Pretty pictures from Cassini's 1 May 2012 Dione flyby
Cassini had a close encounter with Saturn's geyser moon Enceladus two days ago, and followed the fun with an unusually close pass by Dione, the fourth largest moon of Saturn (after Titan, Rhea, and Iapetus). There was some nice Enceladus imaging, but the real eye candy this time came from the Dione encounter, which was reasonably close, at an altitude of about 8,000 kilometers. At that distance, Cassini's camera could get images with pixels as small as 50 meters across! In fact, Cassini did grab lots of images near its closest approach to Dione, a 3-by-3 image mosaic that I did my best to assemble here. It took some warping of images to get features to line up properly, because Cassini was moving quite fast as it flew by, and its perspective on the moon changed from image footprint to image footprint. So don't use this for science!! There's a cool fracture snaking down this mosaic -- I think that may be Latium Chasma, if I'm reading Jason Perry's article about this flyby correctly.
As Cassini flew away from this encounter, it captured a nine-frame mosaic across the visible sphere of Dione, most of this while Dione was crossing the sunlit face of Saturn, so its yellow mass sits in the background, which is very cool. This image is made of only two of those nine frames; I didn't have time to assemble the whole thing. The dark line is Saturn's rings, seen nearly edge-on and from the unlit southern side.
Toward the end of the encounter, Cassini used its wide-angle camera to capture a "movie" of Dione crossing the edge of Saturn's globe. In the middle were three frames taken through three different-color filters, so of course I had to put it together in color. I thought it'd be fun to show you the process I use. The original frames were: W00074069, W00074070, and W00074071, taken through violet, green, and infrared filters.
Here's what happens when you overlay the three frames in an RGB image. It doesn't look completely awful, but the colors don't quite line up. Since a few seconds separated the three images, Cassini's and Dione's motions around Saturn caused the apparent positions of Dione and Saturn to shift. Cassini was commanded to keep Dione still in the center of the field of view throughout the imaging sequence, so it's nearly lined up; Saturn was allowed to drift from frame to frame in the background.
Since most of the image is Saturn and space, the first thing I did was align the three channels on Saturn and its rings. Actually, I used the shadows of the rings as my clue for alignment; because they're so thin and because they bend, they make very accurate markers. What's interesting when you do this is that Saturn's limb -- the edge of its disk -- appears to shift a little from frame to frame. That's not Saturn moving; that results from the fact that Saturn's disk actually changes color as you get close to its edge. Along the edge of the disk you're seeing light being scattered in Saturn's upper atmosphere.
The next step will involve aligning the three images of Dione, cutting them out, and pasting them on top of the messed-up image of Dione in the view above. Here, I have a subjective choice to make. My aligned image of Dione is not going to cover up all of the color-fringed images of Dione in the composite I show above. I could choose to place Dione where it was when the infrared-filter image was taken, or where it was when the violet-filter image was taken, or anywhere in between. If I do the former, I'll have to paint in some of Saturn's atmosphere where the magenta and yellow images appear in the composite above. If I do the latter, I'll need to paint in black space to cover the red and yellow fringes. Since it's easier (and, in my mind, less artificial) to paint black space than it is to paint in Saturn, I chose to place the composite image of Dione where it appeared in the violet-filter image, covering up more of Saturn's disk. We're almost done. It's not perfect, but it'll do.
The last steps are mostly cosmetic. I rotated the image 180 degrees so that Dione's south pole is down. And I messed with the levels of the different color channels to make black space actually appear black, which also has the effect of making the image of Dione appear more dramatic. Finally, I painted out a few blemishes, bright pixels where charged particles struck the detector while the shutter was open. Et voilá!