[ATM] Classical Cassegrain Design Tradeoff Help Needed

[frank at katestone.com.au (Frank)]

Hi All

Are we comparing a F/14.9 Cass with a F/3.9 Newt ???

I don't think this is a valid comparison as the FOV of the Newt
would be much bigger and it would definitely have more coma.

We should compare a F/14.7 Cass with a F/3.9 Newt in which case
the Newt ( I believe) would perform better.

Cheers
Frank Q

----- Original Message ----- 
From: "Michael Peck" <mpeck1@ix.netcom.com>
Subject: Re: [ATM] Classical Cassegrain Design Tradeoff Help Needed


>
> 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.
>