF-like heliocentric orbits. The current communications capability might be inadequate for some of these (it would certainly be inadequate for the heliocentric orbit)
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