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Okay, I've got 250x60s images of NGC 4038 (Antennae Galaxies) so I did crop around two small background galaxies and the fainter antennae. I've uploaded 6 images which, if uploaded correctly should be:
250x60s
100x60s
50x60s
25x60s
10x60s
5x60s
What I've done with these is stacked the cropped area, done a LinearFit on them with respect to the 250x60s and then done the same stretch on all six. Every stage is roughly a doubling. The 5 stack was a Percentile Clip while all the others were a Winsorized Sigma Clip with the default settings. In each rough doubling there is a clear increase in contrast and smoothness while a big drop in noise. Even between 100x and 250x there is a big decrease in the amount of background noise and the faint arm stands out from the background considerably better. Going to 500x would make this even smoother.
I dither every sub with either a Medium or High dither movement in SGP so there is a good statistical sampling. I picked the middle Lum sub (#125) and the background ADU value is ~350 on the CALIBRATED subs.
My LRGB imaging is done at Gain 80 with the QHY163M (the same as Gain 76 with the ASI1600). This gives me a Read Noise of 1.9e- and a Gain of 1.9e/ADU. With LRGB imaging you want your background to be about 10x read noise while with narrowband trying to get to the 3-5x is good but 10x is still better.
The equation is.
(((RN^2)/Gain)*ARN)*AMP
RN= Read Noise in e-
Gain= System Gain
ARN= Above Read Noise (eg. 10x)
AMP= Amplification from 12/14 bit to 16-bit.
The amplification is the difference. To get from 12-bit to 16-bit you do 2^4 which is 16. From 14-bit to 16-bit you do 2^2 which is 4.
(((1.9^2)/1.9)*10)*16 = 304 ADU
As I was getting 350 ADU that means I am 11.51x read noise which is a good sampling.
My point of showing this that stacking 50x various exposure lengths doesn't really help at all as long as each exposure is around the 10x read noise mark. If you're shooting at Unity you will have a RN of ~1.6e- and a Gain of ~1. This means that all you need to do is have your calibrated subs at ~410 ADU and continue at whatever exposure time that is, whether it be 60s or 480s. Doubling or tripling the exposure time required to each that point don't likely do anything other than start saturating more and more stars which will cause havoc when it comes to stacking everything together.
Also, as changing the Gain on your camera only changes the Full Well Capacity and the Read Noise, all you're changing is what background ADU value you need and how quickly your pixel wells fill up.
In short, you are best just finding what the best exposure time is for whatever Gain setting is, your filter and location and sticking to that.
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