Enceladus

General physics and astronomy discussions not directly related to Celestia
Michael Kilderry
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Post #21by Michael Kilderry » 19.01.2005, 09:04

Evil Dr Ganymede wrote:
Michael Kilderry wrote:Yes, but I never said that these are what they would truly look like. The fact that I was saying that I brung out the colours

Agh, english is your first language and you drop a clanger like that.... :(
You brought out the colours. Or enhanced them. But not "brung out", please. Ack.


Well nobody's perfect, and you know what I meant. I find that comment was pretty rude, you also having english as your first language could have been politer in telling me that it wasn't quite right.

Michael Kilderry :)
Last edited by Michael Kilderry on 26.01.2005, 08:31, edited 1 time in total.
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Evil Dr Ganymede
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Post #22by Evil Dr Ganymede » 19.01.2005, 09:19

I've recreated LF's image now - I took these images of Rhea as the red, green and blue channels respectively.

http://saturn1.jpl.nasa.gov/multimedia/ ... geID=29850
http://saturn1.jpl.nasa.gov/multimedia/ ... geID=29851
http://saturn1.jpl.nasa.gov/multimedia/ ... geID=29852

Then made space truly black by moving the bottom of each image's histogram to 110 (this removes a bit of data from near the terminator, but the rest of the data should be unaffected). And then I combined them in an RGB channel, and this is what I ended up with. I saved it as a jpg to save space, but there shouldn't be much compression visible:

Image

Looks like there is a little bit of a reddish tinge in places if you stretch the histograms, but the wispy regions are still definitely white. The "straight line" feature is quite bizarre though, especially considering that we're looking at a curved surface here...

Michael Kilderry
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Post #23by Michael Kilderry » 19.01.2005, 09:27

The wispy features on Rhea can look a bit purple if you increase saturation though.

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Evil Dr Ganymede
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Post #24by Evil Dr Ganymede » 19.01.2005, 09:42

Michael Kilderry wrote:The wispy features on Rhea can look a bit purple if you increase saturation though.


I'm sure you can make anything look purple if you fiddle with the image enough, but that doesn't necessarily MEAN anything. If I increase the saturation on my "stretched" image, I just end up with a speckly mess so I don't have a clue what you're doing to get purple wisps in yours...

Either way, I can't get a purple colour for the wisps without tweaking the image in such a way as to change the data and therefore remove all utility from it. If you're going to do that, then you may as well admit you're not doing anything scientifically useful.

(and I just realised I named the image 'colour-dione.jpg' when in fact it's Rhea. I always get those two muddled up...)

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Post #25by Michael Kilderry » 19.01.2005, 09:52

I didn't say that the purple meant anything, I was just making the point of the wisps looking slightly purple with a saturation increase. I wasn't meaning to make any deep scientific meaning.

The wispy bit that looked a bit purple was to the top of the image, stretching into the darkness, the rest still looked pretty bright still.

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Dollan
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Post #26by Dollan » 19.01.2005, 10:18

That straight line, I forget if there are any higher resolution images of that region. If not, could it be possible that it is some sort of artifact of the image? Perhaps a fault in the data? Or are we indeed looking at a feature that has shown some topography?

...John...
"To make an apple pie from scratch, you must first create the universe..."
--Carl Sagan

Michael Kilderry
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Post #27by Michael Kilderry » 19.01.2005, 11:50

I'd say it's a real feature, I think I've seen it in several images.

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Last edited by Michael Kilderry on 26.01.2005, 08:38, edited 2 times in total.
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Evil Dr Ganymede
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Post #28by Evil Dr Ganymede » 19.01.2005, 17:14

Michael Kilderry wrote:I didn't say that the purple meant anything, I was just making the point of the wisps looking slightly purple with a saturation increase. I wasn't meaning to make any deep scientific meaning.


Then why raise the point in the first place? There's nothing factual in what you suggest, you just fiddled around with the colours somehow to show things that aren't really there. Your image manipulations are very inaccurate, and given that there likely to be some people who read this thread who don't know much about image manipulation or saturn's moons it's very misleading to present them as if they're showing anything that's really there.

lostfisherman
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Post #29by lostfisherman » 19.01.2005, 22:42

The pictures I posted were meant as a general interpretaton of colour, or marked lack of it, as perceived by the human eye. They are I think generally realistic, but were made from uncalibrated and compressed pictures and compressed again to post; they should not be treated as rigourous scientific data.

