Instrument functional description:
The VIS is a set of three low-light-level cameras. Two of these cameras share primary and some secondary optics and are designed to provide images of the nighttime auroral oval at visible wavelengths. A third camera is used to monitor the directions of the fields-of-view of the auroral cameras with respect to the sunlit Earth and return global images of the auroral oval at ultraviolet wavelengths. The VIS instrumentation produces an auroral image of 256 x 256 pixels approximately every 24 seconds dependent on the integration time and filter selected.
The fields-of-view of the two nighttime auroral cameras are 5.6 x 6.3 degrees and 2.8 x 3.3 degrees for the low and medium resolution cameras, respectively. The medium resolution camera was never activated. One or more Earth camera images of 256 x 256 pixels are produced every five minutes, depending on the commanded mode. The field-of-view of the Earth camera is approximately 20 x 20 degrees.
 
Reference: 
Frank, L. A., J. B. Sigwarth, J. D. Craven, J. P. Cravens, J. S. Dolan, M. R. Dvorsky, J. D. Harvey, P. K. Hardebeck, and D. Muller, 'The Visible Imaging System (VIS) for the Polar Spacecraft', Space Science Review, vol. 71, pp. 297-328, 1995.
 
http://vis.physics.uiowa.edu/vis/vis_description/vis_description.htmlx
 
[Note to first-time users:  The first six variables are of primary interest. The displayable 256 x 256 raw image data is in variable 3. The displayable 256 x 256 processed image datais in variable 4. The correct orientation of a displayed image is explained in the description of variable 3 below.]
 
Data set description:
The VIS Earth camera data set comprises all Earth camera images for the selected time period. The raw displayable image counts are in variable 3 while the processed displayable image counts are in variable 4.
 
Full coordinate information is included for viewer orientation. 
 
In addition, a rotation matrix and a table of distortion-correcting look direction unit vectors are provided for the purpose of calculating coordinates for every pixel. See the description of variables 20 and 21 below.
 
To facilitate viewing of the images, a mapping of pixel value to a recommended color table based on the characteristics of the selected filter will be included with each image. See the description of variables 25, 26, and 27 below.
 
A relative intensity scale is provided by the uncompressed count table of variable 30. Approximate intensity levels in kiloRayleighs are given in the intensity table of variable 31.
For detailed information on intensities, see Sensitivities_and_Intensities.txt
 
https://cdaweb.gsfc.nasa.gov/Polar_VIS_docs/SENSITIVITIES_AND_INTENSITIES.TXT
 
Variable descriptions:
 
1,2. Center time
The time assigned to an image is the center time of the integration period within a resolution of 50 milliseconds.
 
3. Raw (unprocessed) image counts
Image pixel counts range from 0 to 255. They are stored in a two-dimensional 256 x 256 byte array. Images from the Earth camera (sensor 0) are conventionally displayed with row 1 at the top, row 256 at the bottom,column 1 on the left, and column 256 on the right. The conventional image display for the low resolution camera (sensor 1) is rotated 180 degrees so that the row 1-column 1 pixel is at the lower right corner and the row 256-column 256 pixel is at the upper left corner. When displayed in this manner, the spacecraft spin axis is oriented to the right in the display, the X component is defined as the center of the image look direction, and the Y component is the cross product of the spin axis and the look direction.
 
4. Cleaned image counts
These are image pixel counts that have been calibrated using the following routines.
 
For the earth camera:
 
Horizontal Smooth EC 4 if Modified Julian Date (MJD) > 3429 (correction required after an event in 2005)
Horizontal Smooth EC 6 if MJD > 4307 (correction required after an event in 2007)
Subtract Cosmic Rays
Subtract Slopes (adjusts for biases across the CCD)
Remove Weave (corrects for interference from low resolution camera)
Flat Field (corrects for other characteristics of the CCD) [Note: depending on viewing geometry, not all irregularities can be fixed completely;  in particular, a wide diagonal stripe may still be visible]
Dayglow Subtract
Nightglow Minimum
 
For the low resolution camera:
 
Subtract Slopes
Flat Field
Smooth Filter
 
The data structure is the same as the Raw image counts. See the description of variable 3 for details.
 
