PDS_VERSION_ID = PDS3 OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = CLEM1 INSTRUMENT_ID = HIRES OBJECT = INSTINFO INSTRUMENT_NAME = "HIGH RESOLUTION CAMERA" INSTRUMENT_TYPE = CAMERA INSTRUMENT_DESC = " Instrument Overview =================== The High Resolution Imaging (Hires) camera combines a lightweight beryllium telescope with an image intensifier-coupled frame transfer CCD imager. Image shuttering is accomplished through voltage gating of the image intensifier. Maximum integration time is 733 milliseconds in 10.67 microsecond increments. Spectral response is limited in the system by the S-2 photocathode between 0.4 and 0.8 microns. Five spectral bands can be selected from a filter wheel which is controlled through the same serial-addressable synchronous interface (SASI). A sixth filter position is allocated to an opaque filter for the protection of the photocathode and image intensifier. The post-FPA electronics allow three gain states followed by 5 bits of offset that span 248 counts in the analog regime to augment the basic 8-bit A/D conversion. Images of the day side of the moon used intensifier gate times with relatively low gain settings on the order of 1 millisecond. Lifetime concerns about the photocathode and micro channel plates in the intensifier unit drove operational settings to low exposures. This resulted in photon shot noise significantly contributing to the overall noise in the Hires sensor. Additionally, the image intensifier imposed a nonuniformity on images. This yielded images that did not compress well using the MATRA optimized rms algorithm. For this reason, JPEG compression, which discards high spatial frequency detail, was employed on the Hires images. The Hires performance specifications are provided below. Calibration =========== The radiometric calibration converts the digitized signal received from the camera (DN value) into a quantity that is proportional to the radiance reaching the sensor. The sensitivity of the CCD focal plane array varies across the field of view but appears to be time invariant during the two month lunar observation period. However, reliable pre-flight calibrations for the Hires camera are not available. The high shot noise due to in-flight use of low gains to preserve the camera system's lifetime, together with spurious 'honeycomb' nonuniformities imposed by the intensifier tube make the task of finding reliable in-flight calibrations most daunting, and have forced a more empirical photometric calibration. The Hires geometric and photometric calibrations were referenced to the UVVIS Basemap Mosaic. The geometric calibration was achieved through a constrained registration of map-projected, nadir-pointed Hires images against the UVVIS Basemap Mosaic, which is the de facto lunar control network. An approximate photometric calibration was performed on Hires mosaic tiles of sufficiently limited extent (1.75 degrees of latitude) that the photometric response was approximately constant. Then, the linear stretch which matches the underlying UVVIS Baseline Mosaic approximates the photometric calibration of the Hires tile. Operational Considerations ========================== The pole-to-pole lunar observations provided scenes with a broad range of viewing conditions, ranging from bright observations near zero phase angle at the equator to very low light-level observations at the poles. In order to properlyrecord an observation with an optimal signal-to-noise ratio it is important to adequately fill the 8-bit (255 levels) dynamic range of the A/D camera output. The integration time (exposure time) and the gain and offset settings of the instrument were adjusted to properly record each image. During the systematic mapping, the gain state of the camera was normally set to lower values for the mid-latitude observations and set to higher levels (thereby increasing the sensitivity of the A/D converter) at the higher latitudes. Integration times were rarely increased as observations approached the poles, nominally being set to 1.067 ms. Lunar observations were broken into 10 latitude bins. Each latitude bin nominally contained fixed gain and offset modes and integration times for each camera/filter combination, although occasional offlooking slews that fell within a bin other than that corresponding to the subspacecraft point resulted in images with differing acquisition settings. The Clementine orbit was designed to provide overlapping coverage in both the down-track (~15% overlap) and cross-track (~10% overlap at the equator) directions. The image overlap is necessary to geometrically control images in cartographic applications. Operational Modes ================= High Resolution Imaging (Hires) camera: The Hires camera combines a lightweight beryllium telescope with an image intensifier-coupled frame transfer CCD imager. Image shuttering is accomplished through voltage gating of the image intensifier. Maximum integration time is 733 milliseconds in 10.67 microsecond increments. Spectral response is limited in the system by the S-2 photocathode between 0.4 and 0.8 microns. Five spectral bands can be selected from a filter wheel which is controlled through the same SASI interface. A sixth filter position is allocated to an opaque filter for the image intensifier's protection. The HIRES camera had four operating modes: 1. Programmable integration time. The range of integration times is 1-733 milliseconds. 2. Gain Mode. The gain mode represents the multiplicative constant applied to the image data passing through the A/D converter. Three gain state settings were available (1,2,5). Gain setting 5, which represented the lowest setting, was used almost constantly during lunar observations due to lifetime considerations of the imaging system. 3. Offset Mode. The offset mode represents the additive constant applied to the image data passing through the A/D converter. 4. Intensifier Gain Mode. The intensifier gain mode represents the multiplicative gain of the image intensifier. The precise behavior of the gain is, however, not known. Camera Specifications ===================== Focal Plane Array: ------------------- Type Si Charge Coupled Device Thomson TH7863-CRU Pixel format 288x384 Pixel size 23x23 microns Readout rate 4MHz Optics: ------------------- Clear aperture 131 nm Speed F/9.5 Imaging: ------------------- Wavelength 0.4 to 0.8 microns Field of view 0.4 deg. x 0.3 deg. Pixel IFOV 18 microradians Point spread 4 pixels Filters 415 nm cw (+/- 20 nm) ('A') 560 nm cw (+/- 5) ('B') 650 nm cw (+/- 5) ('C') 750 nm cw (+/- 10) ('D') 400 to 800 nm broadband ('E') (opaque) ('F') Image intensifier: ------------------- Module General Atomics 0131-Z12-2-009 Useful photocathode diameter 12 mm Luminous gain 1000 fL/fC Limiting resolution 40 lp/mm Gain control 8 bits Camera Electronics: ------------------- A/D resolution 8 bits Frame rate 10 Hz Readout time 27.4 ms Integration time 0.2-733 ms Digitization gain 150,350,1000 electrons/count Offset control 248 gray levels Power 9.5 W Mechanical: Mass 1120 grams Envelope 17.0 cm x 18.1 cm x 36.4 cm " END_OBJECT = INSTINFO OBJECT = INSTREFINFO REFERENCE_KEY_ID = "KORDASETAL1995" END_OBJECT = INSTREFINFO END_OBJECT = INSTRUMENT END