Voyager 2 Wide Angle Camera Description

Index

Instrument Information

      Instrument Id                  : ISSW
      Instrument Host Id             : VG2
      Pi Pds User Id                 : BASMITH
      Naif Data Set Id               : UNK
      Instrument Name                : IMAGING SCIENCE SUBSYSTEM - WIDE
                                         ANGLE
      Instrument Type                : VIDICON CAMERA
      Build Date                     : 1976-12-17
      Instrument Mass                : 13.300000
      Instrument Length              : 0.550000
      Instrument Width               : 0.200000
      Instrument Height              : 0.200000
      Instrument Serial Number       : SN04
      Instrument Manufacturer Name   : JET PROPULSION LABORATORY

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Instrument Description

The Voyager Imaging Science Subsystem (ISS) is a modified version of the slow scan vidicon camera designs that were used in the earlier Mariner flights. The system consists of two cameras, a high resolution Narrow Angle (NA) camera and a lower resolution, more sensitive Wide Angle (WA) camera. Unlike the other on board instruments, operation of the cameras is not autonomous, but is controlled by an imaging parameter table residing in one of the spacecraft computers, the Flight Data Subsystem (FDS) (Science and Mission Systems Handbook, 1987, JPL D-498, an internal JPL document available from JPL vellum files).

The original mission was to Jupiter and Saturn. Voyager surpassed expectations and went on to encounter Uranus and Neptune. As the Voyager mission progressed the objects photographed were further from the sun so they appeared more faint even though longer exposures were used. As the Voyager spacecrafts’ distance from the Earth increases, the telecommunications capability at each encounter decreases. The difference in capabilities from the Jupiter and Saturn encounters and that at Uranus and Neptune was considerable. The reduced telecommunications capability limits the number of data modes that imaging can use. Because of the diminished brightness of the objects being photographed, longer exposure times were used, many beyond the stated maximum of 15.360 seconds. Longer exposure times were all 48-second increments added to the maximum. In addition, the camera was slewed in order to avoid smeared imaging. The light flood state (on/off) was independent of the instrument mode.

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Science Objectives

The overall objective of this experiment is exploratory reconnaissance of Jupiter, Saturn, Uranus, Neptune and their satellites and rings. Such reconnaissance, at resolutions and phase angles unobtainable from Earth, provides much new data relevant to the atmospheric and/or surface properties of these bodies. The experiment also has the following specific objectives: observe and characterize global circulation and meteorology; determine the horizontal and vertical structure of visible clouds; characterize the nature of any colored material which may be in clouds.

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Operational Considerations

To make full scientific use of the image collection, it is necessary to understand the radiometric and geometric characteristics of the camera system and perform corrections to the data.

Each Voyager camera is unique in terms of its calibrated characteristics. Each has intrinsic shading (spatially non-uniform output DNs from flat field target) and exhibits barrel distortions typical of TV cameras flown on previous planetary missions. Because of these characteristics, the cameras were calibrated before launch. The response of pixels to known targets, illumination, exposures, etc. was measured and Calibration Files were generated to remove radiometric and geometric distortions from the flight images. These Calibration Files and detailed information on their use are available through the Imaging Node.

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Calibration Description

The calibration program for the Voyager television cameras consisted of three parts: (1) component calibrations (‘Voyager Imaging Science Subsystem Calibration Report’ July 31, 1978, M. Benesh and P. Jepsen, D618-802); (2) subsystem calibrations; and, (3) system calibrations. In addition, provision was made for in-flight calibrations. Component calibrations were carried out prior to camera assembly. Important measurements include spectral transmittance of the lens and filters, actual exposure times and shading characteristics of the shutter, and pertinent electro-optical properties of the vidicon. After the optics and sensor were assembled it was possible to run calibrations at the camera, or subsystem, level. Particular activities accomplished during this period included radiometric calibrations, focal length measurement, determination of the modulation transfer function, measurement of the geometric distortion, and calibrations required for color reconstruction. System calibrations were conducted after the cameras were installed on the spacecraft. Important tasks included measurement of the Field-Of-View (FOV) alignment and verification of the flat-field light transfer characteristics. Noise measurements were also made at the system level.

A method for in-flight calibration was also available. It consisted of a flat reflective surface which is located about 2 m from the camera. A flat-field light transfer curve can be constructed by orienting the spacecraft so the at the plaque is illuminated by the sun and by photographing it at the proper exposure times. However, calibration data was collected but no new calibration files were generated during either the Jupiter, Saturn, or Uranus encounters or their related cruise periods.

Although the INSTRUMENT_PARAMETER_NAME has been provided as Radiance, the Voyager Experiment Data Record (EDR) data set has not been radiometrically corrected, and thus images do not represent radiance units. In order to convert an image from Data Number (DN) to radiance units, the image must be calibrated. Radiometric calibration files, and selected radiometrically corrected images are available through the Imaging Node.

