Instrument Information
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| IDENTIFIER |
urn:nasa:pds:context:instrument:vsp.lcross::1.1
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| NAME |
VISIBLE SPECTROMETER
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| TYPE |
SPECTROMETER
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| DESCRIPTION |
Instrument Overview =================== The LCROSS visible spectrometer (VSP) is a modified version of Ocean Optics Ltd's commercial QE65000 spectrometer. It has been adapted for space use and is similar to a spectrometer developed for Mars Science Laboratory (MSL). The VSP spectrometer design consists of an FC optical fiber input (0.11 NA) feeding a 25 micron x 1 mm entrance slit, where light is diffracted by a 1-inch f/4 optical cross Czerny-Turner spectrometer with an oversized camera mirror. The 263-650 nm spectrum from the slit is imaged onto a 1044x64 pixel Hammamatsu CCD detector. The data is co-added within the spectrometer, delivering a 16-bit, 1x1024 pixel spectra to the DHU electronics for telemetry formatting. The VSP contains no moving parts. The CCD detector is cooled by an internal Thermal Electric Cooler (TEC), whose set-points are programmable. The VSP power is 4.8 W (TEC off), and could reach 11.8 W (at TEC setting -10 C at operating temperature +19-20 C). Integration time for the VSP is also configurable between 8 ms and 65.5 s. Additionally, the VSP supports two operation modes, single and bracket spectra. The former is a single spectra acquisition of the appropriate requested integration time. The latter is a three spectra acquisition defined by a base integration time and a multiplier that is divided or multiplied by the base integration time to yield a shorter or longer exposure time, respectively. The VSP is fed by 75 cm length 600-micron core-diameter UV/Vis glass fiber attached to a fore-optics unit in the Payload Observation Deck (POD). This fore-optics unit is a fixed two-mirror and one lens system designed to provide a one degree circular field-of-view. Scientific Objectives ===================== The main science objective for the visible spectrometer is to provide wavelength dependent information between 250 nm an 650 nm with a resolving power (lambda/delta_lambda > 100). The VSP has a measured wavelength response of 263nm to 650.25nm, pre-flight. Post-flight, the VSP wavelength response was verified through fits to several lines from scattered sunlight, from 262.58 to 650.30 nm, indicating no significant shift during launch. The LCROSS spacecraft will observe the pre- and post-Centaur-impacted lunar regolith in and outside the targeted region at a spatial resolution and viewing angle unobtainable from Earth. The visible spectrometer will record the sunlit plume evolution, and track the evolution of OH- radicals from sunlight-dissociated water vapor molecules. Specifically, the visible spectrometer will measure the OH- (308 nm) and H2O+ (619 nm) transitions simultaneously which will assess the water vapor production in the ejecta. The visible spectrometer is also used to provide a UV-visible spectra of the shape and evolution of the impact flash event. The visible spectrometer is also used in a star field calibration in the early stages of the mission to check pointing and provide health checks on an temporary aperture door covering the two nadir fore-optics (one feeds the VSP and one feeds NSP1) and the total luminance photometer. Calibration =========== Spectra generated by the visible spectrometer are initially processed by the LCROSS GSEOS (Ground Support Equipment Operating System) which extracted the CCSDS files from telemetry, extracted ancillary housekeeping data provided by the instrument, 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 single mode are saved into one file. The triplet spectra taken in bracket mode are saved into a single file. On-board wavelength calibration is not used, even if available. Wavelength calibration, mapping pixel to wavelengths in nanometers, is provided by cross-checking in-orbit data (using solar spectral lines) against lab reference spectra taken on the ground. Wavelength calibration is applied only to the 1024 pixels of spectral data. 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 pre-flight using a NIST-calibrated reference source and adjusted for any wavelength shifts post-launch, if appropriate. This approach provided a radiance calibration longward of 380 nm. A 'best effort' UV calibration for the unit shortward of 380 nm was provided using an ETU instrument. The calibration steps are described in the LCROSS Instrument Response and Calibration Report in the CALIB directory of this archive. Operational Modes ================= The VSP supports two operation modes - single and bracket spectra. In bracket mode, the spectrometer takes three spectra in quick succession, the first at the commanded exposure time, followed by a shorter exposure then a longer one. The bracketing exposure times are computed by dividing and multiplying the commanded time by a factor (e.g., 2). This factor can be set as well. Each PDS spectra, a collection of 1044 16-bit numbers (raw) or 1044 32-bit PC_REAL numbers (calibrated), is a single spectral acquisition regardless whether it was taken in single or bracket mode. The time stamp will represent the true time the spectrum was acquired. The VSP integration time is a configurable parameter. Typical values used in the LCROSS mission range from 100 milliseconds to 4 seconds. The Thermal Electric Cooler (TEC) has a programmable set-point and a viable range around the ambient working temperature. The set-point as well as the reported temperature the VSP has calculated the TEC is being controlled to are labels in the PDS (.lbl) files. The setpoint is typically set to -10 C, which is in the middle of the range for a VSP operating around +17 to +21 C, the nominal thermal operating environment for this instrument. The integration time is documented with each spectra.
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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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