PDS_VERSION_ID = PDS3 RECORD_TYPE = STREAM OBJECT = INSTRUMENT INSTRUMENT_HOST_ID = VL1 INSTRUMENT_ID = CAM2 OBJECT = INSTRUMENT_INFORMATION INSTRUMENT_NAME = "CAMERA 2" INSTRUMENT_TYPE = CAMERA INSTRUMENT_DESC = " Instrument Overview =================== The Viking Lander camera design was very different from vidicon framing or CCD array cameras. The lander camera was a facsimile camera with a single, stationary photosensor array (PSA) and azimuth and elevation scanning mechanisms. A lander image was generated by scanning the scene in two directions (elevation and azimuth) to focus light onto the photosensor array. The Viking Lander cameras were built by Itek Corp. A number of published papers described the characteristics and performance of the lander cameras. The scientific rationale and early design of the cameras were described in MUTCHETAL1972 and a detailed description of the flight cameras was given in HUCKETAL1975B. HUCK&WALL1976 discussed image quality and PATTERSONETAL1977 described camera performance during the Primary Mission. A summary of the information from these papers is given here as a high-level description of the camera and its operating modes. Science Objectives ================== The major scientific objectives of the Viking Lander imaging investigation were to analyze the geology, cartography, meteorology, and biology of the landing sites [MUTCHETAL1972]. Geologic studies included characterizing the morphology of rocks, soils, and other features from texture and color, determining the size distribution of rocks, and understanding sediment transport. Cartographic studies involved mapping features at the landing sites, measuring surface topography, and determining lander location by comparing features seen in both lander and orbiter images. Meteorological investigations with Viking Lander images determined atmospheric aerosol properties (abundance, size, composition, and distribution) and searched for evidence of dust and condensate clouds. Biological studies with Viking Lander image data consisted of searching for evidence of living things. Platform Mounting Descriptions ============================== Each camera was bolted to the top of the lander body. The camera had a lower stationary section containing electronics and an upper section that rotated in azimuth (i.e., around a vertical axis). The other prominent exterior component was a post assembly that protected the camera window from the Mars environment when the camera was not in use. Major internal camera components were two fused silica windows, a mirror, elevation and azimuth rotation assemblies, a lens, the photosensor array, electronics, and pinlights for calibration. The overall camera height was 55.6 cm. The lower assembly diameter was 25.6 cm and the upper assembly diameter was 14.4 cm. The mass of the camera was 7.26 kg. Below is a table that lists the serial number of the cameras and photosensor arrays as they were assigned to the flight cameras [HUCKETAL1975A]. Camera Serial Number PSA Serial Number -------------------------------------------------------- Viking 1, camera 1 FC-1B M017 Viking 1, camera 2 FC-2A M020 Viking 2, camera 1 FC-3A M015 Viking 2, camera 2 SPARE M019 Operational Considerations ========================== The camera had several features to deal with possible dust abrasion or obscuration. When the camera was not in use, the upper assembly was rotated so that the window was behind the post assembly to prevent exposure to dust. Mounted in the post was a nozzle for releasing carbon dioxide gas to blow dust off the window. In addition, the outer window, known as the contamination cover, was designed to move out of the optical path if it became abraded or dust coated. The contamination cover window was hinged and spring loaded so that it could be moved aside by rotating the camera behind the post with enough force to release the spring lever. During the Extended Mission the contamination cover windows of camera 1 on lander 1 and camera 2 on lander 2 were opened. One drawback of the contamination cover window was that its frame caused a vignetting effect at elevation angles above 25 deg [PATTERSONETAL1977]. Light entered the camera through the windows, reflected off the mirror toward the lens, passed through the lens, and was sensed by one of the photodiodes. The light generated a voltage in the selected photodiode that was digitized by an analog-to-digital (A/D) converter. Below the lens was a black shutter that could be closed to sample photodiode dark current and to perform internal radiometric calibration. Between the lens and photosensor array was a light baffle to minimize internal reflections. The lens had a 0.95 cm aperture diameter and 5.37 cm focal length. Mechanical scanning was done by rotating the mirror around a horizontal axis to scan each vertical line in an image. The entire upper camera assembly rotated around a vertical axis to scan successive lines. Image data were collected while the mirror rotated upward through the elevation range in 512 steps (i.e., pixels were sampled from the bottom of the image to the top). The mirror rapidly rotated down to the start position while the camera turned in azimuth for the next scan. The mirror could rotate from 60 deg below a plane perpendicular to the azimuth rotation axis to 40 deg above it for a total range of 100 deg. The camera could see 342.5 deg in azimuth with the range limited by the post assembly. Image commands specified both a start and stop azimuth such that the width of images varied from image to image. On receiving a command to acquire an image, the camera first rotated to a preset azimuth and then rotated to the start azimuth. The camera stepped in azimuth after each vertical scan until the stop azimuth was reached. The sequence ended by the camera moving back to the preset position and then to the park position with the window behind the post. The preset position at the beginning