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Can dithering replace dark calibration?


Page III

  1. Sigma clipping
  2. Calculation of calibration noise

3.1: Sigma clipping

When you are finished with your sub images you have to align and stack them in your favorite software with a Sigma Clipping/Rejecting or Median function or similar.

Here are the software that I'm used to work with, they are old today but free. I believe most of the astrophoto edit software can do this.

I normally use the standard setting, but for you it could be different.

Even if I haven't done it here you should still do a flat calibration.

Before you do the flat calibration you must adjust the level of your image, replace the bias with a constant. Fitswork which I use always do this to all images by automatic. If the exposures are very long the dark current must also be compensated for, do a measure of a darkimage without the edges, that mean value include the bias and dark current. Why a constant instead of real images? A constant don't have any random noise!

If you want to read more about dithering Lasse H. has provided some links here:

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3.2: Calculation of calibration noise

How much better will the signal / noise relation be with dithering instead of dark calibration?

I got some comments from the forum Cloudy Nights that there was a miss from me about the contribution of noise from flat calibration. I did some dirty math here to correct the influence from flat calibration.
And I also miss the note that gain is equal to one, 1 e- = 1 ADU.
I have added what happens with the S/N at the edges when the optics vignetting, very interesting to see.

Here is an (new) attempt to show a common situation:

CalibrationRandom noise from calibration: Dark/Bias vs Dithering/Median

The weak signal could be a nebula or a galaxy.

Random noise from dark, bias, flat and flat/bias calibration images is thought to be only readout noise, in flat also Poisson noise.

It could be something wrong with the calculation and it's very simplified, I will update it if I find anything wrong with it.

But still you get an idea about why it's better to use dithering instead of dark image calibration under some circumstances.

There are noise added even with dithering and one source of it are that different pixels are used, and they're not exactly the same, it's a static error but since dithering is random it appears as a noise. But anyway, you always use different pixel because the tracking normally don't is that exact. Some cameras are more suitable than others. Specifically, the static errors should not be in big areas, small in the area distribution. There should not be any amp glow as in old cameras. Pixel to pixel variation should of course be low as well.

Here you can download the Excel file above if you want to put in your own values for a test:

You download and use this file at your own risk, I can not take any responsibility if something happens.

Here you can read why we get noise from the object and background itself:

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