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Luminance Stretching

Processing Stretching LRGB Deep Sky
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Karl Theberge avatar
Karl Theberge avatar
Good day folks,

I was wondering if anybody got a good tutorial, video, website,etc. About luminance stretching.

Because no matter how hard I try, I only ruin my images when I attemp to do galaxies.

I get decent result with RGB data, but as soon as I add my luminance layer, it is always too dimm or too bright or all details are lost.

Never been much of a galaxy guy, but I really want to achieve it.

karl
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Reg Pratt avatar
Reg Pratt avatar
I don't have a tutorial handy but this is how I process LRGB generally.

RGB:
Combine channels
Background extraction
Color correction
Decon
Noise reduction
Star removal
Stretch


Luminance:
Background extraction
Decon
Noise reduction
Star removal
Stretch


LRGB:
LRGBCombination
Saturation
Noise reduction (if needed)
Curves

Stars:
Stretch
Saturation
Screen onto starless LRGB


Along the way any small tweaks I see that need to be made are made. If when you combine the L you don't like the result you can easily go back and restretch the L until you get it to brightness level you like. And of course you will likely want to employ masks when doing at least some of the stretching to protect bright areas like galaxy cores.
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andrea tasselli avatar
andrea tasselli avatar
Looking at your gallery I'd say your main issue here is not enough signal rather than stretching the luminance channel. Think in terms of 3x-4x the integration time you have now.
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Karl Theberge avatar
Karl Theberge avatar
andrea tasselli:
Looking at your gallery I'd say your main issue here is not enough signal rather than stretching the luminance channel. Think in terms of 3x-4x the integration time you have now.

*I think you are right...  Ill wait and get more data on my ongoing project.


A few night only doing Luminance filter should do the trick.
trick.

Should I BIN my RGB?
andrea tasselli avatar
andrea tasselli avatar
Karl Theberge:
Should I BIN my RGB?


I don't think it makes any difference, so no.
James Hawks avatar
James Hawks avatar
Chaotic Nebula has a great tutorial on processing L.

https://chaoticnebula.com/pixinsight-lrgb-workflow/
Arny avatar
Arny avatar
having had similar experiences I found out that
- LinearFitting all channels to the one with highest median
- stretching with EZ SoftStretch
really did help.

The LinFit takes care of very different brightness level between RGB and L channels to set them all to the same median,
EZSoftstretch prevents me from overdoing GHS or HT stretching - I am just not there yet :-)

Also it helps to enhance the L channel with some careful HDRMultiscaleTransform to increase the crontrast within the galaxy arms.

But in general: the less, the better :-)

When you stick to that you don't even need to seperate stars for processing, which takes some work and artefacts out
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George Hatfield avatar
George Hatfield avatar
I  usually use a super luminance, where I combine everything (RGBL and often narrowband) with no rejection or weighting.  Then, I use the following procedure:

1.  Remove gradients
2.  Run BlurX
3.  Remove stars with StarX and discard
4.  Run NoiseX on the starless image… use minimum strength to preserve detail
5.  Stretch the starless image
     a.  small arcsinh stretch
     b.  Adjust black point with HT
     c.  Continue stretch with GHS until the median is close to the RGB median; continue to optimize the black point
     c.  Sometimes use LHE to increase contrast
6.  Combine with RGB 
7.  Make minor adjustments… usually in contrast and color in Photoshop.

Adding the luminance almost always improves the appearance of the RGB.
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Karl Theberge avatar
Karl Theberge avatar
Ill try both of your suggestions.

I dont have any clear skies for the next 10 days so plenty of time to experiment.

I know more signal would be better, but I seem to be stuck using small 8 to 16h integration.

Ive seen a lot of poeple getting really good image with less integration time than me…

Thanks to all of you!