We present details of the design, operation and calibration of an astronomical visible-band imaging Fourier transform spectrometer (IFTS). This type of instrument produces a spectrum for every pixel in the field of view where the spectral resolution is flexible. The instrument is a dual-input/dual-output Michelson interferometer coupled to the 3.5 meter telescope at the Apache Point Observatory. Imaging performance, and interferograms and spectra from calibration sources and standard stars are discussed.
The advent of high performance infrared detector arrays together with recent advances in coarse grating fabrication now makes possible the design and fabrication of infrared spectrometers that cover the mid-IR atmospheric windows at high resolution with no moving components. For applications involving small sources at large distances this instrument approach can provide a substantial increase in sensitivity over Fourier transform spectrometers at the same resolution and spectral coverage. We describe the design evolution of the LLNL cross dispersive infrared spectrometer (CDIRS) remote chemical sensors operating in the two atmospheric windows from 2.3 microns to 4.2 microns. The spectral and mechanical performance of the first generation 1 meter focal length prototype echelle grating spectrometer (EGS) is presented. A second generation cryogenic (150 K) spectrometer (mini-EGS) utilizes a high dispersion silicon immersion grating to provide a compact design. This instrument will be flown in the winter of 1995 as part of the instrumentation suite for the DOE airborne multisensor pod system (AMPS) effluent research program.
Arno Ledebuhr, Joseph Kordas, Isabella Lewis, Michael Richardson, George Cameron, W. Travis White, Douglas Dobie, Wesley Strubhar, Thomas Tassinari, Douglas Sawyer, Michael Shannon, Lyn Pleasance, Albert Lieber, Peter Trost, David Doll, Michael Grote
Lawrence Livermore National Laboratory developed a space-qualified high resolution (HiRes) imaging LIDAR (light detection and ranging) system for use on the DoD Clementine mission. The Clementine mission provided more than 1.7 million images of the moon, earth, and stars, including the first ever complete systematic surface mapping of the moon from the ultra-violet to near-infrared spectral regions. This article describes the Clementine HiRes/LIDAR system, discusses design goals and preliminary estimates of on-orbit performance, and summarizes lessons learned in building and using the sensor. The LIDAR receiver system consists of a HiRes imaging channel which incorporates an intensified multi-spectral visible camera combined with a laser ranging channel which uses an avalanche photo-diode for laser pulse detection and timing. The receiver was bore sighted to a lightweight McDonnell-Douglas diode-pumped Nd:YAG laser transmitter that emitted 1.06 micrometer wavelength pulses of 200 mJ/pulse and 10 ns pulse-width. The LIDAR receiver uses a common F/9.5 Cassegrain telescope assembly. The optical path of the telescope is split using a color-separating beamsplitter. The imaging channel incorporates a filter wheel assembly which spectrally selects the light which is imaged onto a custom 12 mm gated image intensifier fiber-optically coupled into a 384 multiplied by 276 pixel frame transfer CCD FPA. The image intensifier was spectrally sensitive over the 0.4 to 0.8 micrometer wavelength region. The six-position filter wheel contained 4 narrow spectral filters, one broadband and one blocking filter. At periselene (400 km) the HiRes/LIDAR imaged a 2.8 km swath width at 20-meter resolution. The LIDAR function detected differential signal return with a 40-meter range accuracy, with a maximum range capability of 640 km, limited by the bit counter in the range return counting clock. The imagery from the HiRes is most useful for smaller scale topography studies, while the LIDAR data is used for global terrain and inferred gravity maps.
The Clementine mission provided the first ever complete, systematic surface mapping of the moon from the ultra-violet to the near-infrared region. More than 1.7 million images of the moon, earth and space were returned from this mission. Two star tracker stellar compasses (star tracker camera + stellar compass software) were included on the spacecraft, serving a primary function of providing angle updates to the guidance and navigation system. These cameras served as a secondary function by providing a wide field of view imaging capability for lunar horizon glow and other dark-side imaging data. This 290 g camera using a 576 X 384 FPA and a 17 mm entrance pupil, detected and centroided stars as dim and dimmer than 4.5 mv, providing rms pointing accuracy of better than 100 (mu) rad pitch and yaw and 450 (mu) rad roll. A description of this light-weight, low power star tracker camera along with a summary of lessons learned is presented. Design goals and preliminary on-orbit performance estimates are addressed in terms of meeting the mission's primary objective for flight qualifying the sensors for future Department of Defense flights.
We describe the design of a silicon immersion grating spectrograph for the remote detection of chemicals in the atmosphere. The instrument is designed to operate in the two atmospheric windows from 2.3 to 2.5 and 2.8 to 4.2 microns at a resolution of 0.1 cm-1. This is achieved by cross dispersing a high order silicon immersion echelle (13.5 grooves/mm) and a first order concave grating operating in a reflective configuration to generate a 2D spectrum in the image plane with diffraction limited performance.
