Kevin J. Heffernan

CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) on MRO (Mars Reconnaissance Orbiter)

CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) is a hyperspectral imager that will be launched on the MRO (Mars Reconnaissance Orbiter) spacecraft in August 2005. MRO’s objectives are to recover climate science originally to have been conducted on the Mars Climate Orbiter (MCO), to identify and characterize sites of possible aqueous activity to which future landed missions may be sent, and to characterize the composition, geology, and stratigraphy of Martian surface deposits. MRO will operate from a sun-synchronous, near-circular (255x320 km altitude), near-polar orbit with a mean local solar time of 3 PM. CRISM’s spectral range spans the ultraviolet (UV) to the mid-wave infrared (MWIR), 383 nm to 3960 nm. The instrument utilizes a Ritchey-Chretien telescope with a 2.12° field-of-view (FOV) to focus light on the entrance slit of a dual spectrometer. Within the spectrometer, light is split by a dichroic into VNIR (visible-near-infrared, 383-1071 nm) and IR (infrared, 988-3960 nm) beams. Each beam is directed into a separate modified Offner spectrometer that focuses a spectrally dispersed image of the slit onto a two dimensional focal plane (FP). The IR FP is a 640 x 480 HgCdTe area array; the VNIR FP is a 640 x 480 silicon photodiode area array. The spectral image is contiguously sampled with a 6.6 nm spectral spacing and an instantaneous field of view of 61.5 μradians. The Optical Sensor Unit (OSU) can be gimbaled to take out along-track smear, allowing long integration times that afford high signal-to-noise ratio (SNR) at high spectral and spatial resolution. The scan motor and encoder are controlled by a separately housed Gimbal Motor Electronics (GME) unit. A Data Processing Unit (DPU) provides power, command and control, and data editing and compression. CRISM acquires three major types of observations of the Martian surface and atmosphere. In Multispectral Mapping Mode, with the gimbal pointed at planet nadir, data are collected at frame rates of 15 or 30 Hz. A commandable subset of wavelengths is saved by the DPU and binned 5:1 or 10:1 cross-track. The combination of frame rates and binning yields pixel footprints of 100 or 200 m. In this mode, nearly the entire planet can be mapped at wavelengths of key mineralogic absorption bands to select regions of interest. In Targeted Mode, the gimbal is scanned over ±60° from nadir to remove most along-track motion, and a region of interest is mapped at full spatial and spectral resolution. Ten additional abbreviated, pixel-binned observations are taken before and after the main hyperspectral image at longer atmospheric path lengths, providing an emission phase function (EPF) of the site for atmospheric study and correction of surface spectra for atmospheric effects. In Atmospheric Mode, the central observation is eliminated and only the EPF is acquired. Global grids of the resulting lower data volume observation are taken repeatedly throughout the Martian year to measure seasonal variations in atmospheric properties.

Compact reconnaissance imaging spectrometer for Mars (CRISM): characterization results for instrument and focal plane subsystems

The Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) will launch in 2005 on the Mars Reconnaissance Orbiter (MRO) mission, with its primary science objective to characterize sites with aqueous mineral deposits hyperspectrally at high spatial resolution. CRISM’s two Offner relay spectrometers share a single entrance slit with a dichroic beamsplitter. The IR focal plane contains a 640 (spatial) x 480 (spectral) HgCdTe FPA with a 980 nm to 3960 nm spectral bandpass. It is cooled to 110 K to minimize dark current, and coupled to a 28 mm long cold shield to minimize thermal background. The spectrometer housing is cooled to -90 C for the same reason. A three-zone IR filter consisting of two broadband filters and a linear variable filter overlays the IR focal plane, eliminating multiple grating orders and providing additional attenuation of the thermal background. The visible focal plane contains a 640 (spatial) x 480 (spectral) silicon photodiode array, with a 380-1050 nm spectral bandpass occupying approximately 106 rows of the detector. A two-zone filter comprised of two different Schott glasses eliminates multiple grating orders. The two focal planes together cover 544 spectral channels with a dispersion of 6.55 nm/channel in the VNIR and 6.63 nm/channel in the IR. The optics and focal planes are gimbaled, and a pre-programmed slew can be used to remove groundtrack motion while superimposing a scan across a target. CRISM will operate in two basic modes: a scanning, high resolution mode to hyperspectrally map small, targeted areas of high scientific interest, and a fixed, nadir-pointed, lower resolution pixel-binned mode using selected wavelength channels to obtain near-global coverage to find targets. Preliminary performance of the CRISM instrument is presented, and is compared with prior system design predictions.

