[X] My Assistant

[ugordan]

I've made some progress with the IR channel on VIMS, have managed to do partial calibration of the cubes. It's still buggy, but at least it's something. Here's a couple of rough results, all using R=5um, G=2um, B=1.26um.


In case of Saturn the green and blue channels were scaled up to match the intense thermal radiation. The cubes were flatfielded, quantum efficiency corrected and divided by solar spectrum, though I don't know if all the steps were carried out correctly. Dark current subtraction remains to be done - it's possibly the reason why the lower left Titan insets have banding in the red channel. I'm actually pretty satisfied with the way these two turned out, they are reminiscent of official VIMS surface composites, including the "blue" material.

Hopefully I'll be able to put something out to convert cubes to PNGs before long. A better solution would be a GUI but that's the tedious part for me.

[remcook]

I assume it would be relatively straightforward to repeat the recent analysis of Adamkovics et al. from telescope data and show the morning drizzle. As far as I can see it's just subtracting images between 2.060-2.070 um (an average might be good enough, maybe) from the same image between 2.027-2.037 um times 0.72 (or some factor close to that depending on the exact shape of the filter used). Although, as is common with Science and Nature papers, it is not quite clear (to me) what they did exactly.

[ugordan]

I don't have access to that paper to really know what they did. Even if that were the case, I wonder if a repeat would be straightforward as I get the impression they were looking at very specific wavelengths on the edge of a methane window and whether or not VIMS' discrete wavelengths would fit the bill. The closest channels are 166 at 2.03424 microns and 168 at 2.06757 microns.

I did try some funky ratios between spectral windows, below are some composites. The left-hand sides show R=5 microns, G=2.7 microns, B=2 microns. The right sides are all ratio composites:
R=5 microns/2 microns
G=5 microns/2.7 microns
B=2.78 microns/2.7 microns & contrast stretched.

The upper image is one of the longest IR exposures, while the lower one is more typical of distant shots so it's more noisy.


In the 5/2 ratio image the contrast between dark equatorial stuff and the rest is practically lost. Tui Regio and Hotei Arcus consistently stand out. Intriguing.

Things at Titan at small scales really start to look strange if you take a ratio of 2.7/2.78 microns. Quivira, for example, looks completely different:


Top left: normal 2 micron reflectance
Top right: 2.7/2.78 micron ratio, heavily contrast stretched
Bottom left composite: R=ratio, B=2 micron, G=( R+B )/2
Bottom right composite: Classic 5 micron (red), 2 micron (green), 1.26 micron (blue, slightly overexposed in raw data)