Visual Imaging Subsystem - Camera A =================================== INSTRUMENT: VISUAL IMAGING SUBSYSTEM - CAMERA A SPACECRAFT: VIKING ORBITER 2 Instrument Information ====================== Instrument Id : VISA Instrument Host Id : VO2 Pi Pds User Id : MCARR Instrument Name : VISUAL IMAGING SUBSYSTEM - CAMERA A Instrument Type : VIDICON CAMERA Build Date : 1974-05-01 Instrument Mass : 22.700000 Instrument Length : 0.937000 Instrument Width : 0.218000 Instrument Height : 0.218000 Instrument Serial Number : SN08 Instrument Manufacturer Name : JET PROPULSION LABORATORY Instrument Description ====================== Each Viking Orbiter was equipped with two identical vidicon cameras. The camera system is commonly called the Visual Imaging Subsystem (VIS). Each camera consists of a telescope, a slow scan vidicon, a filter wheel, and associated electronics. The filter wheel contains blue, minus blue, violet, clear, green, and red filters. The angular field of view as defined by the reseau pattern is 1.51 by 1.69 degrees. Since the scan lines go beyond the reseau pattern in the line direction the active field of view is about 1.54 degrees in this direction. The actual ground area covered by an image varies as a function of spacecraft altitude and emission angle. The digital image is generated by scanning the vidicon face plate. The signal is digitized as a 7-bit number. The image contains 1056 horizontal lines. There are 1182 samples in each line exposed to incoming light. The vidicon samples 11 pixels on either side of the raster onto an opaque mask located at the front of each vidicon. Hence, 1204 samples per line are telemetered to Earth, but only 1182 contain information. Each camera can be commanded to acquire an image every 8.96 seconds. By alternating cameras, an image can be taken every 4.48 seconds. Science Objectives ================== The first objective of the imaging system was to characterize potential landing sites in support of site selection. Additional objectives of the imaging system were to study the photometric and colorimetric properties of the surface discovered by Mariner 9 in order to better understand the geological history of Mars atmosphere. Operational Considerations ========================== Each Viking Orbiter camera contained a filter wheel with five filters and a clear position, i.e., no filter. An image could be acquired using one of the six filters types, which included blue, minus blue, violet, clear, green, and red. The filter wheel could be stepped only one position between each exposure. Three mode parameters or camera states could be set for a given exposure. There was the light flood condition flag. Light flooding was used to reduce the residual image from the previous exposure. Eight small incandescent bulbs exposed the vidicon beyond saturation. There were also two gain states: low and high. The high gain state increased the gain by a factor of 2. There was also an exposure offset that would provide a positive signal (DN) from the camera even for a zero exposure. Calibration Description ======================= 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 vidicon varies across the field of view. The sensitivity of the vidicon also varies with time. Each Viking Orbiter imaging instrument was calibrated before flight. In addition, changes in the calibration over time have been estimated from analyses of images of deep space and dust storms. The radiometric calibration applies additive and multiplicative corrections that account for the varying sensitivity of the vidicon. The resulting values are proportional to radiance factor, which is defined as the ratio of the observed radiance to the radiance of a normally illuminated lambertian reflector of unit reflectance at the same heliocentric distance. The geometric calibration removes electronic distortions and transforms the point perspective geometry of the original image into a map projection. The electronic distortions are barrel- shaped distortions from the electron beam readout and complex distortions from interactions between the charge on the vidicon face plate and the electron beam. The electronic distortions are modeled by comparing the undistorted locations of reseau marks with the actual locations in an image. Section 'VISA' ============== Total Fovs : 1 Data Rate : 2112000.000000 Sample Bits : 7 'VISA' Detectors ---------------- VISA 'VISA' Electronics ------------------ VISA 'VISA' Filters -------------- BLUE CLEAR GREEN MINUS BLUE RED VIOLET 'VISA' Section Optic IDs ------------------------ VISA In modes -------- ... ..D .G. .GD L.. L.D LG. LGD 'VISA' Section FOV Shape 'RECTANGULAR' -------------------------------------- Section Id : VISA Fovs : 1 Horizontal Pixel Fov : 0.001420 Vertical Pixel Fov : 0.001420 Horizontal Fov : 1.692000 Vertical Fov : 1.543000 'VISA' Section Parameter 'RADIANCE' ----------------------------------- Radiance is the amount of energy per time per projected area per steradian. Instrument Parameter Name : RADIANCE Sampling Parameter Name : PIXEL Instrument Parameter Unit : WATT/(METER*METER)/STERADIAN Sampling Parameter Resolution : 0.001420 Sampling Parameter Unit : DEGREE Instrument Detector 'VISA' ========================== Detector Type : VIDICON Detector Aspect Ratio : 1.120000 Minimum Wavelength : 0.350000 Maximum Wavelength : 0.650000 Nominal Operating Temperature : 288.000000 Description ----------- The VIS detector is a Westinghouse 5166 selenium vidicon. It is about 3.7 cm (1-1/2 in.) in diameter. Sensitivity ----------- The saturation current from the vidicon is 43 nA. The residual