Re: ATM Cassegrain

[epabcc@epa.ericsson.se (Bratislav Curcic)]

> You're looking at a real challenge:  twice as many mirrors, each of which
> has to be twice as accurate - at least 4 times harder than a Newtonian.
> The negative secondary is even worse than twice - you'll have to
> make a mirror just for testing it.

All this is valid, but reality isn't as scary as it may look. For
example, you may choose Dall-Kirkham configuration, which has spherical
secondary, and mild elliptical primary (less steep than a parabola).
Still not job for the faintharted. Final touchups can be done on
either primary or secondary, using star test (no addittional optics
needed). The penalty you pay is coma (several times that of an equivalent
f# Newtonian), but if made f/long it won't matter much. And this scope
is by definition narrow field (planetary).
Formulae needed are in Texerau, and perforance data in R&vV.

Don't see the point of making it as a small instrument, though. A 6"
f/longish Newtonian isn't that big, and it will make it VERY hard for
anything in this size to beat it. Once you pass the 12" region,
it starts to make sense having telescope which is not 3 or 4 meters
long.

Bratislav