Green channel subs in imaging, is it worth it ?

[red dwalf]

is it actually worth doing subs for the green channel in RGB imaging ?

i ask as in photoshop and i suppose other post processing software, i can do a synthetic green channel therefore freeing up my time to do more Lum, Red and Blue instead,

thoughts please.

[vlaiv]

You don't need to do green channel. In fact, you can choose which one you don't want to do if you are doing luminance as missing channel can be made by L - sum_of_other_two.

If you are going to go for this approach - I advocate you skip blue as it is channel with arguably worst SNR on most targets, so it's best if you make that one instead of green. Green is rather strong on most targets and you want that so you can have a good SNR in given amount of time.

 

[CraigT82]

4 hours ago, vlaiv said:

You don't need to do green channel. In fact, you can choose which one you don't want to do if you are doing luminance as missing channel can be made by L - sum_of_other_two.

If you are going to go for this approach - I advocate you skip blue as it is channel with arguably worst SNR on most targets, so it's best if you make that one instead of green. Green is rather strong on most targets and you want that so you can have a good SNR in given amount of time.

 

To create a synth blue, what blending mode do you use to subtract sum of R and G from L?  I've tried the obvious (subtract) but it looks very wrong! 

[vlaiv]

1 hour ago, CraigT82 said:

To create a synth blue, what blending mode do you use to subtract sum of R and G from L?  I've tried the obvious (subtract) but it looks very wrong! 

Best to do it with pixel math rather than blend mode. Just make sure that your subs are equally normalized. This means scaled to same exposure length and your flats were scaled to 0-1 range prior to calibration.

You'll get "cleanest" results if you have filters that fill 400-700nm range without overlap - like Baader set:

Otherwise you need to do color calibration to make colors look good.

In above image if you "add" all three bands you'll roughly get luminance band or full band between 400 and 700nm. There is small overlap at 500nm, so this method is not so good if you have OIII target as OIII won't have that teal look.