[Gimp-user] How to fix vignettting, smooth out digital noise and reduce sky glow in astro-photo

[rich404 <forums gimpusers com>]

> Hi, all.
>
> I have a picture of comet 46P/Wirtanen from a few nights ago.
>
> I'm new to GIMP and it's like a space shuttle control panel to me.
>
> Can you please provide step-by-step instructions to
> 1. reduce vignetting in the corners
> 2. smooth out the background noise
> 3. keep stars (the brightest ones are already over-saturated) and
> comet and approximately the same brightness but make the background
> sky darker
>
> I tried playing with levels picking various black/white/mid points but
> wasn't happy with the results.
>
> I also tried gaussian blur but could not figure out how to apply it to
> the photo without bluring the stars. Perhaps some mask needs to be
> created to isolate stars and comet so that the gauss filter can be
> applied to the rest of the pixels and allow to keep the comet and
> stars sharp?
>
> Thanks!

First a caveat, I would not recognise a comet at 100 metres.

Always many ways with Gimp but try: (with screenshots)

1. I would promote the image to 16 bit, might be that already, I see your jpg
has a quality setting of 55 to get a smaller file size. Use your best image.
https://i.imgur.com/BqFWbsG.jpg

2. Duplicate the layer "Layer -> Duplicate layer" and apply a large gaussian
blur to the new layer. "Filters -> Blur -> Gaussian Blur". The value will go
above 100, do not be fooled by the sliders. Round about 200
https://i.imgur.com/llKHYQy.jpg

3. Put that blurred layer into "Difference" mode. That will even out the
background. https://i.imgur.com/J3lfW0R.jpg

4. Rather than destroying your work combine those layers "Layer -> New From
Visible". Apply "Colors-> Levels" to this layer. Use the White point picker and
choose one of those larger white-ish stars. https://i.imgur.com/RRFP2iN.jpg

5. Get rid of the smaller stars/junk "Filters -> Generic -> Erode". If you feel
it needs it, repeat. https://i.imgur.com/dcNks8Y.jpg

Attached a result, again lower quality setting to get the file size down.
Might/might not be what you require.

Attachments:
* http://www.gimpusers.com/system/attachments/1073/original/Comet01.jpg

-- 
rich404 (via www.gimpusers.com/forums)