Re: [ATM] Re: Figuring Help Request: 6" F7 - Which Stroke Next?

[Mike Lockwood <melockwo@uiuc.edu>]

Joe,

COO wrote:
> Hi Gang,
> The link in my previous post doesn't work.
> Here is the correct link:
> http://www.atmlist.net/contrib/coo_mo-at-yahoo-dot-com/mirror2.pdf
> thanks

If all the analysis is correct, you have a slightly undercorrected 
outer zone.  This is a very common condition, even in commercial 
mirrors.  You are very close to an excellent mirror.  Be careful, test 
carefully, and don't rush the remaining figuring.  Think before you 
work.  Realize that, if the tests are accurate, you are tweaking the 
figure.

Many will tell you to stop here, but I won't.  Proceed at your own risk!

Assuming the surface is smooth, has a good edge, and does not need 
much more work, my standard fix is a long, narrow W-stroke, nearly 
COC, MOT.  This is basically the central part of the parabolizing 
stroke.  A few minutes should make a significant difference, one or 
two turns around.  If you are using regular cerium, keep the slurry 
quite diluted and keep the work very wet to slow the action.  Make 
sure the lap is quite well cold pressed.  Keep the strokes slow and 
gentle.

Be careful, and keep and eye on the edge (by observing the diffraction 
ring around the outside edge of the mirror).  This stroke can turn the 
edge, depending on how the lap is behaving.  You shouldn't feel much 
"grabbing" at this size/focal ratio.  During the stroke you should 
feel fairly smooth, constant drag.  The drag may be reduced when the 
mirror is near the ends of the strokes.  Major changes in drag hint at 
contact problems and bad effects.  Stop and repress if this happens.

A slight bit of work with the mirror offset about 2" might lower the 
other slight high spot.  If it were me, I would work both high spots 
with the same stroke, but I don't advise it.  If the edge work is 
successful and the other high spot shows up in subsequent testing, you 
can gently work it down.

My figuring page might be useful, section 1.0 and after:
http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/full_lap/full_lap.html

	Mike Lockwood


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