How to make a Green channel from Red and Blue in Photoshop

[John K]

Hi guys,

Missing a Green channel, but have a Luminance, Blue and Red taken with a monochrome deep sky camera.

Can anyone advise on how to do this in photoshop?

Have tried combining the Red and a 50% Blue layer blended on top in Layers in CS3, but still not working in terms of then using this as a Green channel.

Have tried to follow this http://www.astrosurf.com/cidadao/correction.htm procedure but not using CCDOPS in any way.

Any ideas would be really appreciated!

John K.

[glend (Glen)]

Well luminance contains the green, you could try using a copy of that and in Photoshop mask off the red and blue channels to leave the green.you need fo be able to use clipped layer masking and be able to block the red and blue.
Honestly, i would just wait and shoot a green channel when you can, or build it as a bicolour (red and blue) image, with a luminance or soft light layer, and then play with the colour balance.

[RickS (Rick)]

The only method I've seen for doing this which is based on Physics is the B3E (Ballesteros Blackbody Estimator) process in PixInsight. The maths is described in this paper:

http://iopscience.iop.org/0295-5075/97/3/34008

I know you asked about Photoshop, but I think it's unlikely that such a thing exists because this is not a generic image processing function.

Cheers,
Rick.

[JA]

Depending* on the filter brand:

L = R + G + B + (a little Infra Red)*

so L ~ = R + G + B

So if you take the L and subtract the R and B you get mostly G and a tiny bit of IR (depending on the IR sensitivity of your camera). That should be possible with image processing software

Best
JA

[RickS (Rick)]

Originally Posted by JA

L = R + G + B + (a little Infra Red)*

That's assuming there's no overlap or gaps between filters, so YMMV. One of my colleagues at SRO tried this technique and got very poor results compared with B3E. We were using Astrodon filters which have some overlap between B and G and a large gap between G and R.

Cheers,
Rick.

[John K]

Thanks for the comments everyone.

There is a way to do this in Photoshop, but seems to not be a freely available technique.

http://www.prodigitalsoftware.com/As...l_Version.html

Looks like I'll just get the Green channel next time.

In the meantime have been having fun with a false colour version and some field rotation issues.

https://www.flickr.com/photos/johnka...blic/lightbox/


Clear skies.

John K.