I wish to go deeper with my images but not at the expense of washing out the background. I'm using the ASI6200 camera & I get good results on the C14 at F11 using 60 seconds at 2x2 bin gain 0 offset 50. My Sky is no better than Bortle 7 and perhaps closer to 8.
The question is- do more subs at the same exposure go deeper or does the faint fuzzies captured remain the same but the noise around them decreases?- its hard to tell if more of the same is better than less at a greater exposure.
Please let me know what your experience has been- more subs the same or less subs but greater exposure length with the objective of getting deeper data.
First, better SNR and a “deeper” image are generally synonymous since better signal to noise let’s you stretch more aggressively before the image starts to fall apart.
As to optimal sub exposure duration... I can only give general advice since there are too many variables, but general advice may be enough with this camera. You want your exposure length to be high enough that shot noise swamps read noise. With CMOS vs CCD that usually means a shorter sub since CMOS cameras generally have lower read noise. Under Bortle 7 skies it doesn’t take too much before your histogram starts to move to the right, so you may be fine at one minute. Your camera has a large enough full well capacity that you could probably go quite a bit longer at f/11 before saturating much, so you might do a bit better with longer subs, but if read noise is already much lower than shot noise at 1 min it’s not going to make much of a difference either way. The main advantage you’ll get from longer subs will be less storage and processing. Biggest disadvantage would probably be not as good pixel rejection from fewer subs. Try both ways one night and see. Or use something like PixInsight’s sub calculator to check to see if your shot noise is dominating.
One other thing I’ll mention is that, at f/11, you are probably still oversampled quite a bit at 2x2 binning. That’s about 0.4” per pixel. Unless you have truly exceptional skies or it’s an especially good night with an object high in the sky you might do better at 3x3. I know it feels like throwing away a lot of pixels, but right now they are probably capturing “empty” resolution and just increasing noise. Again, try for yourself. Take two subs, process to taste, and at the end up-sample the 3x3 to the same pixel size as the 2x2 and see if you lost any actual resolution. I suspect not, and Th e SNR will be something like 1.5x better—roughly the same as doubling your integration time.