and end of the sequence prevented the camera from turning through mechanical stops. Internal radiometric calibration could be done by closing the shutter while the camera was in the preset position either before or after the image data were acquired. The camera had two scanning rates so that data could be collected at 250 or 16,000 bits/sec. The photosensor array consisted of 12 silicon photodiodes (or diodes) sensitive to light between 0.4 and 1.1 micrometers. The diodes were arranged in a 2x6 array. There were four broad band, high resolution (0.04 deg angular resolution) diodes, known as BB1, BB2, BB3, and BB4. The distance between the lens and each high resolution diode was different to vary the in-focus distance of each diode. In-focus distances were 1.9, 3.7, 4.5, and 13.3 meters for BB1, BB2, BB3, an BB4, respectively. There was a low resolution (0.12 deg angular resolution), broad band diode known as the SURVEY diode. There were also six narrow band, low resolution diodes for color (BLUE, GREEN, and RED) and infrared (IR1, IR2, and IR3) images. The narrow bandwidth was generated by covering diodes with a set of interference filters, chosen to survive the spacecraft sterilization process. The interference filters had significant out-of-band leaks. Also, the infrared filters were known to degrade due to neutron radiation from the lander RTGs [PATTERSONETAL1977]. The twelfth diode was also a low resolution diode covered by a red filter for imaging the Sun. The in-focus distance for the low resolution diodes was about 3.7 meters. Diodes in the camera generated a voltage proportional to incoming radiance. Each diode, except for the SUN diode, had an amplifier to enhance the voltage signal. The SUN diode did not need an amplifier because of the strong signal when directly viewing the Sun. Voltage from the amplifier was digitized as a 6-bit number in an A/D converter that also performed automatic dark current subtraction. Dark current was sampled after every line in slow scan mode and after every 64th line in rapid scan mode. The A/D converter had 6 gains and 32 offsets so that the full dynamic range of the diode output could be stored in 6-bits. Gain defined the voltage range sampled and its resolution. Low gain (high gain number) covered a wide range in voltage and had low voltage resolution. The offset could be varied from a slightly negative voltage to several volts in 32 small steps. Digital values from the A/D converter could be dumped to the spacecraft tape recorder or transmitted to Earth or an orbiter in real-time (usually done at beginning or end of a transmission link when the bit error rate was relatively high). Each camera had two pinlights in the post assembly. Using a special command, the lights turned on and an image was acquired while viewing the pinlights. This command, known as a scan verification, monitored the mechanical scanning operation of the camera. A third pinlight with four different radiance levels was located between the dark current shutter and the photosensor array. This pinlight was used with the shutter closed for radiometric calibration of the camera. Operation and Sampling Modes ============================ Operation of Viking Lander cameras was versatile in that many parameters for the image could be specified. Commands to acquire an image involved selecting the sampling mode, diode, start and stop azimuths, center elevation, gain and offset, scan rate, and specifying whether automatic dark current subtraction, internal calibration, rescan, or dusting were done. Image start time and whether the image was transmitted in real-time or sent to the tape recorder were also specified. The camera had seven sampling modes: low resolution (0.12 deg) stepping with three diodes (known as triplet mode); low resolution stepping with one diode; high resolution (0.04 deg) stepping with one diode; and four different calibration lamp intensity levels. In triplet mode, elevation scanning was repeated three times with three different diodes at every azimuth position. An unusual sampling mode effect was an elevation shift when the step size did not match the diode resolution. There was a -5.6 deg elevation shift when using a low resolution diode with high resolution stepping and a +5.6 deg shift when using a high resolution diode with low resolution stepping. These non-nominal modes were mostly used to generate high resolution color images or to better resolve the solar disk. For the triplet mode, the specified diode was the first of the three diodes, e.g., BLUE diode for a color image. Start and stop azimuths had to be multiples of 2.5 deg. Center elevation had to be a multiple of 10 deg. The elevation range was determined by the step size with a range of about 20 deg for high resolution and about 60 deg for low resolution since there was always 512 steps in an elevation scan. Rescan was done by inhibiting the azimuth rotation and repeatedly scanning in elevation. Rescan could be activated by commanding the camera to operate for a time longer than it took to scan the azimuth range of the image. Principal Investigator ====================== The principal investigator for the Viking Lander imaging system during instrument development and the Viking Primary Mission was Thomas A. Mutch of Brown University. From the Extended Mission until the end of the Viking Mission, Raymond E. Arvidson of Washington University was the lander imaging principal investigator. " END_OBJECT = INSTRUMENT_INFORMATION OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "HUCKETAL1975A" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "HUCKETAL1975B" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "HUCK&WALL1976" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "MUTCHETAL1972" END_OBJECT = INSTRUMENT_REFERENCE_INFO OBJECT = INSTRUMENT_REFERENCE_INFO REFERENCE_KEY_ID = "PATTERSONETAL1977" END_OBJECT = INSTRUMENT_REFERENCE_INFO END_OBJECT = INSTRUMENT END