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The other thing to also consider with a really fast scope, despite what some US opticians will tell you, is that it's a lot harder to get a sub F4 mirror to the same standard as a > F4 mirror every time. Despite the reputations of a couple of US mirror makers, I've seen sub F4 ash trays from a couple of sources and in both cases the mirror maker wouldn't accept the fact the mirror wasn't up to standard. Suffice to say there's a really good reason Carl Zambuto will not make sub F4 mirrors. That having been said, I've also seen a couple of outstanding F4 mirrors, but there's more pot luck going faster than F4, particularly if the mirror maker is 10,000 miles away and won't acknowledge the problem.
A sub F4 mirror is also exponentially more expensive than a mirror slower than F4.
Cheers
John B
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