LRGB colour processing

[martin_h]

Just a quick image to show that the RGB part of LRGB doesnt have to be a perfect sharp image, as long as the colour is in the right place...

Screen shot of photoshop during processing of the broom showing the RGB part of the image with the Lum layer turned off. The RGB is made of Ha(red) synhtisised green and OIII(blue) with the stars removed...

And the final image with Lum layer added- and colour tweeked

[AndyUK]

Hi Martin - That's a very useful and helpful demonstration - Thanks for posting this up. I've always struggled a bit with RGB/LRGB, usually blaming poor seeing and/or transparency as I frequently find that the stars are bloated in one or more of the channels. However this shows how a good luminance layer can "rescue" an image. Was this a "pure" luminosity layer (using a luminance filter) or did you use Ha... or maybe it was created by mixing the Ha/synth G/OIII using blend mode lighten(?).

Also, looking at the RGB histogram, I note that all 3 channels are pretty much synchronised - When stretching the separate RGB channels from the raw stack (in this case Ha, synth green, OIII), prior to merging, did you work on all 3 at the same time to ensure that the histogram's stayed more or less "in sync"? And if so, is this where you might "repair" any star issues as well, in an attempt to overcome differential seeing/transparency star bloat in one or more of the channels...?

I've also read that the RGB channels can be stretched and denoised almost to within an inch of their lives, as long as you have a good luminance channel. I've not REALLY felt too comfortable doing this to date, and hence I've wimped out the last few times and imaged in NB instead, but I'm now beginning to pine for some star colour so want to get back to (L)RGB soon...

[Martin-Devon]

It's a lovely image Martin - for those of us not quite so advanced as yourself - can you just explain (in simple terms) how you first removed the star layer, and then how you subsequently added back the Luminance star layer - thanks! Presumably this same technique can be alternatively used to apply a H-alpha optimised layer back over a de-starred optimized RGB as well - would be nice to spell out in simple steps the way to do this.

[ollypenrice]

Another demo can be done by going into Lab colour space, blurring a and b channels and then recombining. It's a good way of reducing colour noise and the effect on sharpness is next to zero. LRGB is great!

Olly

[martin_h]

It's a lovely image Martin - for those of us not quite so advanced as yourself - can you just explain (in simple terms) how you first removed the star layer, and then how you subsequently added back the Luminance star layer - thanks! Presumably this same technique can be alternatively used to apply a H-alpha optimised layer back over a de-starred optimized RGB as well - would be nice to spell out in simple steps the way to do this.

I remove the stars with severl iterations of Dust & Scratches in PS

I usualy start with a radius of around 12 and a threashold of aout 100........depends on image

Then follow with several more dust and scratches lowering the radius by 2 and threshold to abut 80.repeat lowering values until you get to about radius 1, threshold 3-10

Then give it an Uber stretch in levels in several iterations and a contrast curve......by now you will have a god awfull image full of noise.

Give it a Gaussian blur - radius 4 and folow wih Dust and scratches again, radius about 24 threshold about 16,followed by another gaussian blur - radius about 3

Give it another contrast curve - without clipping!

Do the same for the other 2 colour channels and merge to form an RGB image, try to ge the histograms the same(ish) for all 3 channels

For the Lum layer I usualy make a blend of Ha,OIII,SII so that the colour data is not all packed into the red (Ha) channel

[AndyUK]

Cheers Martin - It sounds similar to Tone Mapping... Will have to give it a go sometime!

[Interested]

Just bought CS6 so added this page to my list of favorites in Explorer. Thanks Martin.

[Shibby]

Martin, that's a cracking image but, and correct me if I'm wrong, don't you lose all the star colour with this method? Or do you add them back in afterwards?

[Shibby]

Ok, I've just read it again and realised your RGB is actually synthesised from narrowband data. Never mind!