LRGB - Synthesis of Green Channel

[jcm]

Many people using narrow band filters often take only Ha and OIII images and synthesize a green channel.

Is it possible when doing LRGB to take L , R and B frames and synthesize a green channel from the R and B thus saving imaging time ?

John

Doh! this should be in the section below !!!

[steppenwolf]

Yes it is possible, John. Noel Carboni's Tools for PhotoShop have an action to do this for you and it works surprisingly well.

[jcm]

Yes it is possible, John. Noel Carboni's Tools for PhotoShop have an action to do this for you and it works surprisingly well.

Thanks for that , Steve.

I have done very little LRGB work. So I thought missing out the green image , binning the R and B images 2X2 , and synthesising the green might save a bit of time.

John

[narrowbandpaul]

in terms of imaging objects with discrete spectra, like emission nebula, planetary nebula have very little emission in the green, and so not using a green filter is of little concern. The true colour can be represented without a G

For continuous spectra like stars and galaxies including the green is the only true way to accurately reconstruct colour, anything else is a guess.

For example, Noel Carboni's stuff....

i personally dobt believe in using a synthetic green channel that in no way represent the true goings on. In narrowband using a combo of only Ha and O[iII] one can construct the following color palette, which has the combo dictated by physics.

Ha=Red (true) O[iII]=G(true) 1/3Ha+O[iII]=b

the 1/3 Ha is the Hbeta contribution, derived from Quantum mechanics

This gives a pretty good approximation to an RGB image, and no 'synthetic green' where the imager has no idea how its made. It probably has no science in it.

I would include green for continuous spectra and not bother for emission line targets. Mind you, the O[iII] line may be in the green filter, which means you really should include it. If both O[iII] and Hbeta are included in the blue filter, you dont need green.

Hope that helps somewhat.

Paul

[jcm]

Thanks for the extra info , Paul.

Its just I cannot remember seeing any LRGB images that used a synthesized green channel and I was wondering how much different they were from the full LRGB.

Obviously , as you said above , the target choice will effect the outcome.

John

[narrowbandpaul]

i dont think i have seen this either john

i think its more common for narrowband using Ha and OIII as you said above.

only one way to tell, try it!

i take it you are only talking about LRGB and not NBI

what filters do you use?, their transmission function may reveal the answer

paul

[jcm]

I have the Astronomik LRGB and Astronomik Ha/OIII

John

[steppenwolf]

John, just for the fun of it, here is an original and then a synthetic green version of the same image using the same data. I have not attempted to make the synthetic version match but I did slide the gamma control to the right to saturate the image better.

Original:-

Synthetic green version:-

[roundycat]

I had the same idea but was beaten to it! Here is a scrap pic of M42 that I just re-did as R R+B B, no green at all. Hate it!

Dennis

[blinky]

So it seems that depending on the target it may or may not be important to gather 'true' green data....

[jcm]

Thanks for the comparisons.

I will have a go and see how it turns out.

John

[steppenwolf]

Dennis, I hope you don't mind but I have used your data to do the synth. I ran the NC action, adjusted the gamma to right again and then adjusted the colour balance by sliding the Colour Balance, Magenta - Green slider to the left (i.e. towards magenta ad the following is the result. Original image on left, synth image on the right:-

[roundycat]

Hi John, very interesting. FWIW I still don't like it (curmudgeonly old git) but it is amazing what you can do with a bit of perseverance. What is NC action? I take it you did this in Photoshop, I did but didn't bother trying to get it to look 'right'. My blue and yellow effort looks a bit like my narrow band of the Veil using just Ha and Olll and synthesised Green.

I didn't actually address your original question. If it was me I would skip the L and get as much RGB as possible. If you then have time get some L. I think the LRGB route is a good one if you are binning colour but you still need a lot of colour or the L will simply dilute it. I note that some people are using a synthesised L by adding RG and B although I'm not sure what this does to the background noise. I suspect it does not reduce it in the way that a separate L channel would.

Dennis

Edit: seems I mixed up names here, I was concentrating on getting my dinner!

[steppenwolf]

NC Action = Noel Carboni's Astronomy Tools, one of which is an action that synthesises the green channel from the red and blue channels. These actions work with PhotoShop and 'Elements'. Great value and available here