[Shiraz (Ray)]

Originally Posted by leinad

So whether it's 20hours of short 5min exposure stacks, or 20hours of 60min stacks, they can never be equaled.

My understanding of why summed short subs and one long exposure can be the same is that the primary broadband noise is due to shot noise - that comes as part of the signal-plus-background and you get the same number of photons (target, background and noise) in a given period however you choose to collect them. This argument falls over if you have a camera with high read noise - then the read noise can add to the overall noise and make the SNR worse for multiple short subs. However, it is often still possible to use relatively short subs and still keep the total read noise contribution below the level where it makes any real difference.

Originally Posted by RobF

This evidence-based discussion is like a breath of fresh air. My perception of medicine and pathology for the last 30 odd years is that (often younger) specialists are now almost embarassed to express an opinion without being able to reference evidence in peer-reviewed literature. Very different to many years ago when senior specialists and their empirical knowledge were unquestioned.

Opinions from experienced astrophotographers will always be important, but evidence and sold mathematics are great to see, especially as the number of options in range of amateurs continues to expand. And the statement that there's never one perfect 'scope for everyone has never been more true.

I'm just embarassed I've never looked at a decent spreadsheet for exposure times, although I have read up a bit. Must get on with it!

Thanks Rob. I was surprised to find that there was no readily available design methodology for putting together an imaging system. Suck it and see will eventually give a result, but you can waste a lot of time and money getting there in this hobby. A decent model is a lot less costly and can provide insight that is just not available from empirical considerations. There is a place for the wisdom of the elders, but I am personally too old to wait around for understanding to come via that route.

Originally Posted by naskies

Thanks for that, Ray; very interesting. I think it shows that an appropriate image scale is critical for decent sensitivity (as we all know from painful experience or learning from others).

Ignoring the clear case of poorly matched scope/camera (KAF-8300M on a 12-inch RC...), there's not *that* much difference in total integration time amongst the other contenders - a constant factor, rather than orders of magnitude.

Object selection, such as Orion Nebula versus a mag 10 galaxy with low surface brightness, will make more of a difference. However, I'm guessing that some of those scope combinations will have a much deeper limiting magnitude for the same integration/sub time.

Yes you are right Dave (crikey that sounds like HAL from 2001....) - there is not such a large difference between the reasonably well sampled systems - image scale sure is critical.

I am not sure where the concept of limiting magnitude fits in, since it seems to generally be applied to unresolved objects and all of the modelling to date has been on extended objects - will have to do a bit of reading.

Regards ray