dark current is 0.2 nA. The response of the visual imaging subsystem is linear to first order. Analyses of imaging data acquired inflight indicate that the system is linear to within 3% over its dynamic range. Instrument Electronics 'VISA' ============================= Description ----------- About 1000 electronic parts (e.g., vidicon filament voltage and alignment current regulators) were included per camera, divided between the camera head and an auxiliary chassis. (see Wellman et al, 1976) Instrument Filter '1 - BLUE' ============================ Filter Name : BLUE Filter Type : INTERFERENCE Minimum Wavelength : 0.350000 Maximum Wavelength : 0.530000 Center Filter Wavelength : 0.470000 Description ----------- The filter transmittance was measured using a Cary 14 spectrophotometer with a spare set of filters. (see Benesh and Thorpe, 1976) Instrument Filter '2 - MINUS BLUE' ================================== Filter Name : MINUS BLUE Filter Type : ABSORPTION Minimum Wavelength : 0.480000 Maximum Wavelength : 0.700000 Center Filter Wavelength : 0.550000 Description ----------- The filter transmittance was measured using a Cary 14 spectrophotometer with a spare set of filters. (see Benesh and Thorpe, 1976) Instrument Filter '3 - VIOLET' ============================== Filter Name : VIOLET Filter Type : INTERFERENCE Minimum Wavelength : 0.350000 Maximum Wavelength : 0.470000 Center Filter Wavelength : 0.440000 Description ----------- The filter transmittance was measured using a Cary 14 spectrophotometer with a spare set of filters. (see Benesh and Thorpe, 1976) Instrument Filter '4 - CLEAR' ============================= Filter Name : CLEAR Minimum Wavelength : 0.350000 Maximum Wavelength : 0.700000 Center Filter Wavelength : 0.520000 Description ----------- The filter transmittance was measured using a Cary 14 spectrophotometer with a spare set of filters. (see Benesh and Thorpe, 1976) Instrument Filter '5 - GREEN' ============================= Filter Name : GREEN Filter Type : INTERFERENCE Minimum Wavelength : 0.500000 Maximum Wavelength : 0.600000 Center Filter Wavelength : 0.530000 Description ----------- The filter transmittance was measured using a Cary 14 spectrophotometer with a spare set of filters. (see Benesh and Thorpe, 1976) Instrument Filter '6 - RED' =========================== Filter Name : RED Filter Type : ABSORPTION Minimum Wavelength : 0.550000 Maximum Wavelength : 0.700000 Center Filter Wavelength : 0.590000 Description ----------- The filter transmittance was measured using a Cary 14 spectrophotometer with a spare set of filters. (see Benesh and Thorpe, 1976) Instrument Optics 'VISA' ======================== Telescope Diameter : 0.137000 Telescope F Number : 3.500000 Telescope Focal Length : 0.474610 Telescope Resolution : 0.000050 Telescope Serial Number : SN06 Telescope T Number : 5.410000 Telescope T Number Error : 0.020000 Telescope Transmittance : 0.419000 Description ----------- The telescope was an all spherical Schmidt- Cassegrain design employing a Mangin primary mirror. Optical performance was 70% response at 42 line pairs per millimeter. (see Wellman et al, 1976) NOTE: The TELESCOPE_RESOLUTION given above is a line pair resolution, i.e., the geometrical size of a pixel times two. Instrument Mode '...' ===================== Gain Mode Id : LOW Instrument Power Consumption : 22.000000 In sections ----------- VISA Description ----------- (see Wellman et al, 1976) Instrument Mode '..D' ===================== Gain Mode Id : LOW Instrument Power Consumption : 22.000000 In sections ----------- VISA Description ----------- Light flood is off, gain is low, and exposure offset is on. Instrument Mode '.G.' ===================== Gain Mode Id : HIGH Instrument Power Consumption : 22.000000 In sections ----------- VISA Description ----------- Light flood is off, gain is high, and exposure offset is off. Instrument Mode '.GD' ===================== Gain Mode Id : HIGH Instrument Power Consumption : 22.000000 In sections ----------- VISA Description ----------- Light flood is off, gain is high, and exposure offset is on. Instrument Mode 'L..' ===================== Gain Mode Id : LOW Instrument Power Consumption : 22.000000 In sections ----------- VISA Description ----------- Light flood is on, gain is low, and exposure offset is off. Instrument Mode 'L.D' ===================== Gain Mode Id : LOW Instrument Power Consumption : 22.000000 In sections ----------- VISA Description ----------- Light flood is on, gain is low, and exposure offset is on. Instrument Mode 'LG.' ===================== Gain Mode Id : HIGH Instrument Power Consumption : 22.000000 In sections ----------- VISA Description ----------- Light flood is on, gain is high, and exposure offset is off. Instrument Mode 'LGD' ===================== Gain Mode Id : HIGH Instrument Power Consumption : 22.000000 In sections ----------- VISA Description ----------- Light flood is on, gain is high, and exposure offset is on. Mounted On Platform 'SCAN PLATFORM' =================================== Cone Offset Angle : -0.021000 Cross Cone Offset Angle : -0.684000 Twist Offset Angle : 89.770000 Description ----------- (see Wellman et al, 1976) References ========== Klaasen, K.P., T.E. Thorpe, and L.A. Morabito, Inflight performance of the Viking visual imaging subsystem, Applied Optics, 16, 3158-3170, 1977. Benesh, M., and T. Thorpe, Viking Orbiter 1975 Visual Imaging Subsystem Calibration Report, JPL document 611-125, 1976. Wellman, J.B., F.P. Landauer, D.D. Norris, and T.E. Thorpe, The Viking Orbiter visual imaging subsystem, J. Spacecr. Rockets, 13, 660-666, 1976.