Hi Mark,
I strongly recommend reading all the relevant articles from Craig Stark ( original inventor of PHD guiding )
https://www.cloudynights.com/article...g-for-photons/
It should answer all your questions about SNR , light gathering power , focal ratio , focal length, Noise in general , Sampling and so on …
I found them most useful
Cheers
Martin
PS : Some of the information is high tech but hang in there , Craig does eventually bring things into perspective