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Old 06-09-2015, 12:36 PM
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vlazg (George)
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RGB ratios

Hi Spacers,

Is there a way to determine RGB ratios for colour processing?
I've tried eXcalibrator but ended up with some weird ratios
Eg RGB 1:1.48:1.77
Saw something on the net that says to use G2V stars but no method attached.
I have CCDStack, is there a process in this?

Cheers
George
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Old 06-09-2015, 12:57 PM
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There are lots of ways to determine RGB ratios but I don't think any is a magic bullet.

Some of the ones I have tried include:
  • eXcalibrator
  • using a G2V reference star
  • using pre-calculated G2V ratios adjusted for extinction
  • roughly lining up the R, G & B histogram peaks
  • Unlinked stretch in PixInsight
  • ColorCalibration in Pixinsight using a variety of white references (a galaxy, the star field, the whole image)

I didn't find that any of these did a good job in every situation. These days I use whatever gives me a result I like. Sometimes I blend a couple of different versions.

Cheers,
Rick.
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Old 06-09-2015, 03:21 PM
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Quote:
Originally Posted by vlazg View Post
Hi Spacers,

Is there a way to determine RGB ratios for colour processing?
I've tried eXcalibrator but ended up with some weird ratios
Eg RGB 1:1.48:1.77
Saw something on the net that says to use G2V stars but no method attached.
I have CCDStack, is there a process in this?

Cheers
George

Hi George,
I've always wondered about that.
Also - what about using different exposure times for each of Red, Green & Blue
so that the final result is always 1:1 ?

( you would also have to allow for the band pass of the filter used in front of all other filters
such as the CLS-CCD filter that I use &
the quantum efficiency of the CCD chip at different wavelengths )


cheers
Allan
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Old 06-09-2015, 04:48 PM
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Quote:
Originally Posted by RickS View Post
There are lots of ways to determine RGB ratios but I don't think any is a magic bullet.

Some of the ones I have tried include:
  • eXcalibrator
  • using a G2V reference star
  • using pre-calculated G2V ratios adjusted for extinction
  • roughly lining up the R, G & B histogram peaks
  • Unlinked stretch in PixInsight
  • ColorCalibration in Pixinsight using a variety of white references (a galaxy, the star field, the whole image)

I didn't find that any of these did a good job in every situation. These days I use whatever gives me a result I like. Sometimes I blend a couple of different versions.

Cheers,
Rick.
I'm always getting in trouble for my crazy analogies, but it almost sounds like polar alignment. The search for the holy grail of perfection, that isn't ever quite possible!

Rick, I've been in love with "linear fit" within PI recently. I know its not a proper color calibration tool, but it seems to do a great job of getting R, G and B in the same ball park before serious processing begins.
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Old 06-09-2015, 04:53 PM
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I had endless trouble with this. Now I just use Astronomy action tools (Action plug in for PS) http://www.prodigitalsoftware.com/As...l_Version.html

The Construct RGB Image from Channel Files is a few clicks and its done, lifes too short to muck around.
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Old 06-09-2015, 05:03 PM
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Quote:
Originally Posted by RobF View Post
I'm always getting in trouble for my crazy analogies, but it almost sounds like polar alignment. The search for the holy grail of perfection, that isn't ever quite possible!
The problem is that it isn't a single one-off ratio. Atmospheric effects mean that the sensitivity of a scope/camera at different colours is changing all the time.

Quote:
Originally Posted by RobF View Post
Rick, I've been in love with "linear fit" within PI recently. I know its not a proper color calibration tool, but it seems to do a great job of getting R, G and B in the same ball park before serious processing begins.
Nothing wrong with that, Rob. Using LinearFit will just balance the channels. I often use this technique for narrowband. An unlinked stretch does much the same thing.

Cheers,
Rick.
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Old 06-09-2015, 05:22 PM
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Apologies to the OP for a slight off-topic...

Surely this would be dependent on the subject? Something like Eta Carinae is off the charts in red with my modded DSLR, but signal is signal I'm not sure I'd want to balance that out to look nice or is that the point?
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Old 06-09-2015, 06:18 PM
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Quote:
Originally Posted by Bassnut View Post
I had endless trouble with this. Now I just use Astronomy action tools (Action plug in for PS) http://www.prodigitalsoftware.com/As...l_Version.html

The Construct RGB Image from Channel Files is a few clicks and its done, lifes too short to muck around.
Is that what Carboni's tools has grown into Fred, or completely unrelated?
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Old 06-09-2015, 06:19 PM
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Is that what Carboni's tools has grown into Fred, or completely unrelated?
No, thats the one, Carboni's tools. OK, maybe Carbonis name seems to have disappeared.
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Old 06-09-2015, 06:36 PM
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I think that Carboni tool justs combines the channel images into an RGB 1:1:1. I don't remember it dealing with colour balance.
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Old 06-09-2015, 06:38 PM
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Originally Posted by peter_4059 View Post
I think that Carboni tool justs combines the channel images into an RGB 1:1:1. I don't remember it dealing with colour balance.
Thats interesting, and possibly disappointing. I dont know. Seems to do a better job than I do manually.
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Old 06-09-2015, 06:43 PM
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The description of the action says it prompts you to do a colour balance.
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Old 06-09-2015, 08:01 PM
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Quote:
Originally Posted by Camelopardalis View Post
Apologies to the OP for a slight off-topic...

Surely this would be dependent on the subject? Something like Eta Carinae is off the charts in red with my modded DSLR, but signal is signal I'm not sure I'd want to balance that out to look nice or is that the point?
G'day Dunk,

I think the discussion covers both cases. Figuring out RGB ratios for an "accurate" image is not a trivial or easily repeatable process. Coming up with something that looks nice isn't that easy either. But you are correct... they are different goals.

Cheers,
Rick.
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Old 06-09-2015, 09:33 PM
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As has already been stated, there is no "magic scale" that will work in every situation. The Astrodon LRGB Gen II filters are supposed to be 1:1:1 for either Kodak or Interline sensors. Even though the filters may be 1:1:1, it doesn't mean that what you are actually capturing is correct!

That may sound strange but depending on where it is in the sky at the time changes the colour slightly (near horizon or zenith), imaging on different nights and seeing conditions, water vapour or dust in the atmosphere. Of course there is also dust between us and what we're imaging!

PI has some great tools for colour calibration, mostly I guess you just need to be selective about what you are using as your calibration. One good way to go, Aladin Sky Atlas!

http://aladin.u-strasbg.fr

That way you can manually align flux values in RGB (well actually Johnson-Cousins Photometric UBVRI)
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Old 06-09-2015, 10:53 PM
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Quote:
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G'day Dunk,

I think the discussion covers both cases. Figuring out RGB ratios for an "accurate" image is not a trivial or easily repeatable process. Coming up with something that looks nice isn't that easy either. But you are correct... they are different goals.

Cheers,
Rick.
Thanks Rick, good to know it's a common struggle!
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