Old 03-05-2017, 10:54 AM
Star Hunter
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Darkening the sky background

How does one make the background night sky darker if you have over exposed it a bit? Are there any programs that can do this?
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Old 03-05-2017, 11:02 AM
glend (Glen)
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Well, in Photoshop you can through the Curves function apply a linear profile which will usually darken a light ( or over stretched sky). There is also a Darken profile which will make the entire image uniformly darker. You can move the curve graph to tailor what your trying to darken. There is also exposure control, but without seeing an example its hard to advise. It would help to know what you are using now to process your images, and to see a sample. You might have a gradient, or have pushed the sub to far for your sky conditions.
Really rather than trying to repair it might be better to reshoot.
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Old 03-05-2017, 12:17 PM
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rcheshire (Rowland)
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Removing the sky gradient is a first step. StarTools will do that. There is a trial version. Once the background has been extracted, the rest is a matter of stretch and saturate to suit the eye.
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Old 04-05-2017, 03:45 PM
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sil (Steve)
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ideally you cant overexpose the night sky (the black background). you address first why tou are not getting a black background. One is shoot only after astronomical dark, before that you get a shade of blue, brown sky is light pollution so shoot from a dark site and/or use a light pollution filter. Post capture you need to remove gradients and/or adjust curves but you're killing faint signal in the process.
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Old 05-05-2017, 06:01 PM
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DiscoDuck (Paul)
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Most image editing software should allow you to look at and edit the histogram. You'll probably find the histogram shifted to the right, i.e. there's a gap between the lowest data (sky background) and what is regarded as black (value of 0 by default).

You should then be able to slide the "black point" up from zero to redefine what this black point is. Slide it just up to the histogram and your background should be much blacker.

If you later apply some noise reduction (e.g. in Photoshop) you'll find a gap reappear as the lowest values in the image are largely noise. Simply slide it a bit up again after noise reduction and it'll look even better.

The benefits of being able to redefine black with a camera means we can shoot images even in light pollution - wish I could do the same with my eyeball so visual astronomy in the suburbs was possible as well!


Last edited by DiscoDuck; 05-05-2017 at 06:22 PM.
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