Re: [ATM] Inverse Ronchi program

[Mike Lockwood <melockwo@uiuc.edu>]

Ed,

eheiny wrote:
> I may need to evaluate the figure of a 21" f4.2 mirror.  The
> telescope (Newtonian / Classical Cass) has been removed from
> service and put into storage.....  Our club may be able to get the
> primary mirror, but I'm hesitant to start a major project without
> confidence in the mirror quality. While I'm comfortable using the
> Foucault test with smaller slower mirrors, I doubt my skills at
> accurately analyzing a mirror this large and fast with the Foucault
> test.

Foucault testing that mirror should be completely within the 
capabilities of an experienced mirror maker.

Lacking that in your area/club, if you felt like driving it up to 
central Illinois at some point we could test its figure and check it 
for strain here.  (Feel free to email me off list.)

> On a totally unrelated note, are there ways to cut a 23" dia X 4"
> thick blank (Schott borosilicate glass) into two 23" blanks ~2"
> thick?  Are there glass fab shops that could do it?  Would the cost
> be prohibitive?  Is there a high risk of breakage?

Yes, it could be done, but you should test the mirror for strain first 
- look at my strain testing page:

http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/strain/strain.html

The last photo is of another large thick blank (25"x4.5") that is 
currently in my basement.  While stable in shape as is (it's a crappy 
approximation of a parabola right now), I think slicing this blank 
would be quite risky - it could break (or it might not).  Its owner 
may want to sell it - email me off list if you're interested.

As for slicing blanks, Advanced Glass Industries in Rochester, NY (I 
think that's right) is one company that slice large blanks.  It's not 
cheap, but it can be done.

	Mike Lockwood

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