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Enceladus PDS image products
FordPrefect
post Mar 13 2012, 01:01 AM
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QUOTE (Bjorn Jonsson @ Mar 5 2012, 11:10 PM) *
I'm using a slightly modified version of the earliest Hapke function - it's modified to avoid unrealistic effects when the emission angle approaches 90 degrees. The phase effects are interesting and very strong. I even had to reduce the opposition effect a bit to avoid problems with dynamic range.


Outstanding video Bjorn! Amazing! smile.gif

I loved the phase-angle effects too. Could you elaborate on the modified version of the earliest Hapke function or point to where that function can be found? Those zero-phase glares are very prominent on the Moon/lunar surface too, but I suspect generally on any body with rough surfaces.
Thank you very much for any pointers!

Edit: Is this what you've been working with? -> http://selena.sai.msu.ru/Pug/Publications/ms42/m42_60.pdf

Rafael
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Bjorn Jonsson
post Mar 17 2012, 12:47 AM
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The modified Hapke function I'm using can be found in my software and (probably) nowhere else ;-). What I'm doing is a crude (probably), simple and empirical modification: I'm preventing the emission angle from ever getting 'too close' to 90 degrees by multiplying it with a number that is a slightly lower than 1 once the emission angle exceeds ~80 degrees. This number is actually a function of the emission angle and gets a bit lower as the emission angle approaches 90 degrees. This may seem strange but since the patch of surface within a pixel really is never perfectly smooth the average emission angle of the visible 'facets' within the pixel should never get extremely close to 90 degrees. This is simpler (but also less accurate) than the more complicated forms of the Hapke functions and eliminates unrealistic bright 'rims' around some terrain edges or planetary discs.
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FordPrefect
post Mar 17 2012, 01:58 PM
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Thank you very much Bjorn for the insight!
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Bjorn Jonsson
post Nov 5 2017, 08:10 PM
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On November 27, 2016 Cassini at last obtained really good images of Enceladus' north pole during a nontargeted flyby. This is a mosaic of two IR3-GRN-UV3 color composites processed to show Enceladus in approximately natural color and in greatly exaggerated color which reveals compositional variations. A version with a latitude/longitude grid is also included.

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The images comprising the mosaic were obtained at a range of 62,000 to 72,000 km. ISIS3 (qnet/jigsaw) was used to correct the camera pointing. I then reprojected everything to simple cylindrical projection and rendered the images using software I wrote. The final step was to use Photoshop to process the color.

The highest resolution view of the north pole obtained during this flyby is a clear filter image at a range of 32,000 km. The resolution is 190 m/pixel:

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