Recently there are a lot of interests and discussions in short exposure “lucky/high resolution” deep sky imaging. Very good results had been obtained and people have different opinions, but there is a lack of subjective evidence why it works, what are the trade-offs and how to optimize this technique. Personally, I think the best way to compare or optimize this technique is to be based on total integration time and the final signal to noise ratio in the finished pictures. Fortunately, there were similar discussions 10 years ago base on CCD technology at the time, and a few wise men has already figured out the math. Steve Cannistra had a nice write-up on this topic, still pretty relevant today’s topic. He simplified the calculations to a very simple formula: tsub = F^2*R^2 / [Sky*(1-F^2)]. Where F is the ratio of picture quality (S/N) of the finished image to that of the ideal achievable for an object with a given total integration time and sky condition. The beauty of this method is that it is total integration time based subjective image quality comparison with varying sub-exposure time. Details please read his write-up at: http://www.starrywonders.com/snr.html
Putting this formula in a spreadsheet, the attached chart shows how sub-exposure time affects final S/N ratio for today’s low read noise cameras. The sky flux used is 80e/min, estimation of my light polluted suburban night sky.
As you can see, for a camera with read noise of 1e, ~3 seconds of sub-exposure can get 90% of S/N ratio of that of traditional longer sub-exposures with same amount of total exposure time. 0.7s(!) sub-exposure will still be able to get 70% of S/N. Most of the cameras are not there yet, but a few of the CMOS cameras are pretty close.
However, short sub-exposure has trade-offs: all these cases will result in F<1, which means a reduced S/N ratio, that will need increased total integration time to compensate. F>=0.9 is maybe negligible; it is reduced picture quality nerveless. Is this a reasonable price to pay? Personally I think if shot exposure can increase spatial resolution, then it is more valuable than increased integration time. I can always add more subs another day, but cannot increase resolution after it is taken. Expensive mounts required for long exposures have a price tag also.
Limitations of these calculations: thermal noise not considered; assume dim objects, sky flux >>subject flux