PDS_VERSION_ID = PDS3 LABEL_REVISION_NOTE = "2003-09-30 Cornell:carcich 2011-05-04 A.Raugh Incorporated separate CRISP instrument description into this file to fit it into the PDS3 catalog system. " OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = CON INSTRUMENT_ID = CRISPIMAG OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "CONTOUR REMOTE IMAGING SPECTROGRAPH - IMAGER" INSTRUMENT_TYPE = "IMAGER" INSTRUMENT_DESC = " CONTOUR Remote Imaging Spectrograph - Imager (CRISPIMAG) The CRISP instrument is effectively two instruments working independently in a single package. For that reason the same description is supplied for both the imager (CRISPIMAG) and the spectrograph (CRISPSPEC). Instrument Overview =================== CRISP actively tracks the comet's nucleus near CONTOUR's closest approach, taking high resolution images, color images through 10 filters, and an infrared spectral map. Once light enters the reflective telescope, wavelengths shorter than 800 nm are reflected by a beamsplitter to an imager. The imager has a 10-position filter wheel, with one filter for fast, clear- filter imaging and nine filters for measuring color differences indicative of differences in composition of the non-ice portion of the nucleus surface. The digital image is captured by a CCD that provides a field-of-view 1.2deg x 1.2deg (1024 x 1024 pixels). Infrared light at wavelengths beyond 800 nm is sent through a narrow slit into the spectrometer part of CRISP. The slit forms an image of a long, narrow portion of the nucleus. This long, narrow image is dispersed by the spectrometer into 256 different near-infrared wavelengths from 800 nm to 2500 nm. In the resulting two- dimensional image, which is measured by a Rockwell mercury-cadmium telluride (HgCdTe) detector cooled cryogenically to 90degK, one direction corresponds to spatial location along the slit and the other direction corresponds to wavelength of light. CRISP's scan mirror is the critical component that allows the instrument to take high-resolution images (as good as 4 meters per pixel) while passing as close as 100 kilometers from its target at high velocity. The imager and spectrometer both 'look' at a scan mirror. The mirror steers the fields-of-view to track the comet nucleus and take out its relative motion so that unsmeared images can be obtained. The tracking process begins about 10,000 km before closest approach, when the comet is only a few tens of pixels across. At this point the field-of-view is still large compared to navigational uncertainties, so it can be pointed at the comet's position and be assured of capturing the comet. Image processing in CRISP's Data Processing Unit (DPU) autonomously calculates the center of the illuminated part of the nucleus, and keeps track of the location of that point in subsequent images. This series of measurements is used by a program in the DPU that extrapolates into the future where it expects the nucleus to be, and then compares the prediction with the actual position from the image processing software. Both procedures progressively refine the estimated location of the nucleus as a function of time before closest approach. By the time the critical close- up data are taken, a preprogrammed set of observations is matched with the actual flyby time to better than 1 second. The updated three-dimensional estimate of the nucleus's location is used to point the mirror, keep the nucleus in the center of the field-of-view, and take out smear. This approach requires that CRISP's line-of-sight is accurately referenced into inertial coordinates. As a result the spacecraft star cameras, which provide the inertial reference for spacecraft pointing, are mounted directly to CRISP instead of to the spacecraft. At the Encke encounter, in the hours prior to one hour before closest approach, CONTOUR is occasionally tilted by a few degrees so that CRISP can monitor the distant nucleus's rotation unobstructed by the dust shield. Subsequently, the dust shield is oriented to protect the body of the spacecraft from impacts by dust in the comet's coma. Beginning about 6 minutes prior to closest approach, image acquisition is begun at a high rate so that the nucleus' position can be determined and tracked. During this time CRISP's aperture is partly occulted by the edge of the dust shield. CRISP's main period of data acquisition begins only about 50 seconds before closest approach when the dust shield no longer interferes with the view of the nucleus. The mirror keeps the nucleus in the field-of-view and scans the spectrometer slit across the surface, building up an infrared compositional map showing features as small as about 100 meters across. Color images taken by the imager show variations at scales three times smaller. Then a series of clear-filter images is taken as the spacecraft's changing view of the nucleus provides a stereo perspective. The mirror continues tracking the nucleus until it is 30 degrees off the forward direction, about 6 seconds before closest approach, when the highest-resolution images (about 4 meters per pixel) are obtained. Outbound from a comet, the sequence will be repeated in reverse. CRISP Vital Statistics Imager Spectrometer Telescope aperture 100 mm Telescope focal length 683 mm Field-of-view 1.2deg x 1.2deg 0.86deg x 0.003deg Pixel field-of-view 20 mrad (0.001deg) 59 mrad (0.003deg) Wavelength range 450-770 nm 800-2550 nm Spectral channels 10 256 Detector type TH7888A Rockwell PICNIC HgCdTe array Pixels per image 1024 x 1024 256 spatial x 256 spectral Typical exposure times 9-150 ms 0.2 sec CRISP Camera Filters Wavelength FWHM1 Purpose 'Clear' 350 Navigation / tracking 450 40 Nucleus geology, color 490 40 ditto 530 40 ditto 570 40 ditto 610 40 ditto 650 40 ditto 690 40 ditto 730 40 ditto 770 40 ditto FWHM = full width of the wavelength range passed by the filter at greater than half intensity From http://www.contour2002.org/instruments2.html Copyright 2002 NASA Discovery Program Used by Permission " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "N/A" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END