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Getting Decent Looking Stars in the Background of Deep Sky Images

astrophotography beginner dslr imaging observing
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#1 Hard.Boyled

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Posted 20 September 2020 - 05:28 AM

Hey guys, first post here :)
I recently took my first image of a deep sky object (the Lagoon Nebula) and I'm happy with how it turned out for a first try, but the thing that annoys me a lot is that, while the nebula itself looks decent, the stars in the background of the image look like a mess. I was wondering if anyone has any tips on how to fix this issue? I think one of the main problems is that perfect focus wasn't quite achieved? But also in post-processing in photoshop the stars look really gross and ugly, any tips would be greatly appreciated.
Thanks!

 

https://www.astrobin.com/biy2vo/


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#2 Tapio

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Posted 20 September 2020 - 05:47 AM

You are right that focus could be better, so there's something to improve.
There are techniques to deal with stars.
Some shrink stars in post processing, some separate stars and stretch just nebula and put stars back in final stage.
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#3 TOMDEY

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Posted 20 September 2020 - 06:45 AM

Nice image! I believe you have extracted as much as your data realistically allows. Good unsharpened stars provide a severe test of focus, bandwidth, wavefront quality and light pollution. Most (nearly all) of us imagers (traditional film/plates, traditional solid state, or EAA) tend to undercapture, overfilter and oversharpen to (try to) deal with those issues. Alas, it's no free lunch... and artifacts of the poor capture conditions and overprocessing still punch through. The best images start with superior data and are only mildly processed. The resulting aesthetic gestalt proudly boasts "Realistic!"    Tom


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#4 BQ Octantis

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Posted 20 September 2020 - 08:15 AM

I actually use two separate workflows, including captures: one for the nebula and one for the stars.

 

I shoot the nebula at f/2.8. I use a workflow on that stack to bring out the color and detail on the nebula, and then use starnet++ to remove the stars. I shoot the stars with an aperture mask on the lens (~f/3.8; this significantly reduces the distortions in the corners of the image) and use a workflow on that stack to bring out their color.

 

I then add in the stars from the f/3.8 stack to the starless f/2.8 stack. Specifically, in Photoshop I paste in the f/3.8 image on top of the starless f/2.8 image and switch the f/3.8 layer mode to Lighten. I then use Curves (3 points) on the f/3.8 layer to adjust the amount of stars I let back in. For really bright stars, I can feather in more of the periphery from either image as a Luminosity layer with a layer mask.

 

This workflow did wonders on the Lagoon & Trifid this year:

 

https://www.astrobin.../full/bprssm/0/

(Click on "Full resolution" if you want to pixel peep at 100% sensor scale)

 

Let me know if that didn't make sense…

 

Cheers,

 

BQ


Edited by BQ Octantis, 20 September 2020 - 08:22 AM.

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#5 Hard.Boyled

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Posted 20 September 2020 - 09:34 PM

Thanks for all your input, I really appreciate it, especially being so new to this hobby. I'm just trying to learn as much as I can and hopefully improve with each shoot, so insight from seasoned veterans is invaluable! 

 

When I was taking the images I suspected the focus might be out slightly but time was running out so I foolishly thought close enough is good enough, which clearly was not the case. No matter how good anyone's skills are in post, it's no substitute for good data. 

 

Hopefully next image I take I'll have fixed my focussing woes :D 



#6 BQ Octantis

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Posted 21 September 2020 - 12:38 AM

Do you not use a Bahtinov mask for focus? There really is no substitute. I check my focus right before I start a set of subs and any time I take any major action on the setup—typically every hour or so.

 

BQ




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