Re: [ATM] figure stroke ten inch

[Tom Fredrickson <newsforfred@comcast.net>]

On Mon, 01 Oct 2007 17:30:16 -0500
Mike Lockwood <melockwo@uiuc.edu> wrote:

> Hi,
> 
> Tom Fredrickson wrote:
> > I'm working on a 10 inch mirror full size lap. I turned the edge and
> > went to work with short coc strokes to clean it up. Now I need to
> > add correction to the edge. I have been working the edge and it is
> > going the wrong way. I feel I need to work the center and make it
> > deeper.
> 
> As you can tell from the widely varying advice you've received on the 
> list, there are many ways to make a mirror.  Allow me to submit my
> two cents worth......
> 
> A 10" F/5 should be able to be figured with a full-size lap if the 
> pitch is soft enough.  If the pitch is not soft enough, you will get 
> to a point where the mirror just won't correct anymore, and if you 
> force it, bad things may happen.
> 
> The pitch can be made softer by warming it or working in a warmer 
> room.  This should get you to full correction.
> 
> I'm not sure if you've read my full-size lap figuring page, so here's 
> a link - hope it helps:
> http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/full_lap/full_lap.html
> 
> What I say there will also help you judge the hardness of your pitch, 
> which is key in this situation.
> 
> If you have made other mirrors and are ready to try subdiameter laps, 
> this mirror would be a good one to start learning on.  I would use a 
> 6" lap, personally.
> 
> 	Mike Lockwood
> 
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 

Mike,

I agree with your point about pitch hardness. I find my working
temperature needs to be about 80 - 85 degrees. my mirror is 1 1/4 inch
thick pyrex. How long should I let it stand in a water bath? I let it
set for 30 minuets and found the edge would be warmer the the center I
think this caused me some problems.

what I don't like about warming the mirror is testing the mirror is not
as reliable. I have read your web page and find it helpful. I had this
mirror at 1/10 wave but the edge was turned down. I had to mask off 3/8
inch all around to get acceptable results when star testing.

Also had a figure better then 1/4 wave many times but the edge seems to
always be high. That is why I turned the edge with a locolized stroke
to reduce the edge.

Thanks for your help.

-- Tom
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