John Bambury wrote a great little history of most types in
this post, part of a
thread about Plössls.
http://www.iceinspace.com.au/forum/s...30&postcount=6
FWIW the two cheap UO Kellners I have had for nearly 30 years perform
beautifully in my 8" F7 Homemade Newt. Shorter FL scopes, not so
well, as Wavy has said.