Re: [ATM] (no subject)

[aplanatic@aol.com]

Mike, Robert,
 
The mirror is somewhat undercorrected and should continue to respond to standard, full-sized lap parabolization techniques.  You can't tell from the FigureXP report much about the very edge of the mirror.  To determine whether TDE exists, null the 90% zone and look for the classic signs of TDE.  If there is strong evidence of bad TDE then you must decide whether it's better to mask a little of the edge or go back to 40 grit to fix it.  :-)   Just kidding.
 
You're actually quite close to a great mirror.  I usually instruct our students at this point to overcorrect by 10-15% and then bring the mirror back to a parabola by very gentle 1/3 CoC with a little side-to-side.  This tends to make a smooth mirror with less structure.  Don't rush it.  Make small corrections with a well pressed lap and test often.  Good luck!
 
-- Dave
 
-----Original Message-----
From: Mike Lockwood <melockwo@uiuc.edu>
To: atm@atmlist.net
Sent: Wed, 01 Feb 2006 17:06:07 -0600
Subject: Re: [ATM] (no subject)


Robert,

Robert Fifarek wrote:
> I've been figuring a 16 inch f4 plate glass mirror for some time now. I use
> a 9 zone couder mask and Fig XP to check progress on a home made focault
> (fixed source) tester. Star test shows definite turned edge, and my latest
> readings are as follows:
> http://members.cox.net/roboscope1/16inchno9_23FL_Surface.jpg
>  My question, can this be corrected with a 7.5" figuring lap?

Wow, I have seen that plot so many times lately.  It is a common problem.

I would not attempt to fix it with a 7.5" lap.  You will probably get 
better results if you start over.  The outer zone is really rolled, 
which will doom the mirror's performance.  Fix it!

If that mirror were on my test stand, I would take it back toward a 
sphere until I got the edge and outer zone back and then refigure it 
with the 7.5" lap.  The correction stroke with that lap is the broad 
W-stroke described by Texereau, with 1" - 1.5" of overhanging over the 
edge of the mirror at the end of the stroke.  It takes a while with a 
subdiameter lap, but you have good control over the surface.  That 
stroke will tend to correct the outer zones first.

Once the edge correction is nearly there, then you gradually shorten 
your strokes and narrow the region you are working on and correct 
zones closer and closer to the center.

Correcting the outer zones properly is something that only one a few 
commercial mirror makers seem to do really well, and it takes some 
practice to start to get it right.

    Mike Lockwood

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