Quote:
Originally Posted by Peter Ward
I've been mulling over this for a time, and maybe this is not correct.
Is read noise fixed or or random? I suspect the latter, and in fact this is exactly what "skipper CCD''s" utilise by being read-up hundreds of times to get read noise to sub electron levels.
Further, if read noise were fixed, you'd see a gradual pattern build up after summing multiple frames. To date, I've not seen that happen even after summing dozens if frames.
Assuming then read noise is indeed random, would not the noise reduce by the square root of the number of subs.  |
I don't understand what bit you find to be incorrect Peter.
I agree with your statements - read noise is indeed random and the power increases with the square root of the number of reads (ie it reduces by the square root if you take an average). There is also no fixed pattern to read noise with a CCD. However, the equation is based on the noise from a single read event, so, as far as I can see, none of the above applies.
The method is designed to provide a single goal ADU value that you can use to set sub exposure length. If you use it, the read noise contribution in the final image will be less than than 5% of the total noise (ie you bury the read noise under the shot noise). The underlying maths is a recast of that used in the various on-line calculators, but it provides a very quick and universal method for checking exposures on the fly. When you have determined the goal ADU for your camera (once only), you can use it to set subs with any combination of scope, filter etc.