5. Cleaned image data in kiloRayleighs.
Same data as in variable 4, only in kiloRayleighs.
 
6. Sensor number
0 = Earth camera
1 = low resolution camera
2 = medium resolution camera (never activated).
 
7. Half integration time
This is half the length of the integration period for the image, measured in milliseconds.
 
8. Filter 
Twelve filters are available for visible imaging; the filter number, 1-12, is given here. Ultra-violet imaging is done with one filter only, designated here as filter number 0. In addition, the peak wavelength in Angstroms is given for the selected filter.
For detailed information on filter characteristics, see Sensitivities_and_Intensities.txt
 
9. Presumed altitude of emissions
The presumed altitude of the emissions seen in the image varies with the characteristics of the filter used.
 
10. Platform pitch angle
This is the platform pointing angle of rotation around the spin axis, measured from  nadir.
 
11,12. Geographic coordinates
Geographic north latitude and east longitude are provided for all pixels.
 
13,14. Right Ascension and Declination of each pixel These values are given in degrees.
 
15. Altitude along tangent to line-of-sight for each pixel.
 
16. Flag to indicate if each pixel is pointed at the earth.
(0/False 1/True)
 
17,18. Spacecraft position and velocity vectors, GCI
The spacecraft position vector and velocity vector in GCI coordinates are for the image center time as given in variables 1 and 2.
 
19. Spacecraft spin axis unit vector, GCI
 
20,21. Image-to-GCI rotation matrix and look direction vector table
The rotation matrix may be used with the look direction vector table to obtain pointing vectors in GCI coordinates for each pixel. The resulting vectors may be used to calculate coordinates for the observed positions of the pixels.
The general method used is described below in Coordinate_Calculation.txt
 
https://cdaweb.gsfc.nasa.gov/Polar_VIS_docs/Coordinate_Calculation.txt
 
22. Zenith angle of center line-of-sight at presumed altitude 
This is the angle between the geocentric vector through the observed point, assuming the altitude given as variable 8, and the  reverse of the image center line-of-sight vector.
 
23. Sun position unit vector, GCI
 
24. Solar zenith angle at observed point of center line-of-sight.
This is the angle of the sun from zenith at the observed point of the center line-of-sight, assuming the altitude given as variable 8.
 
25. RGB color table
This is the recommended color table to be used with the limits given in variables 26 and 27.
 
 26,27. Low and high color mapping limits
The low and high color limits are recommended for remapping the color table entries, as follows:
For pixel values less than the low limit, use the color at table position 1.
For pixel values greater than or equal to the low limit and less than or equal to the high limit, use the color at table position (pix-low)/(high-low) x 255 + 1.
For pixel values greater than the high limit, use the color at table position 256.
 
28. Data quality flag 
The data quality word has bits set to 1 when the listed conditions are true. Bit #31 is the most significant bit in the word, and it will not be used as a flag. These are the bit assignments:
bit 0 - image data frame sync error
bit 1 - image data frame counters error
bit 2 - image data fill frame flag.
 
29. Post gap flag
The post gap flag has these possible values: 
0 - no gap occurred immediately prior to this record.
1 - the gap occurred because the instrument wasnot in a mode that allowed for the production of images for the selected sensor
2 - the gap occurred because level zero data were missing
3 - the gap occurred because level zero data were too noisy to extract images.
 
30. Expanded count table
The image pixel counts are quasi-logarithmically compressed to the range 0-255. This table gives the average of the uncompressed range for each compressed count value. Table entries 1-256 correspond to compressed counts 0-255 respectively.
 
31. Intensity table
Approximate intensity levels in kiloRayleighs are given for each compressed count value. Table entries 1-256 correspond to compressed counts 0-255 respectively. Intensity calculation is described in Sensitivities_and_Intensities.txt.
 