KEYWORDS: Cameras, Long wavelength infrared, Staring arrays, Cryocoolers, Space operations, Electronics, Sensors, Calibration, Infrared cameras, Control systems
The Clementine mission provided the first ever complete, systematic surface mapping of the moon from the ultra-violet to the near-infrared regions. More than 1.7 million images of the moon, earth, and space were returned from this mission. The long-wave-infrared (LWIR) camera supplemented the UV/visible and near-infrared mapping cameras providing limited strip coverage of the moon, giving insight to the thermal properties of the soils. This camera provided approximately 100 m spatial resolution at 400 km periselene, and a 7 km across- track swath. This 2.1 kg camera using a 128 X 128 mercury-cadmium-telluride (MCT) FPA viewed thermal emission of the lunar surface and lunar horizon in the 8.0 to 9.5 micrometers wavelength region. A description of this lightweight, low power LWIR camera along with a summary of lessons learned is presented. Design goals and preliminary on-orbit performance estimates are addressed in terms of meeting the mission's primary objective for flight qualifying the sensors for future Department of Defense flights.
The Clementine mission provided the first ever complete, systematic surface mapping of the moon from the ultra-violet to the near-infrared regions. More than 1.7 million images of the moon, earth, and space were returned from this mission. The near-infrared (NIR) multi- spectral camera, one of two workhorse lunar mapping cameras (the other being the UV/visible camera), provided approximately 200 m spatial resolution at 400 km periselene, and a 39 km across-track swath. This 1.9 kg infrared camera using a 256 X 256 InSb FPA viewed reflected solar illumination from the lunar surface and lunar horizon in the 1 to 3 micrometers wavelength region, extending lunar imagery and mineralogy studies into the near infrared. A description of this lightweight, low power NIR camera along with a summary of lessons learned is presented. Design goals and preliminary on-orbit performance estimates are addressed in terms of meeting the mission's primary objective for flight qualifying the sensors for future Department of Defense flights.
This article describes the Clementine UV/Visible (UV/Vis) multispectral camera, discusses design goals and preliminary estimates of on-orbit performance, and summarized lessons learned in building and using the sensor. While the primary objective of the Clementine Program was to qualify a suite of 6 light-weight, low power imagers for future Department of Defense flights, the mission also has provided the first systematic mapping of the complete lunar surface in the visible and near-IR spectral regions. The 410 g, 4.65 W UV/Vis camera uses a 384 X 288 frame-transfer silicon CCD FPA and operates at 6 user-selectable wavelength bands between 0.4 and 1.1 micrometers . It has yielded lunar imagery and mineralogy data with up to 120 m spatial resolution (band dependent) at 400 km periselene along a 39 km cross-track swath.
The bidirectional transmittance distribution function (BTDF) of two sets of scratch/dig standard sets were measured. These sets were representative of the inspection standards used in the optical industry to characterize polished surface defects. Measurements were taken with a small (1 mm diameter) illumination beam to maximize signal. The increase in average BTDF that results from a single scratch or dig over the MIL-STD 20 mm diameter surface was then calculated to determine what overall impact a defect will have on system stray light above base surface scattering due to surface micro-roughness. A BTDF measurement was taken with the illumination beam centered on the defect, then with it centered on a smooth section of the reference sample to find the increase in scattering caused by the defect. Results show that dig scattering, when normalized to account for the single dig per 20 mm MIL-STD inspection area criteria, did not catastrophically increase the 0.05 B0 (B0 is the BTDF at 0.57 degree(s)) at 633 nm characteristic of a high quality optical surface. As intuitively expected, dig scattering was angularly symmetric. Scratches, however, scattered highly directionally. Normalized BTDF is substantially increased from a smooth surface's typical 0.05 B0 perpendicular to the scratch axis, but is unaffected in other angles. On average, the scratches may not have increased net surface scattering. Scattering from the defects on the surfaces below the 40-20 scratch/dig level was found to not cause a catastrophic increase in scattering over the level as a well-polished optic (typically 4 angstroms rms roughness). Since comparisons with scratch/dig samples only serve to provide a measure of the localized defects, and fail to be useful in determining the low-level scattering from the surface microroughness, one should not assume that a '40-20' surface is necessarily a low-scattering optic.
Lawrence Livermore National Laboratory (LLNL) has recently developed a wide-field-of- view (28 degree(s) X 44 degree(s)) camera for use as a star tracker navigational sensor. As for all sensors, stray light rejection performance is critical. Due to the baffle dimensions dictated by the large field angles, the 2-part sunshade/baffle configuration commonly seen on space- born telescopes is impractical. Meeting the required stray light rejection performance (of 10-7 Point Source Transmittance, (PST)) with a 1-part baffle required iterative APART modeling (APART is an industry standard stray light evaluation program), hardware testing, and mechanical design correction. This paper presents a chronology of lens and baffle improvements that resulted in the meeting of the stray light rejection goal outside the solar exclusion angle of the baffle stage. Comparisons with APART analyses are given, and future improvements in mechanical design are discussed. Stray light testing methods and associated experimental difficulties are presented.
A prototype wide-field-of-view (WFOV) star tracker camera has been fabricated and tested for use in spacecraft navigation. The most unique feature of this device is its 28 degree(s) X 44 degree(s) FOV, which views a large enough sector of the sky to ensure the existence of at least 5 stars of mv equals 4.5 or brighter in all viewing directions. The WFOV requirement and the need to maximize both collection aperture (F/1.28) and spectral input band (0.4 to 1.1 micrometers ) to meet the light gathering needs for the dimmest star have dictated the use of a novel concentric optical design, which employs a fiber optic faceplate field flattener. The main advantage of the WFOV configuration is the smaller star map required for position processing, which results in less processing power and faster matching. Additionally, a size and mass benefit is seen with a large FOV/smaller effective focal length (efl) sensor. Prototype hardware versions have included both image intensified and un-intensified CCD cameras. Integration times of
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