[X] My Assistant

[Bjorn Jonsson]

This may be a bit specialized to say the least but if anyone can help I'd be grateful. I have now downloaded most of the raw Cassini images from the PDS imaging node plus the coiss_0011 volume from the PDS rings node. Volume coiss_0011 contains calibration stuff like flatfields, masks for removing dust rings and mottling etc.

I have been implementing calibration code which will eventually be added to my IMG2PNG utility, assuming there is interest in this. This utility is mentioned in this thread: http://www.unmannedspaceflight.com/index.p...wtopic=291&st=0 .

My code has been largely based on the CISSCAL source code on volume coiss_0011 but cleaned up a bit and rewritten using C++. This has been very successful despite a wide variety of rather arcane data formats on the coiss_0011 volume (bigendians, littleendians, IEEE and VAX floating point numbers, a weird TIFF file containing floating point data etc.). For example, flatfielding, linearization etc. works very well and in particular, for images of low contrast targets like Saturn and Titan makes a big difference if the images are to be greatly sharpened (no mottling or dustrings).

There is one problem though: Darkcurrent subtraction. This is a problem, the CISSCAL code for this is somewhat complex and may contain some bugs - what it does is to generate 'synthetic' darkcurrent images based on a model taking various factors into account. These 'synthetic' darkcurrent images get written to disk so each of them only has to be generated once. If anyone has 'sample' synthetic darkcurrent images I would greatly appreciate if at least one of these could be emailed to me since without this it is almost impossible for me to know if the images my code is generating are correct.

These images should (assuming my code is correct !) have names like \calib\darkcurrent\darks04228\n_dark027102410100_04228.img where '\calib' is in my case the calib-directory on the coiss_0011 DVD volume (which is lying on my hard disk).

Bjorn

[ugordan]

Can anyone help me on the whole calibration/radiometric correction topic? Is there a guide on the net on how one creates "true" color images from RGB filters (or those similar to them, e.g. in Cassini's case R -> CB1)? I'm basically a n00b when it comes to image processing at the level of calculating radiances and stuff. I'm well aware that the images the spacecraft takes are usually setup so that each exposure maximizes the S/N for that filter combination so, obviously, stacking three channels together doesn't produce the right results.

For starters, I don't want to do all the fancy stuff like flatfielding, de-banding, dark current removal etc, but I do 8to12 bit conversion when necessary as well as bias subtraction.

I've come across a table at the end of Cassini ISS description pdf, it lists values called omega for a given filter combo for both cameras and it's supposed to describe the amount of light each filter passes through. So I've tried compositing images by normalizing exposure_duration * omega for all three filters. The results actually look pretty good when I use this with Titan WAC RGB/RGV frames, I get the same colors as public ISS images turn out. With the NAC, however, things are different - I always end up with an underexposed blue channel and the whole image becomes too orange-reddish. I don't know if there's any more corrections I am missing like quantum efficiencies of the CCD or whether they're already incorporated into omega values.

Like I said, I'm not aiming for very scientifically accurate images, only those that I can say look close to the real thing. Any help would be appreciated...

[Bjorn Jonsson]

I have written a program that calibrates Cassini images, including flatfielding which is very important for low-contrast targets like Saturn and Titan. I will probably make it available soon - I have been too swamped in Cassini images recently to do so.

An underexposed blue channel is a familiar problem. Also in many cases it is necessary to use synthetic color, e.g. synthetic green from CB1 and blue or synthetic CB1 from CB2 and green (or even blue).

[ugordan]

I always thought the following press image was too weird:

It's said to be taken through a RGB combination, but the colors look all but normal. Even if I try to stretch the blue channel in my composites, that still gives me a weird blue fringe in other areas which are naturally bluer (atmospheric limb, etc.). It's as though some of the filters have a gamma function applied when I compare my images to the CICLOPS ones.
As for the blue channel underexposure, I don't really understand it - it should cancel out by a much longer exposure that the BL1 images are usually taken, but it doesn't.

Does it really come down to mixing various filters (in a sense that the R channel in Photoshop is not just RED/CL2 but a mix of both RED and GRN filters?) in sRGB colorspace to get a "true" color image? I'm lost...

