Best number of subs

[Russ]

I'm in Cartagena right now, starting a three week trip in Columbia and surviving with my iPad. I'd like to reflect  a bit on the very interesting results and post some thoughts in the beginning of January. I think that my fellow users of remote telescopes are especially eager to ruminate on this topic, because most of us cannot afford to use this sophisticated and expensive equipment for hours at a time.

[mcgillca]

By coincidence, I've just been working the sub length using the noise to 5% level rule of thumb (drowning read noise in sky background noise). Turns out that for my site (I'm luck - its dark - 21.9mag/arcsec^2) at full moon, with my 8" RC and Atik 460, I would need a 10 hour sub to make this rule of thumb, which is clearly not possible.

I then calculated the difference in total noise in a 4 hour exposure if I took as a series of 10 minute exposures, or one 4 hour exposure - turns out, I can reduce the noise by just two electrons if I did this  out of a total of 40 electrons of noise (95% coming from Poisson noise in the sky background signal).

Given a 27" would have about 12 times the collecting power of my 8", I think a 10" sub is more than long enough! I doubt there will be little practical difference in the noise levels. Redoing the calculations for a 27" RC, I found a reduction of 0.25 electrons out of a total noise of 50 electrons caused by the Poisson noise in the background signal if you took 1 40 minute sub rather than 10x4 minute subs.

I'm sure the images will be wonderful.

Good luck!

Colin