A key method in astrophotography is taking multiple exposures (sub-frames or subs) and stacking them to improve SNR. Which begs the question - given a desired total exposure T, how do you divide that up into multiple exposures.
I should state up from that I am mostly interested in recent DSLR, but of course this does apply to dedicated astro cameras too. I use astro modified Canon 5D mark IV, and QHY 128c, which although cooled, is a Sony DSLR chip with one shot color. These are both low read noise cameras.
In addition to optimal exposure time, there is the related question of optimal ISO setting.
I recently got interested in this issue. I somewhat naively assumed this would have been figured and settled long ago.
Some aspects of exposure constraints are fairly obvious. If you have a fixed mount, and you don't want star trails, then your exposure time is limited by apparent motion of the stars. You can use of the "hundred rules" - i.e. divide 600 by focal length. Or you can calculate the transit time of a star image across a pixel.
Another obvious aspect is that if there is light pollution you must not expose until the image is washed out white, or you will lose detail.
More generally, there are different sources of noise - read noise, photon noise, dark current noise, and potentially others - that one has to worry about.
This issue has been discussed a lot in this forum. Here is an article from 2006 https://www.cloudyni...-in-dslrs-r1543 by Samir Kharusi.
This 2007 article https://www.cloudyni...-exposure-r1571 by Chuck Anstey references an earlier study by John C. Smith, but the link to the Smith paper is dead, and I haven't found the original. Anstey gives a formula that sub exposure time S = lambda* Sqrt[Ttotal]/(2 Etotal)
Where lambda = 15, Ttotal is total exposure time in minutes, Etotal is total noise.
Roger Clark's website has this analysis of sub-frame exposure https://clarkvision....y.and.exposure/, which appears to date from 2016. Clark gives some spreadsheets, but his bottom line recommendation is very simple - choose and exposure such that the histogram on the back of the camera (i.e. after gamma correction) is between 1/4 and 1/3 of the scale, which it states is roughly 3% of the RAW reported ADU. This assumes "modern" cameras with 2 to 4 electrons of read noise.
This analysis was also presented in a 2016 thread on dpreview http://www.dpreview....m-post-57610694. The thread starts off being technical and then devolves into a bitter argument, chiefly between Clark and Jon Rista. The argument seems mostly to focus on issues that were tangential to the sub-exposure issue. So it is not actually very useful - to me anyway - in understanding where they differ on the issue of optimal sub frame exposures.
So, it was a raging debate in 2016, at least for some. More recent accounts, like this one https://www.amateura...which-is-better lay out issues but don't say very much.
I found this 2020 thread https://www.cloudyni...e#entry10302433 , which recommends a sort-of-formula in words:
Shoot a light and a bias. Subtract (either the average value or the obvious skyfog peak, doesn't matter), get the corrected analog to digital units. Using camera data, convert to electrons. Get the read noise, which will be in electrons. Square it. You want the first number to be between 5-10X the second.
This is references to The Astrophotograpy Manual by Chris Woodhouse.
Another post references this article series on Cloudy Nights https://www.cloudyni...ur-camera-r1929
Finally, one of the posts on that thread essentially repeats Clark's recommendation of histogram peak at about 1/3 of range.
So, is that the state of the art?