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Field Of View (FOV)

      Total Fovs                     : 1

      Sample Bits                    : 8

FOV (Field Of View) Shape ‘SQUARE’

        Section Id                     : ISSW
        Fovs                           : 1
        Horizontal Pixel Fov           : 0.003960
        Vertical Pixel Fov             : 0.003960
        Horizontal Fov                 : 3.169000
        Vertical Fov                   : 3.169000

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Parameters

Radiance is the amount of energy per time per projected area per steradian.

        Instrument Parameter Name      : RADIANCE
        Sampling Parameter Name        : PIXEL
        Instrument Parameter Unit      : DIMENSIONLESS
        Noise Level                    : UNK

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Instrument Detector

      Detector Type                  : VIDICON
      Detector Aspect Ratio          : 1.000000
      Minimum Wavelength             : 0.280000
      Maximum Wavelength             : 0.640000
      Nominal Operating Temperature  : 282.000000

The sensor used in the Voyager Imaging Science Subsystem ISS camera system is a 25mm diameter magnetic deflection vidicon (number B41-003, General Electro- dynamics Co.). The vidicon storage surface (target) is selenium sulphur and can store a high resolution (1500 TV lines) picture for over 100 s at room temperature. The active image area on the target is 11.14 x 11.14 mm. Each frame consists of 800 lines with 800 picture elements (pixels) per line, i.e., 1 pixel = 14 microns. One frame requires 48 s for electronic readout. In addition to the normal frame readout of 48 s (1:1), four extended frame-time modes of 2:1, 3:1, 5:1, and 10:1 are available by command. Following readout, light flooding is used to remove any residual image that might remain from the previous frame. At the end of light flooding, 14 erase frames are used to stabilize and prepare the vidicon target for the next exposure sequence (VGR ISS Calibration Report, 1978, an internal JPL document available from JPL vellum files).

Sensitivity

Calibration experiments show the gain map indicating higher sensitivity toward the top of the frame in a generally radial manner. Dark-current ratio results indicate a mean within plus-or-minus 3% of the true linearity of the light transfer function. The required accuracy of the light-transfer functions was plus-or-minus 5% of half-scale signal averaged over any randomly selected area of 10 contiguous pixels. This requirement was consistently met for all Imaging Science Subsystem (ISS) flight cameras. The radiance of the used light cannons was supposed to be plus-or-minus 5% or better of the level to produce a half- scale signal. This criterion was also met. It is not clear whether the color spatial dependence is due to the vidicon, or whether the filters have varying transmissions. In the latter case, the ratios would be independent of light level; and this has been observed to be the case for all flight cameras. Moreover, vidicons have not shown scale variations of this magnitude in the past, so that it is easier to believe that they are due to the spectral filters rather than to the vidicons. The color sensitivity is sufficient to require a separate decalibration file for each spectral filter, which has been done, but not gross enough to cause concern about the quality of the image itself (VGR ISS Calibration Report, 1978, an internal JPL document available from JPL vellum files).

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Instrument Electronics

The Imaging Science Subsystem (ISS) electronics consist of the vidicon support circuits and the signal chain. The vidicon support circuits are the vertical and horizontal sweep circuits, and the various power supplies for the vidicon filament, and the focus and alignment coils. The signal chain consists of the analog signal amplifiers, bandpass filters, and an eight bit analog-to-digital converter. The digital output is sent to the Flight Data Subsystem (FDS) for editing.

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Filters

Spectral measurements at the manufacturer were taken on Beckman spectro- photometers, and verifications at Jet Propulsion Laboratory (JPL) were made with a Cary 14 spectro-photometer. Each test scan was run from 2000 to 7000 Angstroms to check for eventual leaks outside the passband (VGR ISS Calibration Report, 1978, an internal JPL document available from JPL vellum files). For spectral information on each filter, see Danielson, E. G., et al. Radiometric Performance of the Voyager Cameras, JGR, v. 86, Sept. 1981.

Instrument Filter ‘0 - METHANE-JST’

      Filter Name                    : METHANE-JST
      Filter Type                    : INTERFERENCE
      Minimum Wavelength             : 0.614000
      Maximum Wavelength             : 0.624000
      Center Filter Wavelength       : 0.619000

Instrument Filter ‘1 - BLUE’

      Filter Name                    : BLUE
      Filter Type                    : INTERFERENCE
      Minimum Wavelength             : 0.430000
      Maximum Wavelength             : 0.530000
      Center Filter Wavelength       : 0.480000

Instrument Filter ‘2 - CLEAR’

      Filter Name                    : CLEAR
      Filter Type                    : ABSORPTION
      Minimum Wavelength             : 0.280000
      Maximum Wavelength             : 0.640000
      Center Filter Wavelength       : 0.460000

Instrument Filter ‘3 - VIOLET’

      Filter Name                    : VIOLET
      Filter Type                    : INTERFERENCE
      Minimum Wavelength             : 0.350000
      Maximum Wavelength             : 0.450000
      Center Filter Wavelength       : 0.400000

Instrument Filter ‘4 - SODIUM-D’

      Filter Name                    : SODIUM-D
      Filter Type                    : INTERFERENCE
      Minimum Wavelength             : 0.588000
      Maximum Wavelength             : 0.590000
      Center Filter Wavelength       : 0.589000

