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: 2254 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, 08:01 AM
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Newbie Group: Members Posts: 19 Joined: 17-June 07 From: Near Southampton, UK Member No.: 2430 |
All right, it's all downloaded, and I've now posted all the rings and moons images. http://planetary.org/explore/topics/voyager/rawdata.html --Emily I noticed that on this page you have links to the Voyager camera calibration data. I discovered this a couple of years ago and made a spreadsheet to show how the different filters for each colour fit together. I used the *.TAB files at the Ring Node. One thing you will notice is that the centre frequency and bandwidth of the filters are altered from their nominal values by the response of the optical path and the vidicon tube. Another thing to notice is that the response of the orange filter is completely contained within the green filter. So if you were composing a colour composite image using the green and orange pixel values, you would be counting the orange values twice, hence the green minus orange curve. Some double accounting also occurs between green and blue and between blue and violet, but the correction is less obvious. All this and more is in my spreadsheet. I made it using Gnumeric as Excel wouldn't quite do what I wanted. (Mmm, UMSF doesn't like Gnumeric spreadsheets for some reason, and the .xml version doesn't look right.) What you're missing is an approximation to how the human eye would see each filter colour, the RGB ratio for each filter and the area under each curve in the graphs above. These should help in constructing colour images from either the raw data with an appropriate calibration image subtracted, or from the calibrated or geometrically corrected images published by the Ring Node. Some scaling of pixel values is needed to correct for different exposure times, filter response (area under curve) and to avoid saturation. Cheers, Chris |
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