Re: [ATM] Figuring with sub diameter lap

["Jonathan Bishop" <jonathan_bishop@sbcglobal.net>]

Thanks for the advice! Was able to correct my 8"F5. Here is what I did...

Went back to full diameter lap, short w strokes lap on top. This made the
mirror somewhat undercorrected.

Returned to 4.25" lap, lap on top. Used short circular strokes centered on
the 1.5" zone. After several short sessions managed to bring the mirror in
at under 1/10 wave! Still very smooth too.

After dropping (actually carefull placing) the uncoated mirror into my scope
I tried it on a few objects. It performed great! At least in comparison to
the previous time when it was still more than 1 wave off. So all this
measuring and polishing really does work :-). My faith in ATM is restored!

Seveal useful things I learned, reinforcing what I read below.

1) Use soft pitch in subdiameter laps. In fact the main reason for not using
soft pitch in a full size lap, the danger of TDE, disappears when using a
sub-diameter lap, unless of course you are working over the edge.

2) Do two presses. The first I did with fiberglass screen to create micro
facetting. The second was a shorter press without the screen to assure good
contact. You have to use your judgement on how long the second press should
be. Too short and not enough contact. Too long and the microfacets
disappear.

3) Light pressure, smooth circular strokes seem to work the best. I did
sessions of about 5 minutes toward the end.With rouge this was slow enough
to control the figure. Lap on top of course.

Thanks again for the help,

Jon Bishop
----- Original Message -----
From: "Mike Lockwood" <melockwo@uiuc.edu>
To: <atm@atmlist.net>
Sent: Tuesday, March 09, 2004 11:51 AM
Subject: Re: [ATM] Figuring with sub diameter lap


> 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
> > _______________________________________________
> > ATM mailing list http://www.atmlist.net/
> >
>
> _______________________________________________
> ATM mailing list http://www.atmlist.net/

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