[Shiraz (Ray)]

Was faced with a few subs that had brighter sky backgrounds than normal and had to decide if they should be included in the stack. How to decide? If they were too bright, the extra target signal contribution would be wiped out by the excess noise from the bright sky, but if I did not include them I would be missing out on some extra target signal.

Did a simple model and, if read noise is controlled (subs are long enough) then the rule of thumb is to include bright sky subs only if the sky background is no more than 2x the average of the normal sky background.. Note that this is the sky brightness above the bias pedestal produced by the camera.

Algebra shows that this result is general for a wide variety of exposures and provided that the number of bright subs is less than about half of the total.

The attached table shows the ratio of:
SNR with bright subs included /SNR with bright subs discarded
the assumption is that there is a set of 50 subs in total and that between 1 and 20 of them (the first column) have sky brightness higher than the others by factors of 1.5x up to 10x. Where the result is >1, there is an advantage if you include the bright subs (blue), otherwise they should be discarded (pink). For example, if there were 10 subs out of the 50 with brightness of 4x the average and you included them in the stack, the SNR would be reduced to 88% of what it would be if the bright subs were simply discarded - only the 40 darker subs should be used in this case.

This would also seem to give some guidance on imaging when the moon is up, but that will need more work.

thanks for looking - hope it is useful and would appreciate any feedback on validity. regards ray