Oh, and you can also determine the theoretical required exposure length if you have a general idea of what your skyfog flux is (and this is something you can measure over time to get an idea). Exposure length is the required number of electrons divided by the flux. So, in the case of say a 5e- read noise camera being swamped by 10x, with say a 0.1e-/s photon flux (which might be a bit high, even, for a dark site or narrow band imaging):
Exp = Nread^2 * Swamp / Flux
Nread = read noise in e-
Swamp = swamp factor
Flux = background sky flux in e-
So, for 5e- @ 10x and 0.1e-/s:
Exp = 5e-^2 * 10 / 0.1e-/s = 250e- / 0.1e-/s = 2500s
You would need to expose for 2500 seconds, or almost 42 minutes, to swamp the read noise by 10x. If you went with a 3x swamp factor instead:
Exp = 5^2 * 3 / 0.1 = 75 / 0.1 = 750s
You would need to expose for 12m30s if you only swamped by 3x. Now, lets take the ASI1600 at unity, with 1.55e- read noise, swamped by 10x, at say half the flux (smaller pixels):
Exp = 1.55^2 * 10 / 0.05 = 24.025 / 0.05 = 480.5s
Significantly less time than 2500 seconds to swamp by 10x. And if we swamped by only 3x:
Exp = 1.55^2 * 3 / 0.05 = 7.2075 / 0.05 = 144.15s
Significantly less time than 750s to swamp by 3x. Now, I don't really recommend aiming for only 3x unless you have to. For some cameras with 7-8e- or more read noise, you might indeed have to go for less swamping just to be able to get the exposure at all (otherwise you might end up exposing for hours on end for single subs!) Not to mention the practicalities of dealing with airplane, satellite and meteor trails, hot pixels and remnant FPN, column defects, etc. all of which require dithering and pixel rejection, which works best when you have more than just a few subs. With low read noise cameras, getting at least 5xRN^2 should be easy enough, and 10x is certainly within relatively easy reach (just watch your clipping.) This is where CMOS as the potential to be better than CCD, in the ability to swamp the read noise by a lot, without having to use very long exposures or risk losing hours of data due to trails or wind or other issues.
Also keep in mind, variations in LP, light cloud cover passing through, etc. will all affect background sky level as well as transmission rate of deep space photons. So there will be variations in each of your subs around these levels at a given exposure length. Once you figure out your average flux, then you can determine your ideal exposure lengths, and once you determine those, stick with em. You will have some subs that are brighter, some that are fainter, just the name of the game. In the end, though, they will all be normalized and the differences will be rejected and averaged out.
Edited by Jon Rista, 07 November 2017 - 12:21 AM.