Instrument Filter ‘5 - GREEN’

      Filter Name                    : GREEN
      Filter Type                    : INTERFERENCE
      Minimum Wavelength             : 0.530000
      Maximum Wavelength             : 0.640000
      Center Filter Wavelength       : 0.580000

Instrument Filter ‘6 - METHANE-U’

      Filter Name                    : METHANE-U
      Filter Type                    : INTERFERENCE
      Minimum Wavelength             : 0.536000
      Maximum Wavelength             : 0.546000
      Center Filter Wavelength       : 0.541000

Instrument Filter ‘7 - ORANGE’

      Filter Name                    : ORANGE
      Filter Type                    : INTERFERENCE
      Minimum Wavelength             : 0.590000
      Maximum Wavelength             : 0.640000
      Center Filter Wavelength       : 0.615000

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Instrument Optics ‘ISS-WA’

      Telescope Diameter             : 0.057143
      Telescope F Number             : 3.500000
      Telescope Focal Length         : 0.200770
      Telescope Resolution           : 0.000140
      Telescope Serial Number        : WAO-04
      Telescope T Number             : 4.260000
      Telescope T Number Error       : 0.020000
      Telescope Transmittance        : 0.800000

The Wide-Angle camera optics is a 200mm focal length six element lens of the Petzval-type. It consists of five elements plus an additional dust lens, located between the shutter and the vidicon. The f number is 3.5 (VGR ISS Calibration Report, 1978, an internal JPL document available from the JPL vellum files).

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Instrument Operational Modes

Instrument Mode ‘IM2’

      Data Path Type                 : RECORDED DATA PLAYBACK
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 1:1, full frame image, 800 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM2A’

      Data Path Type                 : RECORDED DATA PLAYBACK
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 1:1, full frame, 800 pix/line (Uranus and Neptune only)

Instrument Mode ‘IM2C’

      Data Path Type                 : RECORDED DATA PLAYBACK
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 5:1, full frame, 800 pix/line, lines may be truncated (zero filled) on left or right - data dependent (Neptune only)

Instrument Mode ‘IM2W’

      Data Path Type                 : RECORDED DATA PLAYBACK
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 1:1, full frame, 800 pix/line, last 80 lines are zero (Neptune only)

Instrument Mode ‘IM3’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 1:1, full frame, 800 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM4’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 1:1, centered frame, 608 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM5’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 2:1, Top read out in frame #1, bottom read out in frame #2, 800 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM6’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 1:1, centered frame, 440 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM7’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 3:1, full frame, 800 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM8’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 1:1, centered frame, 272 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM9’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 3:1, centered, approx. 480 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM10’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 1:1, centered frame, approx. 160 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM11’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 5:1, centered frame, 800 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM12’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 5:1, centered frame, approx. 440 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM13’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 10:1, centered frame, 800 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM14’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 1:1, centered frame, approx. 80 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM15’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 2:1, centered frame, top read out in frame #1, bottom read out in frame #2, 800 pix/line (Jupiter and Saturn only)

Instrument Mode ‘IM26’

      Data Path Type                 : RECORDED DATA PLAYBACK
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 2:1, lines 271-536, 800 pix/line (Neptune only)

Instrument Mode ‘IMK’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 10:1, centered frame, 800 pix/line (Uranus and Neptune only)

Instrument Mode ‘IMO’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 5:1, centered frame, 800 pix/line (Uranus and Neptune only)

Instrument Mode ‘IMQ’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 5:1, centered frame (Uranus only)

Instrument Mode ‘OC3’

      Data Path Type                 : REALTIME
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 3:1, lines 301-510, samples 365-453 (Neptune only)

Instrument Mode ‘PB8’

      Data Path Type                 : RECORDED DATA PLAYBACK
      Gain Mode Id                   : LOW
      Instrument Power Consumption   : 14.000000

Scan rate (minor frame:line) is 5:1 or 1:1, data dependent may be IM2A, IM2C, IM2W (Uranus and Neptune only)

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Scan Platform

      Cone Offset Angle              : -0.030800
      Cross Cone Offset Angle        : -0.006800
      Twist Offset Angle             : 0.171102

The measurements recorded below are the coordinates representing the center of the Wide Angle Field-Of-View (FOV) in relation to the center of the narrow angle FOV. As of 5/26/88 the Voyager 2 scan platform offset values were updated and the removal of all extraneous offset values for VG1 and VG2 accomplished. Only the most recently input values remain in the database for each spacecraft. These values are effective for all periods of data inclusive from launch to present. (June 7, 1989).

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References

BARROS1988
BENESH1978
DANIELSONETAL1981
HARCH1987
HARCH1988A
HARCH1988B
HARCH1988C
JPL D-2468
JPL D-498
MARTINETAL1985
NAVE1980
SCIENCEV204N4392
SCIENCEV206N4421
SCIENCEV212N4491
SCIENCEV215N4532
SCIENCEV233N4759
SCIENCEV246N4936
SNYDERL1979
SSRV21N2
SSRV21N3

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