Cassini image database & updates, And notifications of PDS data releases |
Cassini image database & updates, And notifications of PDS data releases |
Dec 25 2006, 08:52 PM
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IMG to PNG GOD Group: Moderator Posts: 2254 Joined: 19-February 04 From: Near fire and ice Member No.: 38 |
I have noticed that the January 1, 2007 Cassini PDS release is already available, see http://pds-imaging.jpl.nasa.gov/data/cassi...assini_orbiter/
I haven't downloaded all of the imaging data and I'm just beginning to digest what I already have but there are some beautiful images of Enceladus' plumes and satellite mutual events, images of the G and D rings and interesting images of Saturn's nightside and fairly good Iapetus images. Radar and VIMS is also available. All in all a nice 'Christmas gift' . |
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Aug 3 2007, 06:36 PM
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Senior Member Group: Members Posts: 3648 Joined: 1-October 05 From: Croatia Member No.: 523 |
I was intrigued by some fairly big brightness differences between different filters in Emily's page. For example the UV images turn out much darker than when I throw them through my software. I decided to do a comparison of narrow angle frames N1507738546_2 (RED,CL2), N1507738663_2 (CL1,GRN) and N1507738491_2 (BL1,CL2). Below are simple RGB composites, modified only to resize and register the three filters:
The left image uses IMG2PNG converted data, the right one is my calibration result. I didn't touch the channel balance in the left case, there are a couple of notes about my results: I do all the calibration steps except dark current removal. In practice this doesn't amount to anything than a slightly brighter background signal in the longer exposed images (blue and especially UV in NAC and violet in WAC case) and is in no way capable of such a drastic difference. If you really kick up the saturation in my image, you'll see the bluish hue likely due to dark current. Other than that, I use the official calibration "fudge factors" found on COISS_0011 with a slight modification for RED/CL2 filters. Early on I realized the red channel turned out consistently too bright with those factors so I worked out it needed to be multplied by 0.949 to bring it in line with other filters. The official calibration factors might indeed be off since they're quite old as a contact from PDS emailed me, in fact the whole calibration procedure was greatly improved since then. I don't have too much faith in other infrared filter factors either. Such factors weren't even derived for WAC and are missing from the calibration volume. The color of Dione's surface in the left case suggests to me it shows the total radiance as seen by the instrument. It reminds me of my VIMS experiments without dividing by solar spectrum as sunlight is actually yellowish. In my calibration procedure the filter transmissivities are divided by integrated solar spectrum and produce reflectance (I hope it's the right term) instead, the intrinsic color of the moon if you will. This is consistent in that it produces gray Mimas, Enceladus and gray non-stained hemispheres of Dione and Tethys. In essence the color they'd appear if shined upon by a big, white light lamp. -------------------- |
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