Just curious - are we talking f/4 or faster or f/5? 
How fast is fast?
I think anything f/5 and lower qualifies.
f/4 seems to be the knee in the price curve. There are lots of reasonable f/4 Newtonians under a thousand, but anything faster costs much more. The other issue is anything faster than f/4 has a large central obstruction which kills contrast. Here is a good mathematical treatment on central obstructions. I think how fast is fast is a personal thing because the question is what are you willing to loose for what you hope to gain.
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I think of f5 or lower as pretty fast, but it all depends on your targets and situation
I’d follow the f5 here as well. Many are using f4 newts too, but when you start getting to f4 or faster your collimation (and other things) really becomes critical and to me it isn’t worth the extra hassle. I’ve shot with f4 and f5 and I really didn’t see that much difference between them at least not enough to warrant the extra time in collimation. Again this is just my opinion here and others may disagree with me.
Dale
Dale
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Just curious - are we talking f/4 or faster or f/5?
ASA H400 f2.4 and ASA UWF 300 f1.3. Others are slow, very slow or super slow.