Tammy D. Henson

Distributed Sensing and Shape Control of Piezoelectric Bimorph Mirrors

Meeting the long term needs of the remote sensing community requires the development of large aperture space-based optical systems to achieve dramatic improvements in resolution and sensitivity. It is possible that ultralarge apertures will be obtained using deployable thin film mirror technology, yet many technological barriers must be overcome to make this approach viable. This paper summarizes an initial research effort into the development of piezoelectric thin film mirrors that can be actively shaped using electric fields applied by an electron flux at selected locations. Recent progress is described in the key areas of mirror figure sensing methods, electron gun excitation, and shape control algorithm development.

Space radiation testing of radiation-resistant glasses and crystals

With the number of cerium doped radiation resistant glasses available to the designer of space optics rapidly decreasing, it is critical to identify and characterize all potential sources of radiation resistant glasses and crystals. Unfortunately much of the data on radiation testing of glasses is quite old and often not completed at very high dose rates as might be experienced by an unshielded space optic in orbit for many years. In addition, many optical glasses and crystals are manufactured today with much higher purity than in the past in order to increase their ultraviolet transmission properties. Consequently these glasses are much more resistant to space radiation than in the past. In this paper we will present gamma radiation effects on the transmission properties of todays fused silica, sapphire, calcium fluoride, barium fluoride, Schott cerium doped radiation resistant glasses, Schott colored glass filters, as well as some infrared glasses with up to a 10 Mrad dose.

Multispectral Thermal Imager optical assembly performance and integration of the flight focal plane assembly

The Multispectral Thermal Imager Optical Assembly (OA) has been fabricated, assembled, successfully performance tested, and integrated into the flight payload structure with the flight Focal Plane Assembly (FPA) integrated and aligned to it. This represents a major milestone achieved towards completion of this earth observing E-O imaging sensor that is to be operated in low earth orbit. The OA consists of an off- axis three mirror anastigmatic (TMA) telescope with a 36 cm unobscured clear aperture, a wide-field-of-view (WFOV) of 1.82 degrees along the direction of spacecraft motion and 1.38 degree across the direction of spacecraft motion. It also contains a comprehensive on-board radiometric calibration system. The OA is part of a multispectral pushbroom imaging sensor which employs a single mechanically cooled focal plane with 15 spectral bands covering a wavelength range from 0.45 to 10.7 micrometer. The OA achieves near diffraction-limited performance from visible to the long-wave infrared (LWIR) wavelengths. The two major design drivers for the OA are 80% enpixeled energy in the visible bands and radiometric stability. Enpixeled energy in the visible bands also drove the alignment of the FPA detectors to the OA image plane to a requirement of less than plus or minus 20 micrometer over the entire visible detector field of view (FOV). Radiometric stability requirements mandated a cold Lyot stop for stray light rejection and thermal background reduction. The Lyot stop is part of the FPA assembly and acts as the aperture stop for the imaging system. The alignment of the Lyot stop to the OA drove the centering and to some extent the tilt alignment requirements of the FPA to the OA.

Optical design for a visible through thermal Infrared multiband imaging system

The optical design and the expected performances of a multi- band imaging system are presented. The instrument is to be operated in a low earth orbit in a pushbroom scanning mode. It has 15 spectral bands with the wavelength range extending from 0.45 micrometer to 10.7 micrometer. The instrument comprises a single mechanically cooled focal plane, a 36 cm aperture, wide field of view off-axis three mirror anastigmatic telescope, and an on-board calibration subsystem. The design has near diffraction limited performance for all spectral bands from visible to long wavelength infrared (IR). It has high throughput and low polarization sensitivity. To minimize the background noise at long wavelength infrared (LWIR) a cold stop is located at the exit pupil to achieve 100% cold shielding.

Optical fabrication and metrology for a visible through thermal infrared multi-band imaging system

The optical fabrication, metrology, and system wavefront testing of an off-axis three mirror anastigmatic telescope will be presented. The telescope is part of a multi-band imaging system which includes a single mechanically cooled focal plane with 15 spectral bands covering a wavelength range from 0.45 microns to 10.7 microns and an on-board calibration subsystem. The imaging system is to be operated in a low earth orbit in a pushbroom scanning mode. The telescope has a 36 cm aperture, a 1.38 degree cross-track by 1.82 degree along-track field of view (FOV), near diffraction limited performance in the visible, and strictly diffraction limited performance from 1.3 microns to 10.7 microns. The primary and the tertiary mirrors are general aspheres which have undergone 80% lightweighting. The secondary mirror is a hyperbola. The primary mirror was extremely difficult to fabricate and test due to its large departure from sphericity, fast f-number, and large off axis distance. The tertiary mirror has a small departure from sphericity and is only slightly off-axis, but it has a very fast f-number also. The surface wavefront measurements for the three mirrors after final figuring and lightweighting are 0.048 waves rms at 0.6328 microns for the primary mirror and 0.025 waves rms at 0.6328 microns for the secondary and tertiary mirrors. The telescope wavefront requirement at the center of the along-track FOV is 0.178 waves rms at 0.6328 microns and at the edge of the along-track FOV is 0.677 waves rms at 0.6328 microns.

Optical assembly of a visible through thermal infrared multispectral imaging system

The Optical Assembly (OA) for the Multispectral Thermal Imger (MTI) program has been fabricated, assembled, and successfully tested for its performance. It represents a major milestone achieved towards completion of this earth observing EO imaging sensor that is to be operated in low earth orbit. Along with its wide field of view, 1.82 degrees along-track and 1.38 degrees cross-track, and comprehensive on-board calibration system, the pushbroom imaging sensor employs a single mechanically cooled focal plane with 15 spectral bands covering a wavelength range from 0.45 to 10.7 micrometers . The OA has an off-axis three-mirror anastigmatic telescope with a 36-cm unobscured clear aperture. The two key performance criteria, 80 percent enpixeled energy in the visible and radiometric stability of 1 percent 1 (sigma) in the visible/near-IR and short wavelength IR, of 1.45 percent 1 (sigma) in the medium wavelength IR, and of 0.53 percent 1 (sigma) long wavelength IR, as well as its low weight and volume constraint drive the overall design configuration of the OA and fabrication requirements.