Recent trends in focal plane array (FPA) technology have led naturally to the development of very large format
remote sensors that require optically fast, wide field-of-view (FOV) imaging optics. Systems that cover broad
spectral ranges, such as multispectral imagers (MSI) and hyperspectral imagers (HSI), require reflective optics to
provide aberration and distortion control without the complication of wavelength dependent errors induced by
powered refractive elements. These large format systems require even wider fields-of-view than offered by the
conventional three-mirror anastigmat (TMA) and four-mirror anastigmat (4MA) designs. Recently, Raytheon has
demonstrated in hardware the first-ever aligned and tested five-mirror anastigmat (5MA) imager. The 5MA was
designed with an F/3.0 optical speed and a 36 degree cross-scan FOV for use with a large format imaging
spectrometer. The 5MA imager has useful features such as: (1) a real entrance pupil to support a full-aperture
calibrator or a small scan mirror, (2) an intermediate image for stray light control, and (3) a real exit pupil for
optimal cold-shielding in infrared applications. A computer-aided alignment method was used to align the 5MA
imager with a final target of balanced wavefront error (WFE) across the full 36 deg FOV. This paper discusses the
design and development of the first-ever 5MA imager and some potential air- and space-borne remote sensing
applications.

Optically fast, wide field-of-view, five-mirror anastigmat (5MA) imagers for remote sensing applications

Methods are proposed for hardening a missile warning satellite against jamming and damage from unlimited running time 1-MW airborne, 1-MJ ground-based, and space-based lasers. The unhardened telescope design is based on a model of a missile warning satellite telescope developed by the American Physical Society Study Group on Boost-Phase Intercept Systems for National Missile Defense. The APS telescope can step-stare or linearly scan. In response to an attack, laser warning devices can close a shutter at the first focal surface. Filter wheels in a collimated section of the optical path, insert filters to protect focal plane arrays and readout ICs from jamming and damage. Neutral density filters are inserted to assess threats. The shutter is reopened. The reduced laser intensity in the collimated section protects the filters. The field of view (FOV) is reduced with a field stop wheel, with differently sized apertures, to allow the system to step-stare between multiple jammers in the normal FOV. A cryogenic gas cools fore-optics heated by lasers or by X-rays from nuclear bursts.

Hardening a notional missile warning satellite telescope against jamming and damage by ground, airborne, and space-based lasers

In research fields such as infrared astronomical observation and space fragments distribution analysis etc, the first and
possibly the most critical evaluation parameter of interest is detection sensitivity. Without adequate detection sensitivity,
it is impossible for infrared detector to detect the target at a distance great enough and play the role of terminal detector
of background. Although cryogenic optical system can be employed for the aim of increasing detection sensitivity up to
the possible maximum extent, research on detection characteristics test method for infrared detector in low temperature
background must be undertaken primarily before the cryogenic optical system is designed and manufactured. In this
paper, some of the fundamentals of detection characteristics test method are presented, and a set of experimental
apparatus is designed and established. Based on this apparatus and by employing an extend plane source blackbody, the
general rule of detection characteristics of infrared detector from normal temperature background to low temperature
background are investigated. The voltage-output signals of every pixel are continuously acquisitioned for F (≥100)
frames under two different blackbody temperatures, and as a result we obtain the evolving regularity of different
performance index, e.g. limiting integration time and detectivity D * . The measurement results reported in the paper
confirm that the integration time can be improved by a large margin in comparison with normal temperature background,
and the specific detecivity D * has the same variation tendency, which can be increased up to one order of magnitude.

Research on detection characteristics test method for infrared detector in low temperature background

The Spatial Infrared Imaging Telescope III (SPIRIT III) sensor on the Midcourse Space Experiment (MSX) satellite observed stray radiation from the lower atmosphere and terrestrial surface, nonrejected Earth radiance, at angles of approximately 2 to 14 deg from the optical axis in measurements of Earth limb radiance. Analysis indicates that direct scatter of terrestrial radiance from contaminants on the telescope primary mirror is the principal source of stray radiation and the bidirectional reflectance distribution function (BRDF) of the primary mirror for the 6.8-10.8-μm radiometer band is (1.11 ± 0.22) x 10 -2 persr at 1 deg off axis with an angular dependence of θ -1.71 ± 0.07 . Similarly, the BRDF values for the three other LWIR radiometer bands (11.1-13.2,13.5-15.9, and 18.2-25.1 μm) havebeendeterminedas(1.03 ± 0.05) × 10 -2 θ -1.66±0.10 , (2.21 ± 0.18) × 10 -2 θ -2.04±0.21 , and (2.59 ± 0.02) × 10 -2 θ -2.10±0.09 sr -1 , respectively. The BRDF values derived from the on-orbit data are significantly greater than prelaunch measurements, and the increase is attributed to particulate accumulation on the primary mirror during the prelaunch period, launch, and the on-orbit telescope aperture cover removal and ejection process.

Midcourse Space Experiment: Off-Axis Rejection Performance of the Infrared Sensor

An all aluminum reflective optical system was tested to evaluate its optical performances at near liquid nitrogen temperature. A special cryogenic dewar was designed and fabricated with an optical window made of quartz glass on the front wall of the dewar. The optical system under test and a reference plane mirror, which were mounted into the dewar and cooled by liquid nitrogen, formed a double pass interferometric test schema together with a He-Ne interferometer of Fizeau type outside the dewar. The test results showed that there was little differences between the wavefront errors before and after the optical system is cooled, and in both cases the optical system had a diffraction limited imaging quality.

Cryogenic test of an all-aluminum infrared optical system

This paper documents the development of the SPIRIT III telescope from the design through its test activities. The SPIRIT III Instrument is the primary infrared instrument on the Mid-Course Space Experiment (MSX). The telescope is an all reflective optical system consisting of twelve mirrors. The nominal collecting apertures is 14 inches. It was designed and built to integrate with a multicolor radiometer and a Michelson interferometer built by the Space Dynamics Laboratory at Utah State University. Key performance features are discussed, and measured test data is presented. The structural/thermal trade-off issues of a satellite-based cryogenic instrument are presented along with a review of the test techniques and test equipment.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

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James J. Guregian

Papers

Development of the SPIRIT III telescope: from design through test