Rev 49 - Aug 9-Sep 14, 2007 - Iapetus I1, The only close flyby of Iapetus |
Rev 49 - Aug 9-Sep 14, 2007 - Iapetus I1, The only close flyby of Iapetus |
Sep 7 2007, 05:46 PM
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Administrator Group: Admin Posts: 5172 Joined: 4-August 05 From: Pasadena, CA, USA, Earth Member No.: 454 |
[Moderator's note: This thread contains images from the Iapetus 2007-09-10 flyby and discussion of them. It was created by splitting this thread which contains pre-flyby discussion]
Another CL-UV3-GRN-IR1 set was posted today, though Iapetus seems to have been hiding from the green and IR filters...here's my best effort at making something from the CL and UV3 images. Lots of topography on the limb! --Emily -------------------- My website - My Patreon - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
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Sep 10 2007, 04:02 PM
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Administrator Group: Admin Posts: 5172 Joined: 4-August 05 From: Pasadena, CA, USA, Earth Member No.: 454 |
Since Iapetus is tidally-locked and Cassini has spent most of the mission much closer to Saturn than to Iapetus, the saturnshine imagery is always the same hemisphere -- the one we've already seen over and over. This time around, we get a daylit look at the other side. Of course, Saturnshine imagery will always only show one hemisphere, the sub-Saturnian hemisphere, regardless of where Cassini is, because that's the only hemisphere that'll get sunlight from Saturn. I think you know this, it just wasn't clear from your post, and in fact I always forget this fact -- you'll never get any Saturnshine images of the anti-Saturnian hemisphere, because that hemisphere never sees Saturn! The point you're making about getting a daylit view primarily of the anti-Saturnian hemisphere is a good one and one I keep forgetting to make.I was struck by how lumpy this body is as seen in this overexposed image. Another thing is the 3 stars visible aren't streaks at all despite longer exposure. Cassini was obviously tracking Iapetus and the stillness of the stars suggests Iapetus wasn't moving in Cassini's windshield much at the time - more or less just growing bigger and bigger. When I was talking with Tilmann at the satellites conference about this, he expressed concerns about the quality of the Saturnshine images because of this very geometry; that Cassini would not be able to use its vaunted tracking capabilities to keep long-exposure images from getting blurry. If it's flying by, from a distance, excellent tracking can keep the same points on the surface precisely in the same pixels on the detector. But if the moon is simply growing in the field of view as you approach it directly, the points on the surface are spreading out, and there's no way to prevent some of them from moving from one pixel to the next, introducing a spreading sort of blur. So they had to keep exposures short enough that this radial smear would be less than one pixel, meaning for the LIMTOPOG001 observation, the exposure was forced to be shorter than 56 seconds. I notice that on Tilmann's website, the detailed information for each observation includes a "Radial Smear Table" for the saturnshine observations.I noticed several 'black' images. I hope these are Saturnshine shots that the automatic contrast stretch didn't handle well and not images where Iapetus got missed. The fact that they are all CL1/CL2 suggests the former may be the case. Tilmann said something to me about this, which I don't remember perfectly, but it had something to do with avoiding every-other-line-truncation issues with these images by forcing them to occupy a smaller part of the histogram than they otherwise might, so he predicted that many of them would show up on the raw page as black images, annoying people.--Emily -------------------- My website - My Patreon - @elakdawalla on Twitter - Please support unmannedspaceflight.com by donating here.
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