Quote:
Originally Posted by RickS
I just ran the numbers on some red data for NGC1097 taken under darkish skies. I was using 15 minute subs which were still nowhere near sky limited (I'd need to do about 1 hour subs to get all the way there...)
Comparing the SNR for a 15 min sub vs a stack of 3 x 5 mins the SNR is 4% better for the bright galaxy core, 20% better for a bright/non-core galaxy area and 34% better for a dimmer area on the outskirts of the galaxy.
To get the same SNR as a 15 minute exposure in the dim part of the galaxy requires stacking more than 5 x 5 minute subs. For the bright/non-core area you need to stack more than 4 x 5 minute subs to get the same SNR. For the galaxy core, a stack of 4 x 5 minute subs has better SNR than the 15 minute sub.
Just a random example, but I think it shows that read noise can have a very significant effect on SNR. It also demonstrates nicely that this effect is much greater for dim targets.
Cheers,
Rick.
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Good post. Thanks for that Rick. I probably should go for longer subexposure legnths. I use mostly 10 minutes but if tracking is behaving I sometimes go for 20minutes. I could go longer though as both my mounts usually work very well. Occassionally minor gremlins seem to interfere but overall they probably could go one hour.
Although a 1 hour exposure is likely to get fatter stars just from the average of PE in different directions. Perhaps mixing in some shorter exposures for stars or not worth the trouble?
Well depth may be the main limitation here.
Greg.