Star colours - test image

[Mac]

Photoshop CS4,
My attempt.
Total time about 5min. (repeatable)
1) Convert Image to 32bit
2) Convert Image back to 16 bit. (I used exposure and gamma method)
3)Adjust exposure and gamma to suit. +6.5 Exposure, +1.5 Gamma
4)Add adjustment layer for colour balance
5) place cursor over known neutral area, (unsaturated star or background layer) you could even use a gausian blur of 255 over the entire image and then sample that, note the adjustment, undo the blur and stick in the adjustment figures
6) Adjust mid tone using sliders, but observing the info tool, to get the CYMK value of the pixel, adjust until colour cast has been removed. -41 on top slider, -25 on middle, 0 on bottom
    (you only need to ever adjust two sliders)
7)add brightness and contrast adjustment layer, and adjust to suit, +20 on both brightness and contrast.

This is my result, which would now be my starting point for further adjustments.



Mac. 

[Mike]

This is a really interesting test image Mark and is great for testing out the stretching functions in image processing software to see what damage they do and also how to do it correctly. If you get it right with the test image  then it theory the same settings should also work for the astro images. It makes me wonder what kind of test images could be created for other parts of the imaging process.

[MarkS]

This is a really interesting test image Mark and is great for testing out the stretching functions in image processing software to see what damage they do and also how to do it correctly.

That's right - it gives a good understanding of what is really going on.

If you get it right with the test image  then it theory the same settings should also work for the astro images.

Yes - but maybe with some minor tweaking

It makes me wonder what kind of test images could be created for other parts of the imaging process.

Interesting idea.

[MarkS]

If you're interested, here's the orginal paper "Preparing RGB images from CCD data" by Lupton et al which advocates using scaling functions that adjust the "intensity" of each pixel whilst preserving the R:G:B ratio (i.e. the "colour")
http://arxiv.org/pdf/astro-ph/0312483v1.pdf

It doesn't have to be asinh - any kind of curve would do, in fact.  Trouble is all "curves" functions in Photoshop, PixInsight etc seem to destroy those ratios - or am I just using them wrongly?  It certainly ought to be possible to apply a "curves" function to "intensity" whilst preserving the pixel's R:G:B ratio.

Mathematically it's not difficult ...

[Edit:  By the way, this isn't restricted to RGB images - it applies equally to "Hubble palette" narrowband images]

[The Thing]

It certainly ought to be possible to apply a "curves" function to "intensity" whilst preserving the pixel's R:G:B ratio.

I have seen selections in curves tools to allow only the luminance to be affected. GIMP calls it Value, shame it's only an 8 bit program. Is this the same thing?

[MarkS]

I have seen selections in curves tools to allow only the luminance to be affected. GIMP calls it Value, shame it's only an 8 bit program. Is this the same thing?

It depends on how it is implemented.  That's why I created the test image - it allows you to verify what the implementation does.

[JonH]

There is a similar option in Iris I think on the contrast adjustment. Can select luminescence, RGB or the individual channels.
I haven't played with it much though.

[RobertM]

PI can do the same - luminance, RGB or R/G/B separately, in fact it's the same with most the tools.

Robert

[MarkS]

PI can do the same - luminance, RGB or R/G/B separately

When you get a chance, would you try applying a PI luminance curve.  I found it affected the RGB ratios in the test image - but I might have been doing it wrong - I'm not yet experienced enough in PI.  On the other hand it could be that "luminance" may not be the quantity we want to adjust.  It is likely that the definition of luminance changes according to the selected colour model.  But I tired changing colour model as well, without getting the results I wanted.  Which is why I finally had to resort to writing a PixelMath function that does what the Lupton paper suggests.

But if luminance curves can be made to work then they would allow far more flexibility than the asinh function.

[Ivor]

I saw FITS liberator was missing from the test group so I had a go


test_rgb by running_ivor, on Flickr

FITS Liberator will let you work on the individual channels and use an broad list of stretch functions, in this case I used arcsinh() however I think some of the others could produce interesting results. I scaled the peak level to 10K, I allowed clipping on the white level in each channel but none on black. I merged the files in PS CS4 and using levels to align the RGB channels.

I've moved the image to flickr and copied the url into the [img] tag however it doesn't come up and the image is set to visable to all what have I missed?

(Edit: The easy way to get images from Flickr into the forum is to use the BBCode option -- Rick)

[MarkS]

Ivor,

The white stars look a bit green to me - I'm not sure why ...

Mark

[Ivor]

Yes I think you're right, it's this damm screen of mine, it's really difficult to do things by eye correctly. I've looked in PS and the green channel is off to the right in the histogram, I've adjusted it now


test_rgb-II by running_ivor, on Flickr

I see someone updated my previous image how do you find the farmstatic link to the image?

(Edit: On Flikr, click "Share" then "Grab the HTML/BBCode" then the BBCode button. Select all, copy and paste. -- Rick)