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.

[ugordan]

Wow, the Voyager data is certainly ress rewarding to work with than Cassini or Galileo. I didn't realize how much more sensitive CCDs really are than vidicons, there's noise all around, even in several-second exposures. Not to mention geometric distortions and smear. Some quick-n-dirty stuff:


Curiously, there's a subtle blockiness in the noise (as well as other regions, but most noticeable there) in this geometrically calibrated data that's very reminiscent of JPEG artifacts, which is interesting. The blockiness appears to be on a 8-pixel block basis too, at first glance at least. I'm wondering if there was a lossy compression step introduced in the calibration procedure somewhere.

[tedstryk]

Wow, the Voyager data is certainly ress rewarding to work with than Cassini or Galileo. I didn't realize how much more sensitive CCDs really are than vidicons, there's noise all around, even in several-second exposures. Not to mention geometric distortions and smear.

Two things I wanted to add. First, a lot of the smear is not due to vidicon. It is due to the lack of reaction wheels and the fact that some exposures were taken with the scan platform still in motion. Also, a lot of the junk in Voyager images is due to after-image effects in the vidicons which can be compensated for. Voyager (and Viking) had lamps that would white-out the camera, making the after-image an image of a solid white frame. This prevented after images of actual celestial objects, as plagued Mariner-9 and earlier probes. However, it also means that there is a lot more background junk in Voyager images. I have found that the so called "calibrated and geometrically corrected" frames have features that appear muted and without definition. However, when I remove the noise myself, I can get a sharper result. I say this to say that there are nice, clean images hiding under a lot of the Voyager frames. For Saturn, granted, Cassini supersedes this imagery, but for Jupiter (thanks to Galileo's limited coverage), Uranus, and Neptune, pulling whatever can be pulled from these images is quite rewarding. For instance, to use a Saturnian example, here is the best view of Enceladus from Voyager from the true raw frames.

[ugordan]

That's a really nice Enceladus image. Very Cassini-like and to an untrained eye probably could be labeled as such.
I'm beggining to find a new appreciation for high quality Voyager image products such as this, seeing how ungrateful the original raw data really is.