Voyager 2 Saturn Revisited, Still a lot to be processed and reprocessed |
Voyager 2 Saturn Revisited, Still a lot to be processed and reprocessed |
Jan 20 2007, 02:36 AM
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IMG to PNG GOD Group: Moderator Posts: 2256 Joined: 19-February 04 From: Near fire and ice Member No.: 38 |
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. |
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Jun 26 2007, 04:26 PM
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Administrator Group: Admin Posts: 5172 Joined: 4-August 05 From: Pasadena, CA, USA, Earth Member No.: 454 |
All right, it's all downloaded, and I've now posted all the rings and moons images. As for Saturn -- there's way too many to post. I'll have to consider what to do with those. I'll probably post a subset of them at some point, but I think I may wait until they've finished going through peer review. Apparently there's still some pretty serious problems with their dark current subtraction, which results in a goodly fraction of the images being badly calibrated, so they have to work on that.
http://planetary.org/explore/topics/voyager/rawdata.html --Emily -------------------- My website - My Patreon - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
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Jul 23 2007, 06:36 AM
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Newbie Group: Members Posts: 19 Joined: 17-June 07 From: Near Southampton, UK Member No.: 2430 |
.... Apparently there's still some pretty serious problems with their dark current subtraction, which results in a goodly fraction of the images being badly calibrated, so they have to work on that. http://planetary.org/explore/topics/voyager/rawdata.html --Emily Now if you've heard that from someone at the Rings Node, please ignore what follows. But if it's from looking at the background shading of some of the thumbnails published at the address above, I think that's an artifact of the processing img2png does. Consider this. The NASA processing subtracts a calibration image, applies further corrections, and scales the pixel values so that value 10000 has the REFLECTANCE_SCALING_FACTOR given in the PDS label, and the values lie in the range -32768 to +32767. Now the dark background pixels have values which cluster around zero, but sometimes there are a lot of negative ones, and sometimes very few negative values. img2png converts all negative values to zero, so if the slight shading that's there is mostly in the negative range it is lost, but if it's mostly just positive, the shading remains. The contrast adjustment that's applied before img2png creates a thumbnail then emphasizes any shading that remains in the image. This is not to say that Bjorn did the wrong thing in picking zero. PNG cannot cope with negative values (nor can any other mainstream image type I've come across for that matter) so one has to do something with them. Adding 32768 isn't an option as that would make the images and the background too bright. Choosing a value depending on the image doesn't always work either as the darker limbs and interiors of the moons may have pixel values lower than the background shading values which you want to remove. In other words, I don't have an answer to this yet. I've also noticed that if there was a reseau mark on the edge of a moon's image, you can get a mid-gray disc apparently sticking out into empty space. Cheers, Chris. |
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