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Yeah, the original person I saw doing the tiny exposures was working with low read noise uncooled cmos cameras. I saw another guy who got a good result (only saw the one) using a camera which, according to what I could find during a google search, had in the area of 10e- of read noise, which made me wonder what I could do with a 5.4e- cooled ccd.
So it looks like Ray was on the money and this was a measurement error. Craig Stark calculates SNR in his article by mean / stddev, which is exactly what I did. However, just then I stacked without any normalisation, and without any rejection (so a straight average) to see what that would do to the SNR, and at that point I saw a stddev that was 4x the mean... so I don't think I can trust these numbers.
So now what I don't understand is, when read noise limited, why would fewer, longer (but still read noise limited) exposures always be better than more, shorter exposures?
The whole reason we stack images is because noise in random but the signal is "more constant" (obviously poisson noise comes into it) so we can "average out" the noise, so I don't find it that intuitive.
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