From: "Rob" 
To: "Richard Schwartz" ,
        
Reply-To: "Rob" 


> The convex secondary is more complicated.  I have never made one, but I
> understand that it has two foci.  One is behind the mirror, and one is in
> front of it.   So if you position a spherical mirror with its center on
the
> rear focus, light rays emanating from the front focus will be reflected
back
> to that same point.  In other words, and ordinary knife edge test for a
> sphere should work there.
>
> But what the hell do I know; after all I never did it.  And the books all
> tell you it is too complicated for an amateur.

That's the impression I got from reading literature from the manufacturers
of R-C scopes..They want you to believe it's to complicated.  I'm not
completely convinced that it can't be done though. I'm going to try a
6" primary to start with..If it goes bad, no biggie...

>
> What is it about the R-C that you find attractive?   Are you planning to
do
> photography?  If so, you might consider an R-C with low magnification.
This
> would give you a nearly flat field.   If you make the primary and
secondary
> of the same radius of curvature, you actually have a flat field.   A large
> central obstruction is of no consequence for a wide-field photographic
> instrument.   Also, pay special attention to vignetting, and you will be
> able to gather useful photometric information.   A camera with automatic
> time and date recording on the film is desireable for this; too bad they
> don't have such a thing in medium format Bronica or Hassleblad.
>
Yes, the main reason I want to build an R-C is photography.. But I also see
other possible advantages. It seems like the best of a cassagrain and a
newtonian merged together with a nice flat field..You get losses only on
the primary and secondary like a newtonian and unlike a cassagrain( extra
losses in the corrector plate) but it's similar in size to a cassagrain..I
do mostly want to do photography with it, but will also do some viewing as
well I imagine :)

> . . . Richard
> ----- Original Message -----
> From: "Rob" 
> To: 
> Sent: Tuesday, February 25, 2003 8:53 PM
> Subject: ATM Ritchey-Chretien telescopes
>
>
> >
> > Hello all,
> >
> > I'm new to the list. Hopefuly I won't get flamed for this..I have looked
> > around and can find very little info on Ritchey-Chretien telescopes in
> > general. I have found enough info to know that I want one :)
> >
> > I'm thinking about trying to grind the optics myself. And hence my
> > questions. I know that the surface of a Ritchey-Chretien is hyperbolic
> > instead of parabolic. What I'm curious about is, how much harder is a
> > hyperbolic surface to grind and test compared to a parabolic surface? Is
> it
> > only slightly harder? Or is it sveral orders of magnitude harder? In
what
> > ways does the process differ (ie same tools for testing and such?). Has
> > anyone here even attemped this before?
> >
> > I really want to build one of these scopes and then do some
> astrophotography
> > with it..I'm thinking I might try the first one with a 6" or
8" primary,
> > just in case things don't work out well. BTW It has been a while but I
do
> > have some previous experience grinding a 6" mirror with my
father (when
i
> > was about 15 or 16 hehe). So I'm not totaly new, but I'm probably just a
> bit
> > rusty hehe....
> >
> > So what do you guys think?? Is this possible or just a pipe dream?
> >
> > Rob
> >
> >
> >
>
>
>

--- BBBS/NT v4.00 MP