Ultraviolet and visible imaging and spectrographic imaging (UVISI) experiment

The ultraviolet and visible imaging and spectrographic imaging (UVISI) experiment consists of five spectrographic imagers and four imagers. These nine sensors provide spectrographic and imaging capabilities from approximately equals 110 nm to approximately equals 900 nm. The spectrographic imagers (SPIMs) share an off-axis parabolic design in which selectable slits (1.00 degree(s) X 0.10 degree(s) or 1.00 degree(s) X 0.05 degree(s)) provide spectral resolutions between approximately equals 0.5 nm and approximately equals 4.0 nm. SPIM image planes have programmable spectral dimensions with 68, 136, or 272 pixels and programmable spatial dimensions with 5, 10, 20, 40 pixels. A scan mirror sweeps the slit through a second spatial dimension and generates a spectrographic image once every 5, 10, or 20 seconds. The four imagers provide narrow-field and wide-field viewing. Each imager has a six-position filter wheel that selects various spectral regimes and neutral densities. Each of the nine sensors use intensified CCD detectors that have an intrascene dynamic range of approximately equals 103 and an interscene dynamic range of approximately equals 105; neutral density filters provide an additional dynamic range of approximately equals 102-3. An automatic gain control adjusts the intensifiers to scenes of varying intensity. UVISI also includes an image processing system that uses the raw data from any single imager to acquire and track targets of various sizes, shapes, and brightnesses. The image processor relays its results to a master tracking system that uses the UVISI data (as well as other data) to point the satellite in real time. UVISI will be launched on the MSX satellite in late 1994 and will investigate a multitude of celestial, atmospheric, and point sources during its planned five-year lifetime.

Measurement of the particulate and water vapor contamination environments of the Midcourse Space Experiment (MSX) spacecraft

We have designed, fabricated, and tested two flashlamp-based instruments that will characterize the particulate and water vapor contamination environments aboard the Midcourse Space Experiment (MSX) spacecraft: the Xenon Flashlamp and the Krypton Radiometer. These instruments will operate as part of suite of instruments to monitor the MSX contamination environment over its five-year mission. The Xenon Flashlamp illuminates particles in the field of view of the UVISI Wide Field of View Visible Imager, which in turn measures the scattered radiation. The particle measurement can detect particles smaller than 1 micrometers and can measure cross-field particle velocities from 0.5 cm/sec to 50 m/sec. The Krypton Radiometer measures the local water vapor density. VUV radiation from an array of RF-excited krypton lamps photodissociates H2O in the fields of view of a filtered radiometer and one of the UVISI Spectrographic Imagers. The radiometer and the spectrograph simultaneously measure the intensity of the resulting OH chemiluminescence. The H2O density is proportional to that intensity. The spectrograph will provide a positive identification of the radiating species. Instrument descriptions as well as ground test and simulation data are presented.

The CONTOUR remote imager and spectrometer (CRISP)

The CONTOUR Remote Imager and Spectrometer (CRISP) was a multi-function optical instrument developed for the Comet Nucleus Tour Spacecraft (CONTOUR). CONTOUR was a NASA Discovery class mission launched on July 3, 2002. This paper describes the design, fabrication, and testing of CRISP. Unfortunately, the CONTOUR spacecraft was destroyed on August 15, 2002 during the firing of the solid rocket motor that injected it into heliocentric orbit. CRISP was designed to return high quality science data from the solid nucleus at the heart of a comet. To do this during close range (order 100 km) and high speed (order 30 km/sec) flybys, it had an autonomous nucleus acquisition and tracking system which included a one axis tracking mirror mechanism and the ability to control the rotation of the spacecraft through a closed loop interface to the guidance and control system. The track loop was closed using the same images obtained for scientific investigations. A filter imaging system was designed to obtain multispectral and broadband images at resolutions as good as 4 meters per pixel. A near IR imaging spectrometer (or hyperspectral imager) was designed to obtain spectral signatures out to 2.5 micrometers with resolution of better than 100 meters spatially. Because of the high flyby speeds, CRISP was designed as a highly automated instrument with close coupling to the spacecraft, and was intended to obtain its best data in a very short period around closest approach. CRISP was accompanied in the CONTOUR science payload by CFI, the CONTOUR Forward Imager. CFI was optimized for highly sensitive observations at greater ranges. The two instruments provided highly complementary optical capabilities, while providing some degree of functional redundancy.

SCHOONERS: absorption and refraction of starlight from space for atmospheric profiles

The Self-Calibrating H2O and O3 Nighttime Environmental Remote Sensor (SCHOONERS) is a compact, integrated UV-IR imaging spectrograph and imager. The instrument has a 25 cm diameter aperture and employs a two- axis gimbaled telescope to provide acquisition and tracking of the star. It also uses a two-axis high-precision vernier mirror to correct for spacecraft jitter and maintain the star within the field-of-view. The imaging spectrograph, covering a spectral range between 300 and 900 nm, measures the varying absorption of starlight as a star sets through the nighttime Earths atmosphere to determine vertical profiles of atmospheric constituents. The relative star position measured by the co-aligned imager not only provides position feedback to the acting tracking loop of the vernier mirror, but also measures the star refraction angle for determining the atmospheric density and temperature profiles. The SCHOONERS scanning platform and its high- precision tracking mirrors provide 44 microradian azimuth pointing stability and 60 microrad altitude tracking accuracy (3(sigma) ). Its built-in image tracking and motion compensation mechanism, coupled with its small size and limited spacecraft resources required, makes it suitable for deployment on existing and future commercial spacecraft platforms as an instrument-of-opportunity after the year 2002. A laboratory facility has been developed to demonstrate the instrument performance, especially its capability to acquire and track a setting, refracting, and scintillating star, to compensate for various degrees of platform jitter, and to provide the pointing knowledge accuracy required for the determination of atmospheric density and temperature. Hardware includes an accurately moving variable intensity point source to simulate the star and motion stages to generate jitter at the instrument. Software simulates the stellar refraction, attenuation, and scintillation for a full end-to-end test of the instrument.