From: "MARY WINSTON" 
To: rob{at}vrf.com
Cc: atm{at}shore.net
Reply-To: "MARY WINSTON" 



Guys,

The only way to end up with a flat field Ritchey-Chretien... is, (1) to
introduce a field flattening lens ahead of the focal plane... or, (2)
design a system in which the primary and secondary mirrors share the same
radius of curvature.  This means, however that the secondary will naturally
be one half the diameter of the primary, or so... I built an 8" and a
12.5", about 20 years ago, using the equal radius method... both were
F/5.6.  I used an optical window to support the secondary on the 8",
which I still have. Photos showed almost perfect images over a 40' field,
and acceptable ones out to about 1.5 degrees... they enlarged rapidly
beyond that though.





>From: "Rob" 
>Reply-To: "Rob" 
>To: "Richard Schwartz"
,
>Subject: Re: ATM Ritchey-Chretien telescopes
>Date: Wed, 26 Feb 2003 17:27:30 -0600
>
>
> > I hope you know that the field is flat only when the curve of the
>primary
> > matches the curve of the secondary.
>
>If I understand things correctly (which I probably don't lol) that is one
>of
>the design criteria for an R-C type scope.. They seem to have very low
>power
>secondarys and the radius of curvature is closely matched. Most of the ones
>I have seen have around 2.7x secondary compared to something like 5x for a
>cassagrain.. I just thought matching the curvature was part of the R-C
>design...Maybe not though :)
>
> >Everybody makes such a big deal out of R-C optics, the cass builders are
> >driven to D-K systems because they are scared away.   But I see nothing
>in
> >the R-C system that is so difficult.  It is certainly no more difficult
>than
> >the parabolic primary and hyperbolic secondary of a conventional
>cassagrain.
>
>This sounds encouraging. The only thing I'm really concerned with at this
>point is trying to grind the secondary. Maybe I should try to buy the
>secondary???
>I've never tryed to grind a cassagrain secondary. Maybe this won't be as
>difficult as it seems...
>
> >However, I do suggest that you keep the project to yourself until it is
> >done.  If anybody asks, just tell them it is a D-K.   Otherwise, you
>invite
> >ridicule and discouragement.   Also, while you are at it, get all your
>old
> >ATM books that have discouraging words about R-C... and BURN 'em.
>
>I'm a loner for the most part. So theres not really many people around to
>tell..Except you guys :)
>I think maybe I'll just burn the pages that have discouraging info on R-C
>scopes.. The rest of the books seem to have useful info I don't want to
>lose
>hehe :)
>
>Again thanks for the info. you guys have been a huge help :)
>
>Rob


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