[Shiraz (Ray)]

I doubt that anyone has seriously proposed 36,000* 1 second subs. However, let's look at something far more practical to see what is going on.

With a camera having a RN of 10e and using 1 hour subs, the total RN over a 16 hour period will be SQRT(16)*10 = 40. If you have a camera with a RN of 2e and take 1 hour subs you will get a total RN of SQRT(16)*2 = 8. the total number of photons detected will be the same in each case as will the shot noise, so the low RN camera will provide higher SNR.

Now try shorter subs on the low read noise camera. If you take 96* 10 minute subs, the read noise is SQRT(96)*2 = 19.6. ie with subs of 10 minutes, the total read noise is still less than that half that of the camera with 10e RN and 1 hour subs. The camera is still exposed for 16 hours so the number of photons detected and the shot noise will both be the same as before. Even with 10 minute subs, the low RN camera will provide better SNR than the 10e camera with 1 hour subs.

Clearly there will be a crossover point where total RN of the 2 cameras is the same. That will be when you have 25x as many subs with the 2e RN camera as with the 10e RN camera. If you have the same total exposure time, you will still have the same signal and shot noise and the 2e RN camera will produce identical SNR to the 10e RN camera - but with subs that are 1/25 as long.

Low read noise does not allow ridiculous results and it certainly does not reduce the total exposure time by orders of magnitude, but it does allow very much shorter subs to be used than previously. The situation for sky limited performance is even more striking and very short subs can be used in some circumstances. However, there is no gain in SNR available with properly exposed sky-limited subs, just the ability to use very short subs.

edit: in your terms, from the dim target equation and the same total exposure, a camera with 2e read noise and one with 10e read noise will have equivalent SNR when:

QP*(T2)*(sqrt(N2))/(R2) = QP*(T10)*(sqrt(N10))/(R10)

now (N2)*(T2) = (N10)*(T10) for the same total exposure, so

(T2)/(R2*R2) = (T10)/(R10*R10) or

T2 = (T10)*(R2*R2)/(R10*R10) = (T10) /25

ie, for equivalent SNR and the same total exposure, the sub lengths can be 1/25 as long with the 2e RN camera