Michael Kilderry,
It is telling that the colours that you "brung out" (I love you for that phrase :) ) are primary or secondary colours, green and magenta and blue, and that they mostly occur near the limb or terminater. If I saw these while putting the original RGB channels together I would have been sure that either the red, green or blue channel was over exposed or taken at a time when Cassinis velocity altered the view of the moon. What you appear to have done is highly exaggerate flaws in my pictures, not resolve real colour information from them. You are just having a bit of fun with it, like me, or I hope you are, but I don't think Mimas will have anything green about the place, sorry.

Evil Doctor

Just to let you know I used the same raw images of Rhea as you, I did nothing to them but convert them to lossless *.pic, so that I could use them with Iris. Guess I should get used to using histograms, your picture does look more natural. I do see a progression to a browny grey when using gamma correction for both images, but it might not mean much. As a point of interest I ran eyedropper over the 3 original raw images and it might be the case that the empty black space in the red one (N00026545) is less over exposed than the green or blue, by one or two points of RGB. Could it be made redder? Something to look into perhaps, I'm a bit tired though...
Regards, Losty

Michael Kilderry
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Post #30by Michael Kilderry » 20.01.2005, 01:38

lostfisherman wrote:Michael Kilderry,
It is telling that the colours that you "brung out" (I love you for that phrase :) )


Thanks Lostfisherman, after Evil Dr Ganymede wasn't quite so polite about the way he notified me about my writing error, that makes me feel a lot better. :D

Evil Dr,

I have already written down in my post with the false colour images that they are not how the human eye would percieve them, and if you think that I should make sure this is said because the average person who doesn't know better might think they're real, then you shouldn't be impolite to people who make a slight error in their english because they didn't know better.

Now may we please move on from this little argument, I'm over it. Forgive and forget.

Michael Kilderry :)
Last edited by Michael Kilderry on 26.01.2005, 08:34, edited 1 time in total.
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Matt McIrvin
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Post #31by Matt McIrvin » 20.01.2005, 03:56

lostfisherman wrote:It is telling that the colours that you "brung out" (I love you for that phrase :) ) are primary or secondary colours, green and magenta and blue, and that they mostly occur near the limb or terminater. If I saw these while putting the original RGB channels together I would have been sure that either the red, green or blue channel was over exposed or taken at a time when Cassinis velocity altered the view of the moon.


Another problem to think about is sub-pixel registration error. Usually, when I superimpose Cassini pictures in this way to create color images, I have to shove the layers around by a pixel or two to get them to register properly, and even that may not be sufficient. For one thing, the spacecraft's position could have changed enough between exposures to produce significantly different perspectives (this happened with the closest Dione flyby), so that aligning one part of the image will misregister another.

A more common problem is that the channels are actually shifted relative to one another by a fractional number of pixels. Getting it to the nearest pixel will be good enough to prevent bad color fringing from happening at the terminator or horizon, but if the brightness is varying over the whole surface, the misregistration will in general produce an extremely subtle color gradient, since it causes the components to vary slightly differently. If the moon is pretty gray to begin with and you then jack the saturation way up, most of the colors you'll be likely to see will just be from these artificial gradients produced by sub-pixel registration errors.

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Post #32by Matt McIrvin » 20.01.2005, 04:04

Dollan wrote:That straight line, I forget if there are any higher resolution images of that region. If not, could it be possible that it is some sort of artifact of the image? Perhaps a fault in the data? Or are we indeed looking at a feature that has shown some topography?


I don't know what it is, but it's real. Voyager 1 photographed it in 1980 and it was already on the USGS Rhea map. See here:

http://planetary.org/saturn/rhea.html
http://planetary.org/saturn/images/rhea_v1_801112_dist0719k_1039x1013.jpg

My best guess is that it is a crater ray. Cassini already photographed a small, bright crater with bright rays elsewhere on Rhea; perhaps there's more than one.

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Evil Dr Ganymede
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Post #33by Evil Dr Ganymede » 20.01.2005, 17:31



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