[ATM] Classical Cassegrain Design Tradeoff Help Needed

[mpeck1 at ix.netcom.com (Michael Peck)]

At 12:52 29-01-04 -0700, Gerald Boutin wrote:
>I am (slowly) moving ahead on design of a Classical Cassegrain telescope.
>The design is for a 350 mm diameter F/14.5.




>What it boils down to is that I don't have a good feel for what effect
>varying the flatness of the focal plane has on visual observing. In other
>words, how flat of a field curvature do I need to get nice sharp points over
>the maximum 1/2 degree field? Although I have done a bit of Googling on eyes

Gerald:

My 12.5" (31cm) cassegrain has virtually the same parameters as what you're 
proposing: the primary is just under f/3.9 and the system is about f/14.7. 
That makes the net field curvature just slightly greater than your f/4 - 
f/14.5 design.

With my 55mm Plossl I can just perceive that stars aren't quite pinpoints 
at the edge of the field if I look there. With my attention at the center 
of the field the edge looks sharp to me. Basically I find the off-axis 
performance of a classical cass to be much more esthetically pleasing than 
a newtonian with the same OTA length, which is going to have considerable coma.

Keep in mind too that a not so well corrected eyepiece has substantial 
astigmatism and field curvature in general.

I'd make design decisions on other factors than field curvature, if the 
telescope is going to mostly for visual use.

Mike Peck


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