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Voyager 2 Saturn Revisited, Still a lot to be processed and reprocessed
Bjorn Jonsson
post Jan 20 2007, 02:36 AM
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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:
Attached Image

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:
Attached Image

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:
Attached Image

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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elakdawalla
post Jan 20 2007, 03:45 AM
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This is great; I can't wait to see what you'll post next! I especially like the sinusoidally wavy stripe across the bright band in the midlatitudes in the second and third images you posted. It should be a lovely map when you're done.

--Emily


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tedstryk
post Jan 20 2007, 05:07 AM
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This is some incredible work, Bjorn. Do you know if they plan to do this with the whole Voyager dataset?

Ted


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Guest_PhilCo126_*
post Jan 20 2007, 02:19 PM
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Guests






Great work to celebrate the 30th anniversary of the Voyager launches of summer 1977 ( Voyager 2 : 20 AUG 1977 , Voyager 1: 05 SEP 1977 )
cool.gif
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elakdawalla
post Jan 20 2007, 05:36 PM
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QUOTE (Bjorn Jonsson @ Jan 19 2007, 06:36 PM) *
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.

Bjorn, I found this notice at the Rings node:
QUOTE
New: A complete set of SPICE C kernels is now available for the Voyager 1 Saturn encounter. More kernels will follow shortly.
Is this what you already had?

--Emily


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Bjorn Jonsson
post Jan 20 2007, 08:45 PM
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No, I didn't know of these - they are new. Thanks for pointing this out. This looks very promising because a subset of the images has been used to correct pointing errors in the original pointing data. The pointing information I have is IIRC typically off by something like 0.1 degrees which is huge since the field of view of the narrow angle camera is ~0.43 degrees. Also I have it only for a small subset of the images.

Since I need this for Voyager 2 at Saturn and only Voyager 1 Saturn is available the big question now is what the exact meaning of "Other C kernels will follow shortly" is wink.gif. I was going to start the 'monster map project' in February but at least I can start by checking if these new kernels are accurate enough (I strongly suspect they are).
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elakdawalla
post May 18 2007, 04:46 PM
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Hi Bjorn,

I tried downloading the zipped versions of the geometrically corrected IMG files for the Saturn encounter and running them through IMG2PNG, but there seems to be a problem; the PNG images are created, but they don't look at all correct. Here's an example image -- the first, as opened by NASAView, the second, as converted by IMG2PNG.

Is there some switch I can use to correct this, or is the current version of IMG2PNG unable to open the *.GEOMED.IMG files?

Attached Image
Attached Image


--Emily


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Bjorn Jonsson
post May 25 2007, 01:22 AM
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QUOTE (elakdawalla @ May 18 2007, 04:46 PM) *
Is there some switch I can use to correct this, or is the current version of IMG2PNG unable to open the *.GEOMED.IMG files?

I didn't notice this message until now - I will fix this in a day or two. This same problem also appeared (and has been fixed) in a bigger program I wrote and on which IMG2PNG is based so it's trivial to fix.
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elakdawalla
post May 25 2007, 03:17 PM
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Great, thanks. It's nice to know I didn't download those 1+ GB files for nothing smile.gif

--Emily


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Bjorn Jonsson
post May 26 2007, 01:46 PM
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I have now fixed this bug. The new version is available here (dowbloading the standalone executable is sufficient when upgrading). In addition to fixing this bug, it is now possible to specify the -s command line option regardless of whether you are using IMG2PNG to calibrate the input images or not (previously -s only applied when calibrating). This is useful for the Voyager images which are very dark (a value of 16 is probably appropriate).

BTW has anyone had problems with some of the the huge Voyager ZIP/GZ files from the PDS Rings Node? For the ZIP files, I typically need to manually download about 5 IMG files after unzipping because WinZIP couldn't extract them and complained they were corrupt. For the first GZ file I downloaded things are far worse because the resulting TAR files I then need to extract the IMG files from are corrupt. I'm reluctant to redownload the huge GZ files until I know of someone who has successfully used them.
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elakdawalla
post May 26 2007, 02:00 PM
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Thanks! I tried the new version on one folder of 100 Voyager images and it worked fine. I'll work more on this later and let you know if I run in to the same problems you described.

--Emily


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elakdawalla
post May 29 2007, 10:38 PM
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Just posting an update here to mention that I've downloaded, unzipped, and IMG2PNG'ed the first five VGISS GEOMED files (VGISS_0026 through 30) as well as #38 and haven't had any problems with corrupt files yet. Seven more to go...

--Emily


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elakdawalla
post Jun 4 2007, 03:28 PM
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Seems I posted too soon. I was really rolling along downloading all this data but came to a screeching halt when I found that the links to the Zip archives simply don't work for CD's 34 through 37 -- i.e. most of the Voyager 2 data. I tried downloading both using Internet Explorer and wget, and neither worked. I've sent an email to the folks at the rings node. I do have all the Voyager 1 data from Saturn, though, so before too long I should have some nice pages with all the pics of the icy satellites. There were only on the order of 100 images of each non-Titan satellite, so that's an easily manageable amount of data. Not sure what I'm going to do with all the Gigs and Gigs of Saturn images though -- I've tossed them on my backup hard drive for now.

--Emily


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Bjorn Jonsson
post Jun 4 2007, 04:11 PM
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Volumes 34-38 are there but you'll need to download them via the ZIP archives page ( http://pds-rings.seti.org/archives/ ) to get links that work.

BTW I think I now know why I had problems with some of the huge GZ files - there is a lot of software out there that cannot properly download files bigger than 2.1 GB. The download appears to succeed but the file is corrupt. Apparently Wget handles these huge files correctly so that's what I will be using from now on.
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elakdawalla
post Jun 4 2007, 04:17 PM
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I tried going to that Zip archives page but still have the same problem with wget, an "Abnormal program termination." It looks like the files have negative file sizes!

--Emily


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