Startrails post-processing in Darktable

blackstar

Senior Member
Hi,

I spent 3 hours in the mid of a chilly night and then mistakenly set iso=800 and 30min to shot a startrails image (extremely over-exposed, see pic1). Then I used darktable to reduce the exposure to make it visible (as in pic2). I just wonder if I can continue with darktable to bring the dark sky at corners to the big central part of the still over-exposed area? (so to show startrails there) Appreciate any help and advice.

2020-04-28 23.27.35-s.jpeg

2020-04-28 23.27.35-exp-s.jpeg
 

blackstar

Senior Member
Hi Needa, Thanks for responding.

I think in the center there are still star trails just kind of washed out by extreme over-exposure even though I have reduced exposure level to the lowest limit... am thinking is it possible to start with the 2nd pic as exposure at 0 EV and draw a circular mask (covering central part) and reduce exposure only in the mask? I am a new user of Darktable and need a lot help in basics. Thanks
 

Needa

Senior Member
Challenge Team
Darktable and need a lot help!

Me too! :)

I don't think you are going to be able to resurrect it, way over exposed. See the color change in the center. What does it look like on the back of the camera.
But if you want to try here are some thoughts.
First duplicate the image several times, so you can see how different versions come out.
No need to start with the new picture you can stack a second instance of exposure on the image you are working with.
Try global tonemap instead of exposure. Move the target slider to the left.

Good luck! Let me know how it works out.
 
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blackstar

Senior Member
Thanks, Needa.

I tried a new exposure instance and a tonemapping. The outcome looks better but the improvement still unsatisfactory. Guess like you said it's gone for now... until next experiment.

2020-04-28 23.27.35_01-exp-s.jpeg
 

blackstar

Senior Member
Needa,

I tried some re-processing: 2 more instances of reducing exposure -- until everything is dark; "Color reconstruction" as the article recommends -- with little effect; "Highlight reconstruction" (also mentioned in the article, but not the primary option) -- with "Reconstructing in CL", suddenly and surprisingly star trails begin to come out from the dark sky! (though some bright, some faint due to focus issue)

So this is! Thank you for letting me know the article and I learned.
2020-04-28 23.27.35_02-exp-s.jpeg
 

Needa

Senior Member
Challenge Team
That is impressive all thing considered. :encouragement: Just bookmarked the page for myself.
 

blackstar

Senior Member
Hi Needa,

Thanks for the encouragement. I think it can still be further improved contrast-wise though not my original intention. I'll try again later when the moon is gone and make sure to get iso and focus right. :)
 
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