Voyager 2 Saturn Revisited, Still a lot to be processed and reprocessed Options

[Bjorn Jonsson]

Emily recently mentioned in her blog the availability of calibrated and geometrically corrected Voyager images. Actually I had 'discovered' this dataset several months earlier but then managed to completely forget about it. Now I decided to do something so I downloaded volume 37 and decided to do some quick-and-dirty processing, mainly to check if it was feasible to do a very high resolution map (probably 25 degrees/pixel to match my Cassini map of the southern hemisphere) of Saturn's entire northern hemisphere by colorizing green filtered images using lower resolution color data I processed several years ago - at the resolution I want only green filtered images are available.

This was successful, opening the door to a new 'monster project': A very high resolution full color map of Saturn's entire northern hemisphere.

First a color composite made from wide angle orange, green and blue images:

This one was made from images C4386547_GEOMED.IMG, C4386554_GEOMED.IMG and C4386608_GEOMED.IMG. I adjusted the color to something more realistic than I initially got and removed some reseau marks in Photoshop that were visible, especially near ring edges and Saturn's limb. Some color fringing was also visible on Saturn's disk due to Saturn's rotation while the three images were obtained; I removed this by cloning the color of adjacent areas. The spokes in the rings presented similar problems.

I then colorized a green filtered image obtained at a similar time as the wide angle images above. This was the result:

The image should be fairly realistic and I was happy with the result, especially because I didn't do this very carefully - something better should be possible.

Finally the same image sharpened with an unsharp mask:

Lots of small scale details are visible, especially near the pole.

I will probably post several additional Voyager Saturn images in the next several weeks. As previously mentioned, the plan now is to do a very high resolution map of Saturn's entire northern hemisphere based on these calibrated and rectified images. This means reprojecting the images to simple cylindrical projection. To do this I need to know the viewing geometry. Does anyone know if this information is available somewhere (or if not, if it's likely to ever become available)? I have some SPICE kernels which give me Voyager 2's location relative to Saturn. These are probably fairly accurate. However, the limited instrument pointing information I have is very inaccurate so it's useless to me. I can reverse engineer the viewing geometry/pointing but it's a lot of extra work.

[edstrick]

I did some systematic experimental work on partial calibration of Voyager 1 wide angle images, in part to better see the Titan wide angle approach sequence. I found that there were several sets of images with identical image parameters that therefore should have had identical patterns of dark current shading.. but had dramatically different dark current shading patterns. Averaging a number of dark images in a "real world" matching set and subtracting that from a matched data image gave essentially perfect dark image subtraction down to random noise levels in the data.

It helped that I could do that in floating point real number image processing, where I had absolutely no problem handling negative data numbers, but the important thing was finding out that there must be some missing image parameter that affects dark current.

The most distinctive things to evaluate dark current matches are the absolute values of dark current at the top AND bottom of the images in what should be black-space parts of images. Both absolute values of dark current and the top-to-bottom gradient varies with imaging parameters inclulding the unidentifiable one.

[ChrisP]

...
The most distinctive things to evaluate dark current matches are the absolute values of dark current at the top AND bottom of the images in what should be black-space parts of images. Both absolute values of dark current and the top-to-bottom gradient varies with imaging parameters inclulding the unidentifiable one.

Thanks Ed. I had noticed how the dark current varies between images and that you have to pick your calibration images carefully to match. The Ring Node mentions this too in the notes about the Saturn and Uranus calibration projects. I see that the corrected Saturn images have passed peer review now.

Another suggestion I read once for removing gross distortions was to chop 10 pixels off each image edge, and to round the corners off at a radius of 480 pixels from the centre.

Cheers,
Chris.