More questions: I need to account for gain, right? 110 is 11 dB of gain according to ZWO's chart, but I seem to recall that that's right about unity gain for a 183. So color me confused again. And what about bias -- should that be subtracted from uncalibrated images? (It was set at 8.)
And this is a 12-bit ADC. Do I then multiply by 4096?
Yep, still pretty befuddled. If I just do the calculation above and use 3.1 as the gain factor (10 dB), I get something like 3 e-/sec/pixel for Bortle 7-ish skies, which seems kinda low, unless I'm misunderestimating the light pollution in my darker site. Then again it's less than an order of magnitude off, which is pretty good for astronomy!
I've been away from the computer all day. Sorry for not following up earlier.
Yes, you need to account for the gain. But you must have the sensor gain in units of e-/ADU rather than the abstract "gain number" setting from ASI. Use e-/ADU rather than db or other measure for this, otherwise the calculation will give you a misleading or wrong answer. You say you are using an ASI gain number of 111. if you look at the charts on the ZWO page for the ASI183MM-Pro, you will see that the sensor gain at a Gain number of 111 is equal to 1.06 e-/ADU. This gain is in native sensor ADU units -- 12 bits in this case. You will need to multiply this by 16 to get to a 16 bit gain number. (This assumes you are examining saved 16 bit FITs files.)
Now refer back to my procedure shown in Post #2. Use this number
(16 x 1.06 = 16.96) (1.06 / 16 = 0.06625) [corrected value] for the gain in step #6. The result of step 6 is the number of electrons in the background of your light frame exposure. Divide by the exposure time seconds as indicated in step 7 and you are done. This is the e-/sec rate of sky flux that you will want to be 10x to 20x times your dark current (which you could also manually measure rather than taking some printed value).
Regarding calibration, this method requires you to use uncalibrated frames. Do not subtract Bias from either. The light frame has a Bias signal in it and the raw dark frame also has Bias in it. Once you subtract these the Bias goes away automatically. It is best to just use raw frames directly as saved by the camera. One dark frame (of the same duration as the light) and one light frame will give you a reasonable answer for sky flux in e-/sec. That will be directly comparable to the dark current you are asking about swamping.
Since you are using the Monochrome version of the camera, don't do any deBayering (?) or other operations. It is best to use an L frame rather than some filtered frame. Just measure the result of subtracting a dark (background) from a Light (background) and then multiplying my the electric gain (e-/ADU) to get the accumulated electrons. You can then compare that to the number of dark current electrons gathered in a dark frame of the same exposure.
I hope this is clearer than my first answer this morning.
The OP was looking to swamp Dark Current not swamp read noise. Since your method of subtracting only Bias from the light leaves the dark current in the result, he is no closer to determining the amount of background he needs to swamp dark current that he wants. The purpose of swamping Dark Current is to determine just how much cooling is really required under light polluted skies.
Edited by jdupton, 26 November 2019 - 04:44 PM.