Dark subtraction

Discussion of using SharpCap for Deep Sky Imaging
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JALabrecque
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Joined: Sat Jan 08, 2022 11:02 pm

Dark subtraction

#1

Post by JALabrecque »

1. I first prepare my Darks in SharpCap using the same settings as my lights, in this case 20 shots, 45s, 121 gain. Looking at the resulting average dark I see horizontal lines and a slight amp glow in the corner - as expected.
2 The loaded Dark as well as Flats are saved in SharpCap to be used on the captured lights.
3. I then take the lights with the same settings.
4. My resulting shots have the same horizontal lines as the darks but no evidence of dust donuts or vignetting seen in the prepared flats.

It does not appear that SharpCap is subtracting the darks. My understanding is that subtracting darks should remove these horizontal lines. Are my expectations too high or am I doing something wrong (loading info incorrectly, not turning something on).

Some help would be appreciated.

John Labrecque
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Re: Dark subtraction

#2

Post by admin »

Hi John,

dark subtraction will not usually remove the horizontal lines that you see in the dark areas of an image - this is because the lines change somewhat with each new frame, so the master dark file you create will have an averaged version of those lines, but each individual light frame will have its own unique pattern of lines that doesn't match the master dark.

Dark frames will be more successful at removing static image problems, like hot pixels, amp glow and background brightness.

You can test if your darks are working correctly by making your master dark, then leave the camera covered and capturing more dark frames. Turn the dark subtraction feature in SharpCap on and off and make sure that it has an effect on the histogram (lower values when on) and that it is making the image darker.

cheers,

Robin
JALabrecque
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Joined: Sat Jan 08, 2022 11:02 pm

Re: Dark subtraction

#3

Post by JALabrecque »

Thanks Robin that makes sense. I did not realize the bands changed slightly between shots.

I assume Band Suppression would be a tool to try to control them? I just tried it on a new set of darks and it seemed to help. Should I leave the band suppression set to where it works for the Darks and take new Lights with the same setting?

If I change exposures will I have to modify the band suppression?

Now just have to figure out what is causing them.

Thank you for the quick response.

John
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Re: Dark subtraction

#4

Post by admin »

Hi John,

you could certainly re-capture everything with banding suppression if you wanted. Alternatively, if you have the individual images saved you could consider using the folder monitor camera (https://docs.sharpcap.co.uk/4.0/#Virtual%20Cameras) to let you re-process those images through SharpCap and apply the banding correction.

My belief is that most of the banding is down to electrical noise within the camera. It's worth testing with different camera settings as sometimes the banding can be minimized by (for instance) using different gains or by adjusting other settings.

cheers,

Robin
JALabrecque
Posts: 23
Joined: Sat Jan 08, 2022 11:02 pm

Re: Dark subtraction

#5

Post by JALabrecque »

Thanks Robin.

Did some experimenting today varying gain, exposure and banding suppression. It was all done with the camera covered so everything I saw was attributed to camera noise as you stated in your response. Leaning toward higher gains (currently use 121 supposedly the optimum setting) and shorter exposures along with banding suppression.

Waiting til the moon leaves evening sky to try things in real life conditions.

Thanks again.

John
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