Supporting software:
Supporting software is available at
 
http://vis.physics.uiowa.edu/vis/software/
 
Included is an IDL program that displays the images with the recommended color bar, provides approximate intensities and coordinate data for each pixel, and and includes multiple options for image manipulation.

Scientific Description of the VIS Instrument.
Instrument Sensitivity and Method of Calculating Intensities.
Method used by Polar VIS to calculate coordinates
Link to Scientific Analysis Software.

Scientific Description of the VIS Instrument
Sensitivity and Intensity Calculation
Polar VIS Coordinate Calculation
Scientific Analysis Software

Polar Visible Imaging System (VIS) Earth Camera Images at ~4 minute cadence

Uncleaned 256 x 256 Ultra-Violet Image (quasi-logarithmically compressed counts)

Image_Counts_Raw contains the unprocessed displayable image.  The counts have been quasi-logarithmically compressed by the instrument.  Approximate uncompressed value for Image_Counts(i,j) isntens_Table ExpandedCount(Image_Counts(i,j)+1).  Approximate intensity in kR is Intens_Table(Image_Counts(i,j)+1).The appearance of the actual count value 255 is rare.  When displaying an image,it works best to use the fill value as an overflow (i.e. brightest) value.

Cleaned and Filtered 256 x 256 Ultra-Violet Image (quasi-logarithmically compressed counts)

Image_Counts contains the cleaned displayable image.  The counts have been quasi-logarithmically compressed by the instrument.  Approximate uncompressed value for Image_Counts(i,j) is ExpandedCount(Image_Counts(i,j)+1).  Approximate intensity in kR is Intens_Table(Image_Counts(i,j)+1).The appearance of the actual count value 255 is rare.  When displaying an image,it works best to use the fill value as an overflow (i.e. brightest) value.

Image_Counts contains the displayable image.  The counts have been quasi-logarithmically compressed by the instrument.  Approximate uncompressed value for Image_Counts(i,j) is ExpandedCount(Image_Counts(i,j)+1).  Approximate intensity in kR is Intens_Table(Image_Counts(i,j)+1).The appearance of the actual count value 255 is rare.  When displaying an image,it works best to use the fill value as an overflow (i.e. brightest) value.

 --> (USE SHORT TIME SPANS) Azimuthal projection to geographic (fixed sun orientation)

 --> (USE SHORT TIME SPANS) Azimuthal projection to magnetic LT and invariant lat

 --> (USE SHORT TIME SPANS) Movie, azimuthal projection to geographic (fixed sun orientation)

 --> (USE SHORT TIME SPANS) Movie, azimuthal projection to magnetic LT and invariant lat

Sensor number: 0=Earth Camera,1=Low Resolution Camera,2=Medium Resolution Camera

Filters #1-12 are visible wavelengths; filter #0 is UV for Earth camera images

Platform angle of rotation around spin axis, measured from nadir in tenths of degrees

X component is look direction,Y component is the spin axis  cross X

Solar zenith angle at intersection of center line-of-sight with Earth

RGBColorTable should be remapped for displaying an image using the low and high limits given for each image in Limit_Lo and Limit_Hi.Image_Counts count values less than Limit_Lo use the color at table position 1.  Count values greater than Limit_Hi use the color at table position 256.  For count values greater than or equal to Limit_Lo and less than or equal to Limit_Hi, the table position is (Count-Limit_Lo)/(Limit_Hi-Limit_Lo) x 255 + 1.At the selected table position C, the color components are Red at RGBColorTable(1,C), Green at RGBColorTable(2,C), and Blue at RGBColorTable(3,C).

MSB will not be used as a flag; see TEXT for other bit assignments

Expanded count table: quasi-logarithmically uncompressed pixel counts

Image_Counts contains pixel counts which have been quasi-logarithmically compressed by the instrument.  Approximate uncompressed value forImage_Counts(i,j) is ExpandedCount(Image_Counts(i,j)+1).

Approximate intensity in kR for Image_Counts(i,j) isIntens_Table(Image_Counts(i,j)+1)