Beautiful mosaic from the Enceladus encounter
Posted by Emily Lakdawalla
19-08-2008 10:06 CDT
In an amazingly quick piece of work, the imaging team has already assembled the 8-frame mosaic captured by Cassini as it receded from its close encounter with Enceladus last week. This is a really difficult piece of work. There are 32 separate camera images in this mosaic -- 8 high-resolution images taken through a clear filter to get the detail, and 24 taken at half the resolution through ultraviolet, green, and infrared filters to get the color.Now, the rovers take mosaics of many more images than this all the time; in that context, 32 images doesn't seem like all that much data. But when the rovers take 32 images for a panorama, they're usually doing it from what amounts to a tripod with a pan-and-tilt mount with very accurately known geometrical properties. Take one set of pictures, pan by N degrees, take the next set, tilt by N degrees, take the next set, and so on. Each multi-filter set taken at one "pointing" overlaps perfectly, without any notable registration error.
Cassini, on the other hand, was flying quickly away from Enceladus while Enceladus was moving in its own orbit around Saturn. So, to begin with, all of those multi-filter sets of images aren't correctly registered. For example, the imaging sequence started with a clear-filter photo taken when Cassini was 4,742 kilometers from Enceladus. Then it rotated the ultraviolet filter into place and snapped the next picture, but by then it was 5,800 kilometers from Enceladus. The different distance means that the ultraviolet image is about 35 kilometers across, while the clear image covers much less area, less than 29 kilometers across. To build each color frame, the component color images had to be resized and coaligned. Then that low-resolution color data had to be overlaid on each clear-filter image.
When it came time to assemble the eight color images into a single mosaic, the rotation of Enceladus also had to be taken into account. To do that, it wasn't enough just to stretch the images; the geometric distortion caused by the rotation of the moon during imaging would really create a warped image. So each of the eight frames had to be tagged with geometric information about where each of its corners were, and where the spacecraft was when it captured the images. Then all the images were stretched -- the official terminology is "transformed" or "reprojected" -- to how they would appear to a viewer located a very long distance above Enceladus at 63.0 degrees south latitude, 281.3 degrees west longitude. (Technically, this is called an "orthographic projection" -- it assumes the point of view of a viewer at infinity.) Once they were transformed to a common map projection, they could finally be assembled into a finished mosaic.
It's difficult work and amazing it was done so quickly! Kudos to Jason Perry.
If you'd like to mess about with the images, I've completed updating my Enceladus flyby raw images page. There was an error in the original map showing the footprint locations for this eight-frame mosaic, which I've now fixed.
One last comment about this mosaic: it is in enhanced color, which helps you see the true color differences from place to place, but doesn't accurately show how Enceladus would appear to the human eye. Here is one amateur's attempt to show how Enceladus would appear if you were there.
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