TL;DR: The design and development of the first-ever 5MA imager is discussed and some potential air- and space-borne remote sensing applications are discussed.
Abstract: 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
Cites background from "Development of the SPIRIT III teles..."
...Table 1 shows some examples of three and four mirror imagers used in remote sensing applications -....
Abstract: 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.
Cites background from "Development of the SPIRIT III teles..."
...5 cm.(33) NSBS is taken to have the Spirit III layout except for changes in the sizes and radii of the mirrors to accommodate the smaller f/D, and the replacement of the gimballed telescope mount with a scan mirror in the aperture....
Abstract: 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.
Abstract: 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.
Abstract: 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.
Abstract: The contamination control of telescopes with the straylight-rejection capability is reviewed to identify the degradative effects of contaminant phenomena on the measurements. Three levels of optical contamination are discussed including bidirectional reflectance distribution function (BRDF), point-source rejection ratio (PSRR), and nonrejected earth radiance (NRER). Measurements of degradation to low-scatter surfaces are set forth for the Zip telescope during storage and for the Cirris 1A telescope performance. PSRR measurements indicate that the Cirris 1A degraded by a factor of 15 during ground testing. A portable external BRDF station is described that measured cryogenic BRDF and BRDF degradation over the life of the Cirris 1A telescope. The optical contamination measurement described are concluded to be important to both determining the causes of degradation and optimizing telescope performance.
Abstract: This paper documents the engineering design, fabrication, assembly, and test activities at SSG, Inc. that produced the CLAES Telescope Assembly. It includes a brief outline of the optical design as given by Lockheed Palo Alto Research Laboratory. Several major design and assembly areas are reviewed to highlight the driving design and performance constraints of the telescope. These include the dip-brazing process utilized on the structural sub-assemblies, and the fabrication process for the three flight mirrors. The telescope system alignment techniques and processes are reviewed, and include an outline of the verification test plan which covers the optical, structural, and cryogenic test procedures for the telescope. Test data is given and compared to the performance specifications. The last topic is a brief discussion of the lessons-learned from this telescope, and the follow-on diagnostic tests that are currently in process at SSG, Inc.
Abstract: This paper documents and investigates the optical performance of metal mirrors at cryogenic temperatures. It also reviews the telescope system level optical performance for several telescope systems designed and fabricated at SSG. These include data on the LAIRTS Telescope (Large Aperture Infrared Telescope Sensor), the CLAES Telescope (Cryogenic Limb Array Etalon Spectrometer), and the SPIRIT II Telescope (Spatial Infrared Rocketborne Interferometer Telescope). A brief discussion of the design and fabrication of these mirrors is included along with a summary of the driving design performance constraints on cryogenic infrared optics. A review of the test techniques and cryogenic test facilities is given. Interferometric testing is the primary tool used to test these mirrors and systems. This section of the paper also discusses the data analysis methods utilized to determine the cryogenic optical performance of these mirrors.