ATM Why are amateur scopes so slow?

[chana@andr.UB.com (Allen Chan)]

Hi,

I have a general optics question.  Why are amateur 'scopes 
so slow compared the giant 'scopes like Keck, MMT....

I was browsing the optics lab at web page the Univerity
of Arizona.  This lab produced the giant multi-meter mirrors.
I notice that virtually all these scope were faster than f/2.
Some mirrors were approaching f/1.  The fastest mirrors in 
amateur scopes tend to be f/4-ish in large Dobsonians and 
f/3-ish in the Cassegrain primaries.

I would like an ultra compact 20" scope, but the slow primary 
mirrors make such a scope too large and bulky.


Allen Chan