Quote:
Originally Posted by Atmos
Combining all of the information that has been discussed thus far basically says that for the absolute best SNR is having a low RN camera with large wells coupled with the longest exposures possible on a target. Take the iKon 231, 2.1e- read noise and 350k e- well depth, easy to take 1 hour subs with very low read noise, 10 hours with this camera and you'll have some of the deepest data possible. You can do 10x1h subs and not even come close to saturating on something like the Helix.
10x 1 hour subs at 2.1 e- leaves a 6.6e- RMS where as a 400x90 with the ASI at 1.3 e- has a 26e- RMS.
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But I think that you need to consider sky noise Colin - the conclusions from M&T's broadband analysis apply in NB if the read noise is low enough.
at Ha, a fully dark sky produces about 0.5 p/s/m2/arcsec2/nm.
lets make up an appropriate scope for the ikon231 camera - with aperture of 1m2, sampling of 0.1arcsec2, 3nm bandwidth and assume 50% optical efficiency (including QE). if you evaluate the photon flux per pixel using those values, you end up with 0.75 photoelectrons/s/pixel for the sky noise.
Now the camera read noise is 2e. The sky noise will overwhelm the read noise when it is about 3x as large, ie to be dominant, the sky noise needs to be 6e rms. You get that noise from 36 photoelectrons. And at 0.75 photoelectrons/s, you get 36 photoelectrons in less than a minute = you would then be sky-limited on any longer subs.
ie, you would not be able to get any advantage from NB subs longer than about 1 minute with the camera in question on a reasonable telescope. (for interest it would be 20minutes at 10eRN)
An ASI1600 reaches the Ha sky limit with a 10 inch scope and 0.75 arcsec sampling in about 10 minutes at high gain under average dark sky. When it is sky-limited, it will go just as deep as any other sky-limited system (including one based on the ikon231). It just takes less total time on a big scope. Very low read noise CMOS chips really are a game changer and we are just starting to scratch the surface on what they can do - even the ASI1600 is way more than a budget entry-level camera, even though it doesn't cost very much.
ref:
https://www.gemini.edu/sciops/telesc...sky-background