Perfect Telescope Diagram Corrected

a nice site -

John Savard wrote:

The diagram of a "perfect" telescope at

http://home.ecn.ab.ca/~jsavard/science/opt03.htm

(well, perfect if you don't mind not being able to observe at 400x; a
small price to pay for building a telescope that acts like a
Schmidt-Cassegrain when you only have sources of parts to build
Newtonians...)

has been corrected.

The correction consists of showing tubes needed to block what would
otherwise be a rather serious stray light problem of the design.

John Savard
http://home.ecn.ab.ca/~jsavard/index.html

lid (John Savard) wrote in message ...
The diagram of a "perfect" telescope at

http://home.ecn.ab.ca/~jsavard/science/opt03.htm

Hi John,

Nice scope - but a bit too long. One other thing sacrificed here
is field size. I run design with 10" f/5 primary and it is still
better than 1/6 wave with parabolic secondary of same size (better
than
1/4 wave s.a. with spherical secondary). But the system is
f/38, with almost twice wider linear field than with an f/8
primary.

Here's more details. For the given configuration, with the focal point
at the primary, needed secondary magnification is given by m=s/(s-f1),
with f1=primary f.l. and s=mirror separation. Keeping the min
secondary
size at 15% of the primary gives separation s=146" and secondary
magnification m=7.68.

Needed secondary radius R2=2s(s-f1)/(2s-f1), or 13.27", which makes an
f/3.3 for 2" secondary diameter. Nedded secondary conic for zero
spherical
aberration is K2=(m-1)^2/(m+1)^2 or -0.59. This means that spherical
secondary
(K=0) would cause about 50% greater spherical aberration than
parabolic (K=-1).
But the system would still be slightly better than
"diffraction-limited".

An interesting idea occured to me looking at the Schwartzschild
concentric
anastigmatic aplant (inverse Cassegrain, shown as a microscope
objective on your page). Since both mirrors are spherical,
corresponding off-axis sections from both, primary and secondary
(still spherical mirrors themselves) would make for un unobstructed
all-spherical two-mirror system, corrected for spherical aberration
and free from coma and astigmatism. The only drawback is that the
secondary would be significantly larger than primary.

Vlad