To: atm{at}shore.net
From: Jim Burrows 
Reply-To: Jim Burrows 


At 21:07 2003-02-25 -0500, Colin wrote:

>Typically the primary is steeply curved, like f/2-f/3, which is not a
>trivial matter.  The figuring needs to be hyperbolic by about 10% or so.

I've made one'o them thangs.  It's a 10"/f8, the primary is f/2.6. 
Not knowing much beyond the fact that one can't figure an f/2.6 with a full
size pitch lap, I tried small tools which were incredibly slow, and ended
up figuring it with deformed HCF laps (wax dolls).  The result is a very
rough mirror.  If I were to do it again (maybe I should refigure this one),
I'd probably use deformed pitch laps.  Testing f/2.6 mirrors ain't very
easy either - Foucault's hopeless, maybe the wire test would work - I used
a modified Foucault test, working at the focal plane.

The secondary turned out to be not as tough; I got mine down to 9 nm RMS. 
I made a 10"f/1 Hindle sphere for testing the secondary - it's not
true that it's useless until the next secondary is started - it makes an
excellent shaving mirror (not too many 1-2 nm RMS shaving mirrors around
).

Texereau's section on cass telescopes has the design numbers for RC, along
with the usual caveat, p. 148, "Finally, it is evident that the more
deformed mirror surfaces of the Ritchey-Chr‚tien are more difficult to
figure, at any rate if we want the diffraction-limited quality taken as a
standard in this book."

Finally, for the perfectionist, the usual conic-section RC designs aren't
exactly aplanatic according to Born & Wolf, "Principles of
Optics", p. 214-217, the true aplanatic surfaces requiring the
solution of a pair of differential equations.  For my 10"f/8 RC, the
difference between conic-section and true aplanatism is 12 nm RMS.  I've
got a couple of programs on my web site for the design and testing of RCs
using B & W's equations:

         http://home.earthlink.net/~burrjaw/public/aplanat.zip
         http://home.earthlink.net/~burrjaw/public/rc_atm.zip

>I hear the RC is more critical to collimate, and the field is more strongly
>curved than the equivalent Cass.

I've always wondered about this.  Since the RC is coma-free, it seems
intuitively that it would be less critical to collimate.  It's true the
field curvature is bigger.  But all the big observatory telescopes are RC,
including the Hubble (I wonder if they knew about B & W?).

         -- Jim Burrows
         -- mailto://burrjaw{at}earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)

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