Re: [ATM] Parabolizing the edge

[Mike Lockwood <melockwo@uiuc.edu>]

Doug,

Douglas Arion wrote:
> What is the best way, on a small mirror, to 'add'
> correction to the edge? Texereau talks about using TOT,
> offset...but there is so much danger of turning the edge
> that it seems dangerous.

Yes, it's risky, especially with faster mirrors.  Personally, I'd 
avoid TOT for this problem.

> I'm finishing up a 6" f/4.5, so
> there is quite a bit of correction to put in, and I need to
> add some at the edge.

This will depend on the hardness of the pitch.  If it is fairly soft 
(channels closing in 1-2 hrs of normal polishing), then long MOT 
strokes (the central portion of the parabolizing strokes) will likely 
correct the edge more.  Press well, and work for few minutes at a time 
with slow, smooth strokes.

Alternatively, if your lap is trimmed well at its edge and is smaller 
than the mirror, then short MOT COC strokes (1/4") will work mainly 
the area inside the edge of the mirror, working the outer 1/4" or so 
less.  In effect, you will be turning the edge by deepening the 
center.  Again, press well, work for a short time, and then test.

If you still have more correcting to do in the center, modify the 
parabolizing stroke (MOT) to spend more time in the central portion, 
while still doing some edge correction.  (This is shown in Texereau - 
method of the full size tool for reducing the edge.)  This method will 
keep the mirror the smoothest.

	Mike Lockwood

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