Problem with dark subtraction

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Ukjay1971
Posts: 11
Joined: Fri Mar 03, 2017 3:09 pm

Problem with dark subtraction

#1

Post by Ukjay1971 »

I am using Sharpcap ver 3.0.3906.0 and am having an issue with dark subtraction making the subs worse, not better.

My camera is an Altair IMX178c

The two images attached are 4sec subs stacked without alignment (staking inside to test) over 60 second exposure. The quieter one has no dark or flat subtraction applied, the noisier one has 100 darks averaged and subtracted.

Any ideas what's going on?

No darks
Stack_15frames_60s_no_darks.png
Stack_15frames_60s_no_darks.png (130.72 KiB) Viewed 3022 times
100 darks subtracted
Stack_15frames_60s_with_dark_subtraction.png
Stack_15frames_60s_with_dark_subtraction.png (133.31 KiB) Viewed 3022 times
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admin
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Re: Problem with dark subtraction

#2

Post by admin »

Two possibilities here

1) different settings when taking your darks compared to when using them - this always leads to problems because you are subtracting the wrong amount of dark signal

2) [more likely] Live stacking auto stretches the displayed image, so the one where you are using darks has less noise really, but because the hot pixels are gone the peak value is much lower, which means that everything gets stretched more, so it looks noisier. This only happens because if you are stacking black frames - if you had some stars there or a galaxy etc then the effect would go away.

To get a fair comparison of noise levels with/without dark subtraction when live stacking, look at the 'Max Pixel Value' in the status tab of the live stacking - that is the accumulated brightness of the brightest pixel in the stack. I expect you will find it is much higher for your non-dark-subtracted stack than for your dark subtracted one.

cheers,

Robin
Ukjay1971
Posts: 11
Joined: Fri Mar 03, 2017 3:09 pm

Re: Problem with dark subtraction

#3

Post by Ukjay1971 »

It's not (1) as I'm careful to keep settings identical so (2) sounds like the answer. I'll run it on Sat night if the weather stays as it is.

Thanks Robin.

Jay
Ukjay1971
Posts: 11
Joined: Fri Mar 03, 2017 3:09 pm

Re: Problem with dark subtraction

#4

Post by Ukjay1971 »

An update, Robin:

I changed back to RAW8 3040x2048 1x1 binning and reshot. It works perfectly, just as you described, Infact, it works better than I had hoped - my Altair IMX178c (which I have struggled to like if I am honest) is now firing out 60 sec subs with NO AMP GLOW! This is new territory for me and opens up a new chapter in DSO shooting (I hope).

Many thanks and I hope I can continue to fine tune my settings to get the most out of this camera :)
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Re: Problem with dark subtraction

#5

Post by admin »

Sounds like an excellent result - do share some of your images if you have a chance :)

cheers,

Robin
Ukjay1971
Posts: 11
Joined: Fri Mar 03, 2017 3:09 pm

Re: Problem with dark subtraction

#6

Post by Ukjay1971 »

Here is a stack of 90x60 secs, 20 dark, 50 flat on the Crescent Nebula. Shot on an Altair Hypercam 178c in Sharpcap @300 gain, 1x1 binning 8 bit mode.

Sharpcap managed the amp glow pretty well - only a small crop needed - I'l still trying to dial in the exposure time as the stars are saturated even at 60 secs.

Any advice / comments / tips always appreciated :)
Image
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Re: Problem with dark subtraction

#7

Post by admin »

Did you adjust the gamma (pull it downwards) on the live stacking histogram? Doing that strengthens the nebulosity relative to the stars. I applied a gamma of 0.4 and a black level shift in Paint.Net and pulled out a good deal more nebulosity on the image. I'll post that on Facebook.

cheers,

Robin
Ukjay1971
Posts: 11
Joined: Fri Mar 03, 2017 3:09 pm

Re: Problem with dark subtraction

#8

Post by Ukjay1971 »

Ah ok, cool. Be nice to see that :)
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