Re: [ATM] figure stroke ten inch

[Tom Fredrickson <newsforfred@comcast.net>]

On Mon, 1 Oct 2007 19:12:38 -0700
Tom Fredrickson <newsforfred@comcast.net> wrote:

> 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
> _______________________________________________
> ATM mailing list http://www.atmlist.net/
> 

Hi all,

I wanted to add a quick note of results from warming the mirror and lap
to soften the pitch. I set them in water at about 80 degrees for 1 1/2
hours. did W stroke as before for 3 turns around the mirror result in
over corrected center and high edge. also more roughness and ripple as
contact was still not good.

three more turns after a long press COC MOT 1/3 wave edge still high.
so problem was hard pitch. now it is contact. need to smooth the over
all roughness.

new readings in mm 0, .91, 1.36, 1.93, 2.15 

-- Tom
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