[Bjorn Jonsson]

Actually "underexposed" is not what's happening in the blue filtered images - when calibrating the final result looks identical regardless of exposure time (at least in theory).

As for synthetic color, yes you do something like R=a x CB2 + (1-a) x G when using CB2 and green to get synthetic red.

[ugordan]

Actually "underexposed" is not what's happening in the blue filtered images - when calibrating the final result looks identical regardless of exposure time (at least in theory).

That would be my reasoning also. I gather the only difference with varying exposures would be in the final calibrated image S/N ratio and whether there would be overexposed areas in the image.
I still don't understand why, then, the BL1 channel turns out too dark when I do the above normalization. Is there an additional factor that needs to be taken into account? Perhaps a correction for the bandpass of the filter combo? IIRC, one of the blue NAC filters has a pretty narrower bandpass than the other.
Is there actually ANY deterministic way to produce a true color image from the RED/GRN/BL1(BL2) images or is it in the end just guesswork and finding out what looks right?

[ugordan]

I've been experimenting with "deterministic" ways to produce true color images. I've also been playing around with procedures to calibrate the images, adding flatfielding to my rudimentary converter.
My problem is, using NAC RED/GRN/BL1 filters and integrating the trasmittance with QE and solar spectrum, I wind up with composite images that tend to underexpose the blue channel. If I try correcting for the Sun's spectrum, I get results which, for Saturn, give more "natural" looking images, but make the icy moons look too blue.

I've attached two sample images (they weren't taken at the same time, but were both taken through CB1/GRN/BL1 filters)

The left image is the result when the formula cisscal uses is applied. Also it's flatfield-corrected. The right one is using a correction for the fact the Sun emits less in the blue end and it's not corrected for the flatfield (note the dust ring on the rings).

Which of the two would you say looks more like true color?

[Bjorn Jonsson]

I've been experimenting with "deterministic" ways to produce true color images. I've also been playing around with procedures to calibrate the images, adding flatfielding to my rudimentary converter.
My problem is, using NAC RED/GRN/BL1 filters...

I have also been experimenting with calibrated color images using CB1/GRN/BL1 - I assume that by RED you mean the red filter and not CB1?

The color I get looks somewhat different from yours, possibly because I'm using CB1 and not RED. Some time ago when I was processing images of Jupiter I had to add 'fudge factors' to get realistic color (the main problem was the dark BL1 image). Interestingly, after applying these same fudge factors to a color image of Saturn the color becomes very similar to your left image.

[ugordan]

I have also been experimenting with calibrated color images using CB1/GRN/BL1 - I assume that by RED you mean the red filter and not CB1?

The color I get looks somewhat different from yours, possibly because I'm using CB1 and not RED. Some time ago when I was processing images of Jupiter I had to add 'fudge factors' to get realistic color (the main problem was the dark BL1 image). Interestingly, after applying these same fudge factors to a color image of Saturn the color becomes very similar to your left image.

You're right, the above image was taken through the CB1 filter. There aren't that many RED images taken by NAC as yo've noticed a while ago. I've found a case where both a RED and CB1 filtered images of Saturn (the same view) exist and compared them, they indeed turn out very similar, the CB1 being only slightly brighter and the "blue cranium" is the only place noticeably brighter in CB1 than in the RED.
As I said before, I have a feeling the imaging team enhances their true color images of Saturn to make them look more bluish. If you take a look at the small moons (Mimas and Enceladus) often visible in those shots, they're colored distinctly blue, while the shots of them alone are obviously predominantly whitish or grayish in Mimas' case.
Perhaps volcanopele could shed some light on the "true" color issue of Saturn?

[Bjorn Jonsson]

If you haven't done so you should take a look at this HST image of Saturn:

http://heritage.stsci.edu/1998/29/index.html

This is by far the most realistic spacecraft image of Saturn that I have seen, based on what Saturn looks like when seen visually through a telescope but even so this is not a simple problem. The problem is that even though I have seen Saturn through a fairly big telescope I'm not sure exactly what it would look like from a distance of a few million km. The color saturation increases with increasing telescope size with Saturn becoming more yellowish (instead of whitish) when using a big scope.