It took me while to find it, but off-axis reflectors are a minority
sport for ATMs.

The Schiefspiegler or Kutterscope is a typical off-axis reflector, and
at long focal lengths can use spherical mirrors and has perfect colour.
But at large apertures they become more difficult to make. Seem to have
been very popular in Germany and Holland (where i first came across
them).

I would recommend a look at Dave Stevick's excellent Weird Telescopes
page for more info about off axis reflectors.
http://bhs.broo.k12.wv.us/homepage/a...vick/weird.htm

And good software if you want to design your own.

Enjoy,

Callum