Quote:
Originally Posted by Bassnut
. Are you saying I need to increase subs to several hrs for best S/N?.
Have I missed something?, my 50 ADU background after calibration, is that what you mean by "above bias"?.
What about blooming on an NABG cam!. Several Hr subs would be unusable?.
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yes, yes and yes.
I am not saying this method is necessarily useful for all NB applications (it derives from LRGB considerations), but it is still correct - the basic rule for NB is "as long as possible".
If there are other competing considerations, then clearly they must be taken into account. However, the suggested method
will give you the best possible SNR for any equipment. If you cannot use long enough subs for other reasons, you will be losing out on SNR due to the read noise. For example, your quoted sub background of 50 ADU will presumably correspond to something around 50 electrons. the shot noise will therefore be about 7 electrons, which will be a lot less than a single read event with a typical big pixel camera - your system is dominated by read noise. The only thing you can do to improve the SNR (apart from increasing the overall imaging time) is to decrease the number of reads by increasing the sub lengths.
Just for interest, have you ever tried really long NB subs.
regards Ray