Rev 230: Jan 7-23, 2016, Mimas and small satellites Options

[Bjorn Jonsson]

Rev 230 featured a fairly close flyby of Mimas. In fact this was the second closest Mimas flyby of the entire mission (the closest one was in February 2010 and resulted in great images of Herschel).

During this flyby of Mimas Cassini obtained a mosaic of Mimas' trailing hemisphere through several filters. This was possibly the most spectacular mosaic Cassini has ever obtained of Mimas because Saturnshine is especially apparent in these images. The result is beautiful images of a gibbous Mimas where the nightside is also clearly visible. Here is a four frame mosaic of images obtained at a range of ~40,000 km on January 14, 2016:



North is up and the mosaic is centered near longitude 270°. Saturnshine is very strong; it is several hundreds of times stronger at Mimas than Iapetus (where some of the best known Saturnshine images were obtained). I suspect Saturnshine is exaggerated here since the images were probably processed aboard the spacecraft to make dark pixels brighter (comparable to gamma correction) and for accurate results it is necessary to 'undo' this. But these images may actually be more similar to what the human eye would see than more 'correct' images. At least that is my experience with Voyager and Cassini images of Jupiter/Saturn compared to what I see through a telescope.

The mosaic was rather difficult to assemble due to the relatively big Cassini/Mimas relative motion and I had to do some not so pretty (to say the least) things at two locations at the limb to force a seamless match. But the result looks nice (and this was a good opportunity to at last learn to use Hugin properly). To minimize changes between adjacent images in the mosaic I used two GRN and two IR3 images (normally I'd use images obtained through a single filter for everything when doing monochrome mosaics but this was OK since Mimas isn't a very colorful object).

I may do an improved version in the future by reprojecting the images on a shape model of Mimas once PDS versions of these images are released.