Quote:
Originally Posted by Atmos
From that graph it looks like around the 130s mark would be a sweet spot.
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At 130s you've definitely got the lion's share of the SNR curve going for you. Because I'm lazy and the chart already shows 144s instead of 130, I'm going to use that as the basis for the following.
144s gets you 94% of the SNR of a 240min sub.
96s gets you 91.3% of the SNR of a 240min sub.
50% extra sub length for 3.7% extra SNR - I'd personally rather take the shorter sub.
Having said
that, it's not all up side for shorter subs either. I suspect it may be trickier to process (signal you care about is closer to the noise floor), and it means a lot more exposures, so more storage, more computation to process etc.
Edit: Didn't see your edit before my post. Very good point. That's something I've been thinking about and haven't fully factored in. I lost a huge amount of time on the first night due to (1) stuffing up the backlash on automatic focusing, making it do multiple runs and (2) auto focusing too frequently given the new sub length. When combined with extra download time and dithering/settle time, it made a really big difference, and not a good one.
My camera reports 2sec download time at full res, though I haven't timed it. I settle my dither for 5 seconds < 1px... definitely wouldn't be more than 10s per frame for dither+settle.
So lets forget about AF, which I can do as a function of time rather than sub count, and say each 96s net sub costs 96 + 10 (dither) + 2 (download) = 108s. 8hrs nets me ~267 subs. 267 @ 96 = 25,632s of net integration time, out of a possible 28,800, or 89% efficiency.
And if I go back to 480s net subs, 480 + 10 + 2 = 492. 8hrs nets me ~58 subs. 58 @ 480 = 27,840s of net integration time, out of a possible 28,800, or ~97% efficiency.
So compounding the loss of the "inactive time" (down 8%) and the SNR (down 6.8% from 98.1%), I'll net about 85.7% of stuff vs the 480s. I'd prefer to be around 90%, so that's a bit low and might be enough for me to up the sub length a little bit, but not much.