Re: [ATM] refiguring a 20 inch Pyrex f/2 cassegrain

[Jim Burrows <burrjaw@earthlink.net>]

At 2005-08-07 15:17 -0700, Guy Brandenburg wrote:

>Has anybody ever tried refiguring a 20 inch Pyrex f/2
>(or thereabouts) Cassegrain? I understand that the
>primary on the one I have in mind - it belongs to the
>Hopewell Observatory, of which I am a member - is
>supposed to be parabolic, and that it's not figured
>very well.

I'm currently refiguring the mirrors of a 10"f/8 RC cass.  The primary's 
f/2.6.  You'll have to think about what test to use - Foucault's hopeless 
at that f/.  Early on, I used Focal Point Foucault (FPF) in which you match 
the returns from single-zone Ritchey masks on ground glass using a 
microscope.  It works (I guess).  I've finished the primary refiguring 
using 2D webcam Hartmann which is also a fair amount of struggle at that 
f/.  The current struggle is to program the reduction of images using 
Hartmann masks on the sphere for Hindle tests of the secondary.

It's interesting - the Hindle test, sphere plus very hyperbolic secondary, 
is blessed <g> with a LOT of coma - you have to be very careful with 
collimation for each test run, which is a nuisance, or somehow evaluate and 
throw away the test-generated astigmatism.

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

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