Re: [ATM] figure stroke ten inch

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

I am answering three messages (Tom, Jerry, Anthony) here so this is a 
little long.....

Tom Fredrickson wrote:
> 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.

You want to get the pitch warm enough to make it slightly softer. 
80-85 degrees sounds OK.  The pitch should not be so warm that it is 
super soft.  A fingernail should make a larger dent than it did before 
heating, but you should not be able to make a dent with a fingertip. 
The idea is to get the heat deep into the pitch and tool substrate. 
For an 8" tool you only need to heat them for 10 minutes, I'm 
guessing, but more won't hurt if you keep the water temperature 
steady.  Then you will have 5-10 minutes to work until things start to 
cool off, but that is enough to get some figuring done.

You can sometimes get away with warming the mirror.  Just warm the 
lap, press quickly (soft pitch gets in contact quickly), and work 
until the lap starts to "feel" harder.  Softer pitch will have a more 
uniform drag as the mirror goes from hanging off the lap quite a bit 
to centered on the lap.  As the pitch gets harder and the asphericity 
of the mirror increases, the difference in drag as a function of 
mirror overhang will increase.  This tells you that the lap is having 
trouble staying in contact.

> 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.

Warming the mirror shouldn't make a difference in testing reliability. 
  You just need to wait longer until the mirror cools off to test it. 
  You should wait several hours at least.

> 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.

The hardest thing to do is to learn to correct the outer zone or two.
  It takes time, also.  Many commercial manufacturers hate taking the
time and leave undercorrected outer zones (sometimes not corrected at
all).  The P-V ratings may look OK, but the transverse error is quite
high.  Many amateurs have the time to correct the outer zones, but 
then they look at the P-V ratings and they think they are done!

Jerry wrote:
> There is much on the internet that discourages ATM's from trusting
> Foucault measurements. Too much. Don't let those who did not do it
> enough themselves to learn it talk you into joining them in burying
> their heads in the sand.

I couldn't agree more!

Anthony Anconetani wrote:
 > Mike,
 > I've found your full size lap figuring page to be very helpful. You
 > should consider writing up a sub-diameter page with a figuring walk
 > through that is similar to your full size example. I'd also be
 > interested in how to pick the tool diameter. -Anthony

Maybe in the future, but that is not a small task.  Thousands of hours 
of practice and learning are not easily (and sometimes not willingly) 
expressed.  It is best to develop your own techniques so you 
understand what you are doing.

As for the figuring example, join and read the Zambutomirrorgroup 
Yahoo group.  Carl has posted some figuring examples.  Then take a 
moderate focal length mirror (F/5 to F/7) in the 8-12" size range, a 
soft pitch lap 50-70% of the diameter of the mirror, and start 
experimenting with W-strokes of various overhangs.  If you are patient 
and good at Foucault testing, you'll figure out how it works.

Mike Lockwood

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