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Right. Flat darks and bias frames


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I just did a big test with a load of different combinations of calibration frames.

I couldn't tell the difference between those processed with flat darks, and those with bias.

Further research implies that they're essentially the same thing.

So, for a DSLR:

  • Is there any case where I should use both flat darks and bias?
  • If not, am I actually making my images worse by using both? In which case, which one should I use?

That's all!

Thanks, Brendan

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All images you take include a bias signal. Unless you need to scale (=alter) dark frames, there really is no need to use bias frames. For ccd and dslr images however, bias frames can (sometimes) replace dark frames, ie flat darks and ordinary darks. Whether that's the case for you, only experimentation can tell.

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1 hour ago, BrendanC said:

Further research implies that they're essentially the same thing.

Simple way to check if bias and darks are in your case same thing would be to stack bias to one stack (regular average will do) and stack flat darks to other stack.

Subtract the two and analyze result.

If result is pure Gaussian type noise with mean value of 0 - then yes, these two are essentially the same.

In reality however, they are not. Difference is in dark current. How much dark current there will be depends on your sensor and flat exposure. Some people use short flat exposures (bright flat panel, fast optics) and dark current is minimal in that case, but sometimes people use longer flat exposures - slower optic, sky flats or not very bright flat panel, maybe using paper to further diffuse it or mechanical shutter - that creates motion shadow in fast flat exposure. In any case - longer flat exposures or hot sensor susceptible to dark current will have substantial dark current even in flat exposure and it will show.

If you get any sort of pattern in above result - that usually means Bias that is not stable. If you get non zero mean value - that means dark current build up even on short flat exposures.

In either case - flat darks not equal bias.

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12 minutes ago, BrendanC said:

Very good! I like the idea of subtracting one from the other.

Until I get around to doing that, presumably there's no harm in continuing to use both? 

No harm, but not much point either.

As bias is contained in both in flats and flat darks - you can either remove it from both or let it cancel out (since flat darks are subtracted from flats) - result will be the same.

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