Author Topic: Color is evading me  (Read 1639 times)

Offline speach

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Color is evading me
« on: 2018 October 08 13:01:02 »


NGC253 taken 6/10/18 in bortel 4. Can't seem to bring out the star forming regions, and the focus is a little off. This was the first time I'd imaged this galaxy. Any ideas would be helpful.

Offline ngc1535

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Re: Color is evading me
« Reply #1 on: 2018 October 08 13:39:31 »
I have many ideas- too many in fact. I think a little more information is necessary in order to be helpful. For example- was this by any chance a OSC (one-shot color) camera? If so, part of the issue may be the incorrect bayer matrix pattern assignment for the RGB pixels. Was this an LRGB image? If so, there may be an issue with brightness of either the color image or the luminance when you blended with LRGBCombination.

So... maybe more info from you?
-adam

Offline msmythers

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Re: Color is evading me
« Reply #2 on: 2018 October 08 14:58:33 »
I'm with adam, many questions but I still used your jpg and was able to get this with an extreme push using an extracted L as a mask and the ColorSaturation tool. It looks like the core of the galaxy is probably saturated but that is a guess since this is from a jpg.



Mike

Offline speach

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Re: Color is evading me
« Reply #3 on: 2018 October 08 15:12:29 »
I have many ideas- too many in fact. I think a little more information is necessary in order to be helpful. For example- was this by any chance a OSC (one-shot color) camera? If so, part of the issue may be the incorrect bayer matrix pattern assignment for the RGB pixels. Was this an LRGB image? If so, there may be an issue with brightness of either the color image or the luminance when you blended with LRGBCombination.

So... maybe more info from you?
-adam
Yes it was a osc, Canon 1100d modified and the correct rggb matrix was applied

Offline ngc1535

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Re: Color is evading me
« Reply #4 on: 2018 October 08 18:52:13 »
Hi again,

How many exposures?
What were your basic processing steps?

-adam

Offline speach

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Re: Color is evading me
« Reply #5 on: 2018 October 08 22:03:28 »
53 good ones @120 sec 1600ISO no darks, bias and flats, Stacking, then Dy crop, Dy background extraction, background  neutralization, color calibration, clone stamp (to remove 2 dust bunnies) stf and histogram (to make it non linear) then fiddling about trying to get a bit of color.

Offline ngc1535

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Re: Color is evading me
« Reply #6 on: 2018 October 09 00:59:23 »
Hi,

I did not see image registration in there (before stacking). I assume this was just an oversight.

1. A good thing to test- does your stacked image (using AutoSTF with unlinked channels) have some color variation that your more processed image does not? This can be a good hint that something along the way clobbered the color.
2. Perhaps there is some logic to the contrary.... but I would recommend doing the background neutralization and color calibration before DBE. Based on your steps... these are the ones that could likely lead to pale images. I would highly recommend Photometric Color Calibration (PCC)- very very good stuff that one.
3. RAW images that you get from your camera are not necessarily white balanced. You will need to apply the correct RGB ratios. PCC will figure this out for you... but depending on what you do with your choice of previews and methods for color calibration- there are some possible pitfalls that will not quite get the ratios you want (and lead to a blanched image).

-adam

 
« Last Edit: 2018 October 09 07:59:45 by ngc1535 »

Offline speach

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Re: Color is evading me
« Reply #7 on: 2018 October 09 20:51:24 »
Answers 1: very very little
             2: PCC won't solve the image

Offline msmythers

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Re: Color is evading me
« Reply #8 on: 2018 October 09 21:58:52 »
Hi

Can you upload the stacked but not processed image to one of the file storage sharing sites like Dropbox or Google drive. It would be very helpful to see the image before any type of post processing processing.



Mike

Offline Andres.Pozo

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Re: Color is evading me
« Reply #9 on: 2018 October 10 04:12:15 »
Perhaps there is some logic to the contrary.... but I would recommend doing the background neutralization and color calibration before DBE.
I am curious, why do you think that it is better to do BN and CC before DBE? In my experience color calibration works better after removing the gradients of the image.

Offline ngc1535

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Re: Color is evading me
« Reply #10 on: 2018 October 10 15:12:41 »
Perhaps there is some logic to the contrary.... but I would recommend doing the background neutralization and color calibration before DBE.
I am curious, why do you think that it is better to do BN and CC before DBE? In my experience color calibration works better after removing the gradients of the image.

Andres,

BN and CC attempt to work from a reference of some sort (as does PCC using a star catalog). The idea is to take an image and remove offsets due to the differing sky brightness in each channel and then applying coefficients to each channel (globally, across the entire image, every pixel). So for BN you choose a bit of sky (a reference) and apply non-local (global). Same is true for CC where the coefficients are computed. If, you select a white reference such as a galaxy- this determines your coefficients for the white balance ratios. Using bright sources (structure method) is a different method that also will find the ratios.

DBE models changes to the background (and possibly more, depending on your tolerances) based on many local samples. This model could influence the above. Lets say for a given preview location for BN- I can certainly get multiple results at this location if I do DBE first- it depends on what I did in DBE that will determine my measurement for BN. There are no constraints on the DBE model.

Now you might argue that if you do CC before DBE and you choose that little spiral galaxy in your picture-  the gradient will impact the result. This is very true. And, as I am guessing, it might lead you to say "Ah! I need to do DBE first." My response is to simply choose a method that makes the gradients small compared to the measure you are making. Structure detection is a good choice as you would expect significant structures are brighter than gradients.  Let me be clear- if the statement that gradients are not much smaller than significant structures- then I fear this is an acquisition issue (glare, bad flats...etc etc). PCC is exactly the answer by analyzing stars which not be influenced in any significant way by gradients (given the assumption above).

So, BN and CC first (as well as PCC) because the reference if valid applies globally to the image- it is a constraint. DBE is unconstrained based on the great number of free parameters (choice of samples, tolerance..etc)- so it should be applied afterwards. I basically see BN+CC as a linear operation- whereas DBE is not.

-adam

Offline speach

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Re: Color is evading me
« Reply #11 on: 2018 October 10 15:54:51 »
https://www.dropbox.com/s/xepls11yxtpdny0/integration.xisf?dl=0


Hope this works as it's the first time using dropbox

Offline msmythers

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Re: Color is evading me
« Reply #12 on: 2018 October 10 16:25:12 »
Well done. It's downloading right now!


Mike

Offline speach

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Re: Color is evading me
« Reply #13 on: 2018 October 10 16:52:24 »
Well done. It's downloading right now!


Mike

That's only had Dyn/crop applied

Offline msmythers

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Re: Color is evading me
« Reply #14 on: 2018 October 10 17:19:11 »
No problem, that should be fine.

Now I need someone who is smart to explain what's happening in the center of this galaxy. I've done this type of quick sanity check processing on hundreds of images and have not seen this before. What I thought was a post processing clipping error with the jpg from earlier in the post is in the linear image. The steps I took are simply ABE with attention to not sampling in the galaxy or brighter stars. Then BN and CC using the normal preview method. This is where I ran into problems. When I applied an exaggerated color saturation, the center of the galaxy was the wrong color and the outer spirals were very green. I then decided to use the arms as my white reference and now the arms look more correct and the stars look fine but you can see the center is just odd and missing some green to balance to a more normal galaxy center. I've never seen this with a linear image so someone smart needs to look.

As far as overall color the image seems fine. Just a bit noisy but I've seen and taken much worse.


Mike