Calibration frames

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Nick James

Hi Sheridan,

It is true that a ratty dark frame will add noise to the output image but you don’t use ratty dark frames. Since darks are much easier to obtain than light frames you can collect hundreds of them when it is cloudy and median stack them. You then use the clean dark in your calibration process. The same applies to flats and biases. With a DSLR flats are very easy to obtain since you can use short exposures on a bright sky, I even do them in daylight. It is true that if you median clip stack dithered images you can get away without darks but you still need to calibrate the light frame so that you can properly apply the flat. If you don’t you can get significant zero point errors, particularly if you are stacking a lot of short exposures. You can possibly get away without using a flat in dark locations but not where I live.

All of this may be too much for beginners, although programs such as Deep Sky Stacker make it easy to do, but my problem with Graham’s website is that he appears to dismiss calibration altogether and I think that is wrong.

Going back to the original subject I have read David’s review again and it seems perfectly reasonable to me. A book review will always involve some degree of personal opinion and the Journal would be a much worse place if we only ever published positive ones.

Doing anything generally lends to the risk of criticism. That is part of life and it is something that you get used to.