Instrument Information
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| IDENTIFIER |
urn:nasa:pds:context:instrument:nsp2.lcross::1.1
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| NAME |
NEAR INFRARED SPECTROMETER 2
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| TYPE |
SPECTROMETER
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| DESCRIPTION |
Instrument Overview =================== The LCROSS payload contains two near-infrared spectrometers (NSP1/NSP2). Their electronics units are identical, but they have different fore-optics designs. Near infrared spectrometer #1 (NSP1), also known as the nadir NSP, is located inside the Payload Observation Deck and is orientated along the +X axis in LCROSS spacecraft coordinate system. The payload bore-sight is defined as the central position of the NSP1 bore-sight throughout the mission. During the final stages of the mission, the LCROSS spacecraft's +X axis is orientated towards the lunar impact site. The payload's bore-sight (center of the NSP1 one-degree field of view (FOV)) is targeted towards the expected Centaur impact location. In comparison, the fore-optics feeding near infrared spectrometer #2 (NSP2), also known as the solar-viewer or occulting NSP, is orientated along the -Z axis in LCROSS spacecraft coordinate system. This fore-optic is expected to collect radiance from the sun in its generous approximate 130 degree FOV during the final hour of the mission. Both spectrometers provide 1.2-2.4 micron spectral coverage at low resolution. Both spectrometers are manufactured by Polychromix, a company whose primary spectrometer line is designed for material analysis and chemical sensing. The spectrometers are designed to take a single SMA NA=0.22 fiber as input instead of a slit. The diameter of this fiber defines the resolution of the instrument. Within the electronics unit of the spectrometer, light collected from the fiber (plus fore-optics) is reflected off a grating to spread frequencies across an innovative, electronically-tunable MEMS device. This MEMS device reflects selected frequencies onto a single, TEC-cooled InGaAs sensor element. The combination of frequencies merged together changes according to a timed sequence, and the resulting measurement stream is decoded on the ground to recover the individual frequencies. The result is a highly sensitive, low power, and inexpensive spectrometer. The spectrometer contains no moving parts other than the MEMs device and the peak power for each NSP is 2.5 W. NSP2 is fed by a 1.45 m length 600-micron core-diameter low-OH glass fiber attached to a fore-optics that is located external to the Payload Observation Deck (POD) housing. The NSP2 fore-optic is a diffuser providing ~130 degree field-of-view. Scientific Objectives ===================== The two LCROSS near infrared spectrometers (NSP1/NSP2), by providing R~100 spectra over the 1.2 to 2.4 micron wavelength region, are the primary method to measure the amount of water vapor in the ejecta. The LCROSS spacecraft will monitor spectral bands associated with water vapor, ice, and hydrated minerals covering the first overtones of the symmetric and asymmetric stretches of water. This band, relatively free from interferences, is more brightly illuminated by sunlight than the fundamentals near 3 microns, more than compensating the weaker absorption of the overtones. The regions near 1.4 and 1.9 microns, normally obscured by terrestrial atmospheric background in spectra from icy surfaces, will provide a sensitive indication of water vapor from ice or hydrates. The remainder of the spectral band will reveal the nature of ice crystals and mineral hydrates. NSP2 with its unique constant light source (the sun) provides a continuous mapping of the ejecta cloud in absorption by conducting an occultation experiment, until its FOV goes below the crater rim. The data set derived with NSP2 data provides a different viewing geometry through the ejecta cloud compared to the nadir-looking NSP1. Calibration =========== Spectra generated by the near infrared spectrometers are initially processed by the LCROSS GSEOS (Ground Support Equipment Operating System) which extracted the CCSDS files from telemetry, applied a fixed mathematical function (the Hadamard transform provided by the manufacturer) to convert 256 mask positions to 100 spectral elements, and converted them to an ASCII comma-separated file with some metadata and with MET-based packet timestamp encoded in the filename. Spectra taken in Flash or Diagnostic Mode are saved into one file appended by subsequent sampling. Wavelength calibration, mapping pixel to wavelengths in microns, is provided by cross-checking in-orbit data against lab reference spectra taken on the ground. Wavelength calibration is applied only to the 100 pixels for Hadamard Mode, and a look-up table providing a the wavelength range across each mask position in Flash Mode. Conversion from raw data values [DN] to a scene spectral radiance [W/m^2 um sr], with errors, is performed using a response curve measured by the instrument in-flight using a data set from the preimpact sequence on Oct 9, 2009 ~11:00 UTC with the Sun as the source and providing a correction for the cosine angle to the it (~15.3 degrees for the data set used). This calibration was validated against pre-flight laboratory test of the same instrument, using a NIST-calibrated reference source, although with a different fore-optic configuration, whose attributes were measured independently to correct out. In the lab-based comparison data, particular attention has been paid to addressing and correcting for any water contamination. The flight calibration for this instrument is derived from the solar spectrum measured on orbit during the last day of the mission. The calibration steps are described in the LCROSS Instrument Response and Calibration Report in the CALIB directory of this archive. For Hadamard spectra, a separate additional raw (pre-Hadamard transform) ASCII file containing the original mask positions, will also be part of the PDS delivery. The data for the NSP2 is also self-calibrating in orbit as reference spectra (sun, no ejecta) can be used for a constant reference source. Operational Modes ================= Both spectrometers have three modes: Impact Flash (IF), Hadamard Spectrum (HS) and Diagnostic (DI), which are configurable by sending a command to the unit. The nominal Hadamard Spectrum Mode provides a 100-pixel spectra at 1.7 Hz rate with continuous coverage across 1.2 to 2.4 microns. The Impact Flash mode provides a five point (with a dark mask) sampling at known wavelengths across the spectral range at a 72 Hz sampling rate. The mask wavelengths, set approximately around the water bands, are described in the Instrument Response and Calibration Report. The instrument mode will be documented with each spectra or mask product.
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| MODEL IDENTIFIER |
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| NAIF INSTRUMENT IDENTIFIER |
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| SERIAL NUMBER |
not applicable
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| REFERENCES |
unk
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