Re: [ATM] Figuring question

[Mike Lockwood <melockwo@uiuc.edu>]

Lance,

10 zones for a 12.5" mirror is a bit high.  If you really want some 
testing practice and you want to check your skills, try a 7-zone mask. 
  But, assuming the readings are OK, you should first worry about the 
outer zones.

In FigureXP, go to the surface error page and drag the ROC slider a 
bit to the left.  Notice how the outer part of the mirror gets 
flatter.  Move it to the left until the outer zone is flat.  At that 
point, all parts of the mirror inside of the outer zone are high, with 
a peak of about 260 nm near the center.  This is a common problem, and 
this is the perspective I would use when figuring the mirror.

You have quite a soft lap.  So, you need to be gentle, and let the 
weight of the mirror on top of the lap (which I assume is full-size) 
do the work.  In other words, don't push down on the mirror very much 
- you may need to use the edges of the mirror a bit to help move it 
along.  Strokes are slow (one forward and back movement about every 
two seconds) but steady, and smooth.

However, since you have a soft lap, this can be a little bit risky - 
you might lose the edge depending on how you work.  (If that happens 
the solution is simply to re-pour the lap after hardening the pitch a 
bit.  But I think you will be OK if you work gently.  If the edge is 
already turned, then you should let us know.)

Here's what I recommend - you will want to focus on the center part of 
the classic W parabolizing stroke (working MOT, of course).  That is, 
you will be doing mainly long strokes with the mirror very close to 
center-over-center.  This will tend to correct the outer zones more. 
See my full-size lap figuring page at:
http://bi-staff.beckman.uiuc.edu/~melockwo/mirror_making/full_lap/full_lap.html

Look at the end of section 1.2 for the "Edge Correcting Stroke" shown 
in a sketch.  My advice would be to try it for a short time (10 mins) 
to see if it adds correction to the outer zones.  Avoid moving the 
mirror very far off to the side, as this will deepen the center more, 
and we don't want to do that just yet.  Don't do longer than 4/5D strokes.

If that doesn't add some edge correction, another technique is to do 
some very short (1") strokes either MOT or TOT.  That may also 
increase the correction in the outer zones.  We are only concerned 
with the outer zones - don't worry about the center so long as it is 
high in the error plot as we have viewed it.

Be prepared for this to take a while - the outer zones take time to 
correct, and this is why few commercial opticians do it properly. 
Also, detecting the changes may be small enough to strain your testing 
skills, but keep at it.  Be patient.

	Mike Lockwood

lance clarke wrote:
> Here are the details of my mirror:
> diameter= 12.5"
> thickness= 1"
> focal length=62" F5
> glass = supposedly 96% silica glass (i'm not totally conviced though...looks 
> too much like plate)
> I'm definately totally polished out.
> Pitch lap is soft (now)...it was just right a couple of months ago. Now we 
> are in a bit of a heat wave.  However, I've been trying to figure since 
> March.  In the cold weather it took about 2 hours for the channels to close. 
>   Now, I'm not sure, since I don't do spells long enough to determine this.  
> I'm using gugolz 55.
> The lap is probably 3/8" thick.


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