Re: ATM Cassegrainian? Advice needed

[astillma@nit.AirTouch.COM (Anthony Stillman)]

>Is it possible to build an f/6 Cassegrain?

By this I assume you mean f/6 EFL (Effective Focal Length).  Yes, its
possible.

Either your primary has to be very fast or your secondary magnifacation
must be very small, or both.  Say f/3 with m=2  or f/4 with m=1.5

Coma for a Classical Cassegrain is the same as that obtained with the
same focal ratio of the Cassegrain's effective focal ratio.  An f/6's
coma is not that bad.

Field curvature can be controlled by your choice of mirror radii.  Infact
if you make the radius of curvature of the secondary equal and opposite that
of the primary your field will be flat.  Unfortunely, even small
values of m will result in both very long back focal lengths (the distance
from the secondary to the focal plain) ((no doubt some will say the bfl is
the distance from the back of the primary to the focal plain, but thats
not what I mean here)) and large secondary diameters.  The later being bad
for image contrast.  I have a flat field cassegrain I'm working on now, but the designed 
use is forgiving to large secondaries.  Still you should be able to 
select a system with resonalable field curvature.

Astigmatism.  This is the killer in cassigrains where coma doesn't kill
you first.  Astigmatism is m times worse than a corresponding Newtonian.
For an f/6 coma will be your enemy not astigmatism, especially if you keep
your multiplier small.

Really, the most dificult problem I believe you will incounter in making
the glass is the deformation coefficient of the secondary. 
If I remember the formula correctly, it is ((m+1)^2/(m-1)^2). 
Also, placement of the components, that is the C-word, collimation could get
a little tricky. Yes, everyone I know I'm understating this.

So, Diego, go read Texereau (How to Make a Telescope), then design your scope.

Good luck
Anthony