Imaging types: stop reading now. The visual observers might like to read this link and ponder the implications of the expansion rates of the ED and fluoro glasses being 18X worse...
http://neilenglish.net/why-the-class...stable-images/
Personally, the most memorable telescope I have ever used was an 1880's Cooke refractor, with a superb 4.25" f/16 objective that produced absolutely perfect textbook diffraction images at high power. I've used much bigger scopes since, but the sheer perfection of that Cooke refractor was a real pleasure.
My 102mm f/7 ED refractor is good enough to show nice Airy disks but its nowhere near as good as that old refractor. Now I understand why.
Maybe there is still some point in owning an f/15 Unitron !