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Kraken Mare through polarisers, Cassini looks for lake shine
ngunn
post Oct 29 2014, 09:12 PM
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On visual inspection I can't say I recognise lake outlines there, but that's not too surprising given that the filters chosen for those images were optimised for atmospheric study rather than for surface features. Also the image orientation was rather unfortunate, putting most of the lake surfaces near 45 degrees from each polariser. But I'm thrilled that somebody has finally had a go at this, so thank you very much!! smile.gif smile.gif I will continue to look for suitable image pairs for this treatment. (There are a few further up the thread that might be worth a try.)

I would like to know one thing about the subtraction process you used. Does it preserve the sign of the differences or just their magnitudes? That would greatly affect what I'd be looking for in any future attempts.
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ngunn
post Oct 29 2014, 10:17 PM
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For anyone puzzled by this off-beat thread here's a short rationale. I'm looking for shiny surfaces. All around you, anywhere, day or night, indooors or out, you have no difficulty identifying shiny surfaces. Some things look matt, others a bit more burnished and others are clearly mirror-like. I'm looking for an image from Cassini that shows the surface of Titan in this way. I want to say to my students "Look, there's a shiny surface. Its obviously wet". The scientists already know its wet and have other priorities so images that serve my purpose will arrive only fortuitously if at all.
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Greenish
post Oct 29 2014, 10:56 PM
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Here's my take, FWIW:
0. This is useless and invalid if they adjust the contrast on the web jpgs differently for each image. I don't know if this is done for Cassini or not.
1. I registered the images using translation only. I have no idea how much the camera moved or if it rotated between frames, but I assumed the size & orientation were the same between pics. May or may not be accurate.
2. I then converted to floating-point values & subtracted W00090641 from W00090640. I contrast-scaled the result symmetrically about zero. Black shows negative & white positive. Beware blockiness & other compression artifacts in the original JPGs.

Attached Image


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ngunn
post Oct 30 2014, 10:01 AM
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Positive and negative are distributed as expected there, though the signal is clearly from the atmosphere in this case. That's interesting in itself and a nice demonstration of the method so, again, thanks for giving it a go.
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ugordan
post Oct 30 2014, 11:50 AM
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QUOTE (Greenish @ Oct 29 2014, 11:56 PM) *
This is useless and invalid if they adjust the contrast on the web jpgs differently for each image. I don't know if this is done for Cassini or not.

It is done. Doing any kind of manipulation that aims to bring out very subtle differences seen through separate filters is a futile effort, IMHO. Raw jpegs are contrast-stretched and clobbered by compression artifacts and loss of dynamic range.


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nprev
post Oct 31 2014, 08:42 AM
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Incredible mosaic with specular reflection from Kraken Mare:

http://www.jpl.nasa.gov/spaceimages/details.php?id=PIA18432


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A few will take this knowledge and use this power of a dream realized as a force for change, an impetus for further discovery to make less ancient dreams real.
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FOV
post Oct 31 2014, 02:45 PM
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I agree nprev, wonderful image(s) of sun glint on Kraken Mare.
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ngunn
post Oct 31 2014, 03:35 PM
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Spectacular, wonderful, so beautiful in the chosen colour scheme - and definitely shiny!! Words could never do it justice. What a journey we have been on in the few short years since Kraken Mare itself was first glimpsed.

EDIT: From the image caption -This particular sunglint was so bright as to saturate the detector of Cassini's Visual and Infrared Mapping Spectrometer (VIMS) instrument, which captures the view. . . . . Because it was so bright, this glint was visible through the haze at much lower wavelengths than before, down to 1.3 microns.

Reading this makes me wonder if the specular reflection of the sun is so bright that it allows Cassini VIMS to 'see' beyond the normal limits of the methane windows for Titan. A less restricted infrared spectrum could potentially yield a wealth of new information about the atmosphere and maybe the surface too, at least the wet bits.

MOD NOTE: Dedicated thread for this particular image now here.
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ngunn
post Sep 21 2019, 10:18 AM
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Please excuse the old thread revisited but it's been more than a decade since I started enquiring about this topic. I am delighted to see someone taking a proper look at it now.

https://meetingorganizer.copernicus.org/EPS...2019-1900-2.pdf
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