RE: ATM 18 inch telescope

["Good, Donald" <dgood@aha.org>]

Search the ATM archives at http://astro.umsystem.edu/atm/ for Mersenne which
is another type of folded Newtonian that uses a convex parabolic secondary
to transfer the image down to the bottom with the help of a smaller
secondary (refractor) scope to refocus the image.  "Hole in the primary" or
"diagonal tertiary" are options for placing the final image location.  The
advantage is that the secondary obstruction is the same as or smaller than
(if you can live with the side affects) a normal diagonal secondary.  You
can even get a constant height, sit in a chair, comfortable viewing location
with one of these arrangements.

Don