[ATM] Cassegrain Secondary Magnification

["Alfredo Rainho Neves" <arainho@snowsoft.biz>]

Hi Richard,

Ok. I had not realized the field of curvature problem! In relation to the
secondary, I could just make a D-K which would make making the primary a bit
easier and the secondary would just be spherical. This is to be a visual
scope for planetary work; what are the limits in field of curvature that
would be acceptable visually?

Regards,

Alfredo



-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net] On Behalf Of
Richard
Sent: Sunday, February 24, 2008 3:39 PM
To: Alfredo Rainho Neves
Subject: Re: [ATM] Cassegrain Secondary Magnification

Hi Alfredo,

Sunday, February 24, 2008, 5:52:10 PM, you wrote:

ARN> I am trying to design a short tube, long focal length cassegrain
(planetary
ARN> scope) and would like to know what are the disadvanteges of having high
ARN> secondary magnification factor. For example what would be the
disadvantage
ARN> of having an F1.5 mirror in an F15 cassegrain (10x secondary
magnification)?
ARN> I do realize that an F1.5 parabolic primary is a challenge. Anyone care
to
ARN> comment.


I've not even looked aberrations, but the two things that leap
straight to mind are Field Curvature and  manufacturability of the
Secondary.

Take a 10" scope with your parameters above. The Field will have a
radius of curvature of 2.3 inches, which is very poor. Also, the
secondary is going to F/1.5 too, but convex and hyperbolic so it will
be a very difficult object to make and figure.

-- 

Best regards,
  Richard in the UK

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