Can I used scaled dark frames with a CMOS sensor?

[billyharris72]

Hi all:

Question exactly as it sounds. I was reading up on the relative advantages of CMOS and CCD and came across the fact that noise from a CMOS chip is not Gaussian, but is heavily skewed. Does this affect the use of dark frame scaling, for example if using 1 minute dark frame to correct 2 minute subs on a DSLR? It's not something I have actually tried to do (I always shoot at least a few darks on the night) but would be handy (and interesting) to know.

Thanks,

Billy.

[AKB]

AFAIK, the average of a number of frames should still be good to use, its the higher-order statistics (variance, skewness, kurtosis,...) which take on a different value.  

Because of this, subsequent approaches to noise reduction should theoretically, perhaps, be treated differently.  But for many, the conventional least-squares formulation to minimisation and best fitting is probably good enough.

 

[andrew s]

Billy darks should be as long as or longer than lights. If not you add unnecessary noise. 

Regards Andrew

[Filroden]

I've found that with uncooled CMOS (I.e. DSLRs) then calibration was better without darks and just use bias, flats and hot pixel removal. With cooled CMOS then scaling a longer dark to a shorter sub only worked if the exposures did not include amp glow. As darks are so easy to take in daylight it was much easier to build a library of darks for any exposures I might use.

[Xplode]

13 hours ago, Filroden said:

I've found that with uncooled CMOS (I.e. DSLRs) then calibration was better without darks and just use bias, flats and hot pixel removal. With cooled CMOS then scaling a longer dark to a shorter sub only worked if the exposures did not include amp glow. As darks are so easy to take in daylight it was much easier to build a library of darks for any exposures I might use.

I agree with this, i found darks a total waste of time when i used a DSLR.

Instead i used dithering as that will help remove fixed pattern noise like hot pixels when enough exposures are stacked, it starts to do it's thing at 10+ exposures with at least 3 dithers.

The more dithers the better, but dithering can also take away a little more valuable time than wanted so dithering once every 3-5 subs is often best.