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Ray,
I assume we are talking about sky limited exposures ? and that this is about DSO style imaging where we have faint targets with relatively low flux levels.
Re " . . . to make the subs long enough that shot noise in a sub (from the sky background . . .) "
Is it the Shot Noise or is it actually the Sky Noise itself that we are using to hide the Read Noise ?
I think its the Sky Noise
I'll have an attempt at this !
The purpose of determining a sub exposure time (using this method) is usually to provide the imager with a minimum exposure time that satisfies the purpose of reducing as much image noise as one has control over - that is BTW after all the normal image image calibration and gear tuning has been done.
Its not "The" time or the best time or even the maximum time - its just the minimum time that satisfies a particular basic goal given the local sky conditions, the imaging system and the camera system.
Afterall - there are many other constraints or preferred goals that might also dictate a longer or shorter exposure time for a given situation !
More specifically, since we cannot control Sky Noise at our given site (other than by relocating !), the goal of producing a Sky Limited exposure is to minimise camera Read Noise as a significant contributor of total image noise.
ie to reduce or minimise the effects of noise over which we have some control compared to the noise over which we have no control.
This is achieved by limiting Read Noise to about 3-10% of sky noise so that the total contribution of Read Noise which is a 'fixed' constant determined by the cameras CCD and electronics is swamped by the higher level of Sky Noise over which we have no control.
The 'fixed' value of Read Noise, as provided by the camera manufacturer, is always present no matter how short or long the exposure, but it isn't truly fixed since its also random in nature.
By using statistical methods with multiple exposures in our stacking we can also reduce the effect of Read Noise, since its not scaled with the exposure times and nor is it consistent per pixel across multiple exposures - ie its random
So this process is just another tool to use to improve our SNR, along with our normal calibration processes and everything else.
As mentioned if you want faint detail and the flux rate is low, then you have no option but to expose for very long times just to capture enough signal to get above the read noise or even get above the quantisation levels to even register as one bit !
So of course more bits of faint flux data is needed to get more detail - is a given, rendering this minimum exposure time as meaningless.
My 2c
Rally
PS
Greg,
Passing clouds at a 'Dark site' dont add noise they only reduce signal and therefiore dont really affect your image after normalisation !
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