From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


I like the Ritchey test.  With a good sphere, the defects are not too hard
to see on the flat.  In Mackintosh's ATMT there are two great articles on
using this test.  Setup is not too bad with a laser pointer and adjustable
holders.

Ritchey used it to make a 40" flat with a 13" spherical mirror. 
The 40" flat was to make a 40" Ritchey-Chretien scope.  I assume
Ritchey got the flat pretty accurate.  He was one of the heroes.

Colin

-----Original Message-----
From: John Sherman [mailto:shermj{at}netzero.net]
Sent: Wednesday, February 12, 2003 9:52 PM To: CSC
Subject: Re: ATM Tesing a spheriod?







Hi Colin,

>Ah, Yes.  Easy to test is right.  I labored through probably 20 hrs
>of figuring to get a good sphere 8" f/15.

Wow, only 20 hours? You're really fast!

>I wanted the best reference sphere
>for the Ritchey-Commons test.

How do you like this test?

>Are you going to flex it or use it as a sphere?


I made it and made a flex Herschelian, long ago. Before they said anything
about wedge. The sphere was really nice, and it was "easy"
(sort've), but at the time I had the hands-on help of my uncle, who had
been making mirrors for 65 years or so. The scope failed cuz of
astigmatism, and it was too fast for the application (f/5, meaning it was
an off-axis section of an f/2.3 parabola). But I had just done the whole
thing for the fun of it, so that didn't really matter. Now I know that
wedge causes astigmatism in a flex job. The program allows me 0.0023"
(on-axis flex)  and the mirror has 0.012". Yes, I can flatten it out,
but then I'd also want to correct the 1/4" TDEZ (turned-down edge
zone, with accompanying TDE). This time around I was just after a real good
sphere to take pictures of for my website. Someone who knows these things
looked at it and estimatied I have about 1/40 wave wavefront.
www.johnspics.com/a page 4. I got ~7.4 inches of a real good sphere, so I
masked off the rest. He figured the TDEZ is only a 1/4 wave/front, and it
would make a fine scope even without masking. This time around I had very
little help, so I spent lots of time floundering around. It took way too
long. Now that I have the pictures, and don't have the time to play, I'm
content to let it rest. But maybe some day I'll flatten out the wedge and
make a 7.5" flex job, on-axis this time. The rest of the scope has
been sitting in the corner, waiting, for many years now....

Have fun, eh?

John

--- BBBS/NT v4.00 MP