[ATM] Figuring with sub diameter lap

[melockwo at uiuc.edu (Mike Lockwood)]

Hi,

This message is for those who want to learn to use subdiameter laps more 
effectively.

I've figured five mirrors with subdiameter laps (8" F/3.9, two 12.5" 
F/6's, 14.25" F/4.5, 16" F/4) and each time I was rewarded with great 
mirrors and smooth surfaces.  It is a wonderful way to control the 
figure and see it change in a logical manner.

I have learned some lessons, most of them the hard way.  Hopefully these 
suggestions will help others avoid the pitfalls I have encountered.  My 
suggestions for using subdiameter laps are:

1)  Use pitch that is very soft.  This helps keep the surface smooth.
2)  Use a slower-acting polishing compound, like rouge or zirconium, or 
a refined cerium product.
3)  Use elliptical or circular strokes that avoid stopping the lap's 
movement.
4)  Keep the strokes SLOW and the speed steady.  Use light pressure.
5)  Keep the work wet (use plenty of polishing compound).
6)  Press often (soft pitch presses into contact quickly).  Contact is 
VITAL for this to work properly.  If you feel the contact degrading, 
stop and press.
7)  Make sure you start and stop in the same place as you work around 
the mirror, i.e., don't work only 1/2 of the mirror.  (This can cause 
astigmatism, of course.)
8)  Be careful working near the edge.  You MUST have good contact in 
order to do this or you can easily turn the edge.
9)  Avoid exactly center-over-center strokes unless you want to dig a 
hole in the center of the mirror quickly.  (Been there, dug that hole!)
10) Be patient.  These methods change the figure slowly, but also 
smoothly and predictably.  When you understand the process, working to 
1/10-wave wavefront error is straightforward, though time-consuming. 
And the mirror will be SMOOTH!

As others have mentioned, for the overcorrected 8" F/5, a full-size lap 
with the normal stroke is the fastest way to get back to something that 
is close to a parabola.  However, after this you can use the subdiameter 
lap to tweak the figure to perfection.  Using a computer program (I 
really like FigureXP, but other excellent programs are available) really 
helps you visualize the changes in the surface.

Another advantage of this technique is that detecting the small changes 
you make in the mirror's figure will challenge your testing ability, and 
you will become much more adept at Focault testing.

If the 4.25" lap from your first mirror has been sitting around for a 
while, it may be too hard to work well as a subdiameter lap.  If you 
feel intermittent contact with this lap, this is probably the case. 
Either pour a new lap with softer pitch or warm up the mirror and lap in 
warm water before working with it.

Sorry for the long posts, but it's nice to share some of the things I've 
learned in a very intense 16 months of mirror work.

	Mike Lockwood

         "When it comes to pitch, CONTACT IS EVERYTHING."


Jonathan Bishop wrote:
> Hi,
> 
> I am currently figuring an 8" F5 mirror using my old
> 4.25" lab left over from when I polished my first
> mirror. The defect I am working on is a sever
> overcorrection, so I am attempting to reduce the 70%
> zone.
> 
> So far it is going well, which is encouraging. I am
> wondering whether many others use a sub diameter lap
> to figure with. It seems reasonable as you can easily
> direct you polishing to the zone you need to work on.
> 
> Jon Bishop
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