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Old 27-09-2016, 09:05 AM
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Crushellon (Tim)
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Bias subtracted darks for flat frames.

Hi all,

So I've done flat frames for the first time and I'm just wondering if I should use bias subtracted dark frames (length of the darks is the same as my light frames) or if I should just take dark frames to match the length of my flats. I played around with bias subtracting my darks last night and the results didn't resemble the original dark frames at all. I'm just not sure if that's normal or not.

I used pixinsight to stack the bias frames in image integration and calibrated the darks with the bias using image calibration.
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Old 27-09-2016, 10:02 AM
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Slawomir (Suavi)
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Hi Tim,

The easiest and most reliable way is to use Batch Preprocessing script in PixInsight. It automatically does stacking of bias, dark and flat frames and effectively calibrates the lights. Master Jedi Rick also uses this script in his workflow so it must be good
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Old 27-09-2016, 07:16 PM
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Camelopardalis (Dunk)
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Tim, the bias is there in every shot the camera takes, so this must be subtracted from every image we take, light, dark or flat, before we can do anything else useful with them.

Depending on the behaviour of your camera, the bias may be the primary source of noise for short exposures. For longer exposures, dark noise may become more significant. Once the bias is subtracted it may well look different as it should be leaving behind just the dark noise. Clear as mud?
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Old 27-09-2016, 07:31 PM
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RickS (Rick)
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Quote:
Originally Posted by Slawomir View Post
The easiest and most reliable way is to use Batch Preprocessing script in PixInsight. It automatically does stacking of bias, dark and flat frames and effectively calibrates the lights. Master Jedi Rick also uses this script in his workflow so it must be good
Thanks, Suavi

Tim: unless you're doing something quite exotic, BPP is usually perfectly good enough to do the calibration and registration... but always do ImageIntegration manually!

I wouldn't, and don't, bother to do short darks to match the flats. If you throw the flats, normal darks (or master dark previously created with BPP) and bias frames (or master bias previously created with BPP) into BPP it will do the best it can. If the darks don't help improve the SNR of the master flat you'll see warning messages and they won't be used:

"Warning: No correlation between the master dark and target frames"

Cheers,
Rick.
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