Focal Ratio

[GlassWalker]

I... want... one... but why do I get the feeling that's well into the "if you have to ask, you can't afford it" territory?

[Naemeth]

For visual, the only reason for a fast system is to make it compact, for example a big dobsonian which can be used without a ladder. Such systems are demanding for eyepieces and collimation, and a coma corrector is needed.

For imaging you can gather a lot of light and cover a wide area. The new TEC 300VT, 300mm f/1.44 astrograph giving a 7-degree field is astonishing! See:

http://www.baader-pl..._300mm_7deg.pdf

That is amazing! I do also get the feeling that if you do have to ask the price, you can't afford one. I think people are probably offered to buy one. 41kg is a serious load on most mounts, but I'm sure the Mesu Mount 200 could support it.

My initial train of thought was intended to spark a bit of discussion as to what has already been done or what is possible and it's limitations, and it seems to have done that. I consider myself much more enlightened, mind you, I'd probably have to at least do a degree in Physics to understand a few aspects of Optics that I do not know currently... or I at least get that feeling!

[FraserClarke]

I suppose it's do-able with enough money, isn't the new E-ELT an F/1 Newtonian?

No. The E-ELT primary mirror is F/1 (actually, I think it's now a bit faster, about F/0.9) -- but there is no 'prime focus'. The system f/ratio is F/17.5 by the time you get to a focus.