Instrument Id : ISSW
Instrument Host Id : VG1
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 : SN06
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) (Voyager 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 Voyager 2 went on to encounter Uranus and Neptune. As the Voyager mission progressed the objects photographed were further from the sun so they appear 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 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 the 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 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 that 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 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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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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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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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 25-mm 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 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 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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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
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
Filter Name : GREEN
Filter Type : INTERFERENCE
Minimum Wavelength : 0.530000
Maximum Wavelength : 0.640000
Center Filter Wavelength : 0.585000
Instrument Filter '6 - METHANE-U'
Filter Name : METHANE-U
Filter Type : INTERFERENCE
Minimum Wavelength : 0.536000
Maximum Wavelength : 0.546000
Center Filter Wavelength : 0.540000
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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Telescope Diameter : 0.057143
Telescope F Number : 3.500000
Telescope Focal Length : 0.200465
Telescope Resolution : 0.000140
Telescope Serial Number : WAO-06
Telescope T Number : 4.140000
Telescope T Number Error : 0.020000
Telescope Transmittance : 0.800000
The wide-angle camera optics is a 200 mm. 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 stop number is 3.5 (VGR ISS Calibration Report, 1978, an internal JPL document available from JPL vellum files)
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Instrument 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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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)
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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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Scan Platform
Cone Offset Angle : 0.031500
Cross Cone Offset Angle : 0.024700
Twist Offset Angle : 0.275000
References
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Last updated 26 August 2003