To: atm{at}shore.net
From: "Matthew L. Brown" 
Reply-To: "Matthew L. Brown" 



>Charles,

  Since no one has answered you, I'll give it a go.

1. I just finished grinding two 4.25" mirrors using 1" tiles.  It
worked fine.  I decided to trim the tiles on the edge, but I think you
might not need to do it.

2. In these archives I've read of people doing mirrors as thin as 18:1. 
Your mirror would be 17:1, so I suppose that qualifies.  Note that relative
stiffness goes UP as things get smaller, so that's in your favor.

3. You would probably want to grind the back side of the mirror blank with
#320 or #500 to prevent distracting reflections during figuring.

4. The nice thing about little mirrors is they go quickly.  I've learned an
awful lot working these two  little mirrors.  The converse is you have to
have a light touch figuring.  Things went much better for me when I
switched to rouge.  With cerium oxide I kept overshooting.

5. Before you start, you probably want to check for residual stress.
http://www.megspace.com/science/sfe/i_fa_cir.html#selecting


=Matt




>Hello all,
>
>I have recently begun my journey into ATM. With my 8” f/8 “well underway”
>(ok, so far 2 hours rough grinding and several months prior of plotting
>and planning -- followed by yet more plotting and planning) and a cheesy
>50mm f/4 refractor built from scrounged parts, a new project beckons to me…
>
>A friend got at auction a box of optics/photography equipment. What
>interested me were the 5 clear optical glass filters 4.25” by .25”. Only
>one of them is marked: the word “CLEAR” with a triangle inscribed in a
>circle to the left. The rest are all unmarked.
>
>I would like to make an f/4 or f/5 newt, mostly to learn to figure a fast
>mirror. Sagitta would be between.064” and .052”, depending. Is this
>reasonable in a .25” thick blank? What would be the best way to proceed
>with grinding? Using another filter as the tool or pour a tile tool
>(should I try to find .5” tiles?)? Should I build up the thickness of both
>tool and mirror before I start grinding?
>
>Advice appreciated!
>
>Thanks,
>Charles
>
>

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