Rev 139 - Oct 4-28, 2010, Titan, Mimas, Pallene, Dione and Rhea Options

[charborob]

Nice shot of Titan and Tethys (?): http://saturn.jpl.nasa.gov/multimedia/imag...4/N00164575.jpg
Part of a sequence that would make a nice animation (hint, hint).
Titan is darker than usual on this image. What would be the reason for that?

[ugordan]

Part of a sequence that would make a nice animation (hint, hint).

One of the Cassini Scientist for a Day observations, though I'll admit I was expecting to see Enceladus in the last frames.

Titan is darker than usual on this image. What would be the reason for that?

Tethys is about the same brightness in all 3 filters while Titan darkens with shorter wavelengths. Also at this phase angle ice is still significantly brighter than the atmosphere. At high phase angles the situation would be reversed.

[Juramike]

HiPassLRGB image of Dione taken October 18, 2010. HiPass and Luminance layers both used the BLU filter channel. Colors tweaked to get "close" to Planetary Photojournal image PIA07744 and some of Gordan's images.

[pat]

Nice shot of Titan and Tethys (?): http://saturn.jpl.nasa.gov/multimedia/imag...4/N00164575.jpg
Part of a sequence that would make a nice animation (hint, hint).
Titan is darker than usual on this image. What would be the reason for that?

The images are exposed for Tethys/Enceladus rather than Titan. As ugordon has said these are part of a "Scientist for a Day" observation, the main concern was to make sure Tethys/Enceladus didn't saturate.

[pat]

One of the Cassini Scientist for a Day observations, though I'll admit I was expecting to see Enceladus in the last frames.

The full observation doesn't seem to have hit the ground yet, there are only 29 frames out of a planned 65. I'm hoping the rest of the data is in Thursday's downlink. It would be a shame if it arrived during the ~25 min power outage at the DSN dish that was being used for yesterday's downlink -- if thats the case its gone, never to be seen.

[ugordan]

Well, 30-ish frames in 25 mins sounds about right... One can still hope.

[pat]

The images are exposed for Tethys/Enceladus rather than Titan. As ugordon has said these are part of a "Scientist for a Day" observation, the main concern was to make sure Tethys/Enceladus didn't saturate.

Actually I've dug deeper and the "dark" Titan is mainly down to another cause. Its the automatic compression applied to the raw images again. While Titan is the only thing in the frame the image gets scaled using the brightest Titan pixel as the reference, as soon as Tethys enters the frame its the brightest Tethys pixel thats the top reference. Also this whole observation was done in 12 rather than 8-bit which makes the difference even more dramatic. So you start with an apparently bright Titan and then Titan is much darker as soon as Tethys enters the frame (Tethys being significantly brighter than Titan).

[ugordan]

Also this whole observation was done in 12 rather than 8-bit which makes the difference even more dramatic.

Ahh, so the entire observation (including the CB3 with the awesome cloud) is linear, no LUT encoding? Useful bit of info, thanks

[ugordan]

Mid-transit composite. Titan's disc is actually from earlier on, when there was no Tethys in the frame so better s/n ratio in the jpegs was available.



No alien bases were harmed during production of this image.


A ratio of CB3/BL1 frames:


I wonder how familiar a sight it would be to stand there on Titan's surface and look up at those clouds.

[elakdawalla]

Gordan, when I look at the frames from earlier in the sequence you can actually see that huge band of clouds in the red filter image and even, to a limited extent, in the green. If you used those images to make the color for Titan, why isn't the band of clouds showing up?

[Phil Stooke]

The brightest horizontal stripe does appear to be there, it's just so faint due to the low contrast.

(EDIT-yes indeed, if you do a high pass filter and a contrast stretch of that the clouds are there)

Phil

[ugordan]

Primarily because those raw images are contrast-enhanced due to you viewing them on a sRGB screen and they were uncorrected for that. The effect is most pronounced when Cassini returns 12 bit DNs without LUT encoding. That's what Pat was talking about. The 8bit LUT encoding employs a square-root encoding which is nonlinear and is somewhere inbetween 12 bit and gamma-correct nonlinearity of the sRGB colorspace. Had the same images been encoded onboard using the LUT table, you'd see them as having lower contrast in these raws.

I merely applied a gamma correction, knowing the source raws were in fact linearly encoded (so similar to what you'd get with calibrated I/F data). Had I not done so, this is what the result would look like:

[elakdawalla]

Thanks for the explanation -- it's funny what seems so prominent in one filter just goes away (from view with one's own eyes anyway) once you've done the color processing. In a way, an argument for why spacecraft need this monochrome detector/filter wheel arrangement to reveal what's going on.

[ugordan]

Yes, the clouds are fairly inconspicuous even in that second version both because Titan is darker than Tethys and also because most of the visible energy picked up by the eye is in the green range, and that is where the clouds already start to fade from view.

[ngunn]

this is what the result would look like:

Why is it that I actually prefer that one? It's a subjecive thing I suppose. It makes me feel present in a way that the other version doesn't and I have no idea why. Anyhow I just want to congratulate and thank everybody on the Cassini team including the scientist for the day for these wonderful views of the changing weather.