GIMP help (Manual stretching/colour management)

[Endolf]

Hi

I'm new to photo processing and astrophotography in general. I've been playing around with ASTAP for stacking and was able to get something ok. I've now taken a look at Astro Pixel Processor. I've managed to get a passable image of M81/82 and M101, I'm now having a go and reprocessing some of my earlier images I did in ASTAP. I've got an image of M42 that I've processed with it. In ASTAP the colours were correct, it was just very dim in it's native FITS result. I was able to do a couple of sets of level adjustment, one that got the nebula details and one that darker that showed the details in the core. I could then blend the images in GIMP, sort of manual HDR.

I've got APP to process the image now, which seems to show a lot more detail. If I save the auto stretched version in APP I get an image with the detail but a blow out core, if I save the non stretched version I am having troubles in GIMP as the colours are off. I'm not familiar enough with photo processing or GIMP to be able to adjust the colours, I've tried adjusting the levels, but can never get a neutral look. I've attached some photos that show the issue, any advice or links to tutorials would be greatly appreciated.

Thanks

[JamesF]

Which version of GIMP do you have installed?  Could be that you really need a later version to get the full 32-bit support.

James

[Endolf]

2.10, it does support the 32 bit FITS images.

[happy-kat]

@alacant is good with GIMP perhaps they'll see this, as does another member but I can't get their name right.

[alacant]

Hi

You can process 32 bit in gimp [1] but I'm not sure why you'd want to do that. Make big paper prints maybe? 

The best way to import into gimp for post calibration is to save your stack as 32 (or preferably 16) bit tiff without compression. 

Colour balance is the single most difficult -for me at least- part of post processing, so stick at it; do the colour last of all. Perhaps use the CMYK colour balance module before resorting to curves?

But hey, don't sweat. What used to have to be e.g. red, can now be the once forbidden green. Or violet. Or whatever you choose. Fashion.

Good luck and HTH.

[1] Works for the Linux version. I haven't tested any other platform.

[han59]

You probably are better off  by stretching in APP or ASTAP.  In GIMP you could stretch using curves, but most likely like in Photoshop you will loose colour of the faint objects:

See e.g.:

http://allthesky.com/articles/colorpreserve.html

or here

https://www.markshelley.co.uk/Astronomy/Processing/Colour_Preserving_Stretch/colour_preserving_stretch.html

 

And explained in this little graph:

Input:

blue=5
red=10

The input colour ratio  is then 10/2 is 2.

The curve gives these values after stretching:

Output:

blue=0.26
red= 0.38

De output colour ratio is then 0.38/0.26, about 1.5. De result is less colorful.

 

A better method is colour preserved stretching as implemented in ASTAP:

in de range of 0..1:

luminance:=(red+green+blue)/3

luminance_stretched:=gamma_curve(luminance)

red_output :=luminance_stretched * red
greenn_output:=luminance_stretched * green
blue_output:=luminance_stretched * blue

The ratio between red, green and blue so the colour stays the same. Only the luminance is stretched.

 

Han