[Shiraz (Ray)]

A generally accepted rule for choosing the sub length is to make the subs long enough that shot noise in a sub (from the sky background) will overwhelm the read noise that is added at the end of the sub exposure. A widely used equation for determining an appropriate sub length is that from Starizona, which is:

Sub length = constant*(readnoise*readnoise)/skyflux

The key thing about this equation is that the sub length is proportional to the square of the read noise (for a given sky flux). To put some numbers on this, if a system using a camera with a read noise of 10 electrons requires 10 minute subs, a similar camera, but with a read noise of 5 electrons, would only require 2.5 minute subs for the same conditions (half the read noise requires a quarter the sub length). You could use longer subs, but you wouldn’t gain anything practical in signal to noise ratio, since read noise has already been taken out of consideration once subs are longer than 2.5 minutes.

The new breed of low read noise chips (eg from Sony) really can operate very effectively with short subs – the game is changing.

The other thing to note from the equation is that the sub length is inversely proportional to sky flux – if the sky is bright, you can use shorter subs than if it is dark. Shorter subs do not help with SNR under bright sky, but they do not make it any worse - the shot noise from the sky is so high that it doesn’t matter if you add a bit more read noise.

All this goes out the window with NB - the sky flux in the equation above will be very small and the basic rule is that subs should be as long as possible - low read noise chips will have better SNR, but even they can benefit from long subs in NB.

thanks for reading - discussion welcomed. regards ray