Open DGodfrey54321 opened 3 years ago
Photosounder
commented
3 years ago Thank you for the insights. I'm mainly interested in the calibration data. I dug a bit and found that calibration comes from files such as "FICOR77_VG1_NA_VIOLET.DAT", however it seems impossible to locate such files anywhere.
DGodfrey54321
commented
3 years ago A lot of data is available on the various NASA Planetary Data System (PDS) sites – main page at https://pds.nasa.gov/ . Part of the confusion is discovering what data sets are available where, particularly for missions like Voyager which studied many different objects. According to the PDS "Planetary Atmospheres Node" page for the Voyager Imaging SubSystem (ISS) at https://pds-atmospheres.nmsu.edu/data_and_services/atmospheres_data/Voyager/iss.html the Jupiter and Saturn data has the Raw data (EDRs) hosted on the "Cartography and Imaging Node" (at https://pds-imaging.jpl.nasa.gov/), while the processed data are available on the "Ring-Moon System Node" (at https://pds-rings.seti.org/). The latter has a page for the Voyager ISS at https://pds-rings.seti.org/voyager/iss/index.html I found that the easiest way to locate images was actually to use their 'Viewmaster" application at https://pds-rings.seti.org/viewmaster/volumes/ to browse my way down the directory tree. The various sub-directories have a slightly confusing structure, however the read-me document ( replicated in many locations - one is at https://pds-rings.seti.org/holdings/volumes/VGISS_5xxx/VGISS_5101/AAREADME.TXT ) explains it well. It was this that enabled me to locate you’re FICOR77_VG1_NA_VIOLET.DAT file amongst the calibration files at at https://pds-rings.seti.org/viewmaster/volumes/VGISS_5xxx/VGISS_5120/CALIB/MIPL and the equivalent file for Voyager 2 can be found at https://pds-rings.seti.org/viewmaster/volumes/VGISS_5xxx/VGISS_5214/CALIB/MIPL
I must admit this seemed amusingly odd. In the ‘good old days’ to calibrate an image I first had to find the 1/2 inch magnetic tape containing the appropriate file – only two would fit on each tape – load that onto disk along with a relevant dark current image and then run FICOR. Being able to download a calibration file instantly from across the world in seconds and store it in memory seems like magic. Sometimes this IT stuff works really well!
HOWEVER you may not need to use the calibration files, etc. as it turns out that the images have all been batch processed, and both the raw and calibrated versions of the files are available on the site. By starting at:
https://pds-rings.seti.org/viewmaster/volumes/VGISS_5xxx
and proceeding based upon the range of Flight Data System (FDS) times you are interested in, one can obtain the raw and processed versions of images in both VICAR and jpg formats. As an example consider the Voyager 1 Jupiter image with FDS count 16348.42 (the Wide Angle (WA) part of a Both Simultaneous (BSIMAN) shuttered pair). The JPG version of the raw image is available at
https://pds-rings.seti.org/holdings/volumes/VGISS_5xxx/VGISS_5115/BROWSE/C16348XX/C1634842_RAW.JPG
and the jpg version of the corrected data is at
https://pds-rings.seti.org/holdings/volumes/VGISS_5xxx/VGISS_5115/BROWSE/C16348XX/C1634842_GEOMED.JPG
While the VICAR format versions are at
https://pds-rings.seti.org/holdings/volumes/VGISS_5xxx/VGISS_5115/DATA/C16348XX/C1634842_RAW.IMG
for the raw image, with the corrected ones at
https://pds-rings.seti.org/holdings/volumes/VGISS_5xxx/VGISS_5115/DATA/C16348XX/C1634813_GEOMED.IMG .
The documents describing the processing details are available at
https://pds-rings.seti.org/viewmaster/volumes/VGISS_5xxx/VGISS_5115/DOCUMENT
Hope this Helps – D. Godfrey
The Voyager images were produced by two bore sighted vidicon tubes (one narrow angle and one wide angle), and have a range of distortions. To correct for the varying ratio between the incoming radiation and the digital readout, the images were calibrated. This was traditionally done, at JPL and other institutions, using a program called FICOR77. To the best of my (distant) recollections this required: a) The relevant calibration file, containing a mapping between intensity and read out across the image. I believe there was a different calibration file for each filter - or maybe it was camera mode. And, b) An appropriate dark current image. As the residual background can vary with time, I believe due to its sensitivity to background radiation.
In addition to the radiometric calibrations, geometric corrections were also necessary. This is because the vidicons include electron optics, and the image can therefore be distorted by magnetic fields. To correct for this the various reseaux marks on the image are located and compared with their known locations on the face-plate. The image is then warped to fit the original locations.
If there are requirements to accurately position or mosaic images then additional navigation information is required - provided either by the relevant SEDR (Supplementary Experimental Data Record) data or the NAIF/SPICE system (see https://naif.jpl.nasa.gov/naif/index.html) . Even this is not sufficient on its own as the position of the scan platform is never known quite accurately enough. This is inferred by locating the edge of the plant in a simultaneously shuttered wide angle image.
Traditionally most the of the above corrections were carried out using version of the JPL originated VICAR system. A guide to this system can be found at: https://www-mipl.jpl.nasa.gov/vicar_os/v1.0/vicar-docs/VICAR_guide_1.0.pdf . and an open source version is apparently available at https://www-mipl.jpl.nasa.gov/vicar_open.html. The following list of Voyager specific programs is taken from the above Vicar pdf:
VGRCDCOPY Convert a VGR image archived on CDROM to a VICAR image VGRFILLIN Fill in data gaps in VGR (EDR) images CAMPARAM Copy camera params from VGR label to TAE local variables RESLOC Locate reseau on VGR images RESSAR77 Remove reseau from VGR images OSBLEMLOC Convert VGR blemish locations from image to object space FICOR77 Radiometric correction of VGR images FIXVGR Scale VGR images to correct for FICOR77 scaling error PHOTLIST Print phase, incidence, and emission angles for a VGR image
I hope the above is of some use. I have processed a fair number of Voayger images myself ( see references 1 and 11 of https://en.wikipedia.org/wiki/Saturn%27s_hexagon ), but not recently.