[ATM] Tom's back - Parabolizing Plate Glass 12.5" x 0.73" f/5.5

[Mike Lockwood <melockwo@uiuc.edu>]

Tom,

Polk, Tom wrote:
> About three months ago, I helped me cure an oblate sphere. Your
> remedy worked wonderfully (that prompted me to look at your web
> page closely for more ideas).

Glad it worked.

> Last week I resumed worked on the project and achieved the results 
> attached. Not to bad: minor (I think) dog biscuit, minor (I think)
> TUE and gentle zoning.

You still have some leftover oblateness and a slight depressed zone
outside of the central bump, but the amplitudes of the defects are
small.  The dog biscuit is of more concern to me.  It might indicate a
lap that is too hard, a polishing stroke that is too forceful or 
rough, contact problems, or that the mirror/lap is getting too dry.

> I read your "Figuring with a full sized lap". The question I have
> is will this work for a thinner mirror: 12.5", 0.73", f/5.5 thick
> plate glass. In the photo, Zone4.jpg, you can see stress at the
> 12:00 and 4:30 test stand pegs (mirror is tilted just forward of
> its center of gravity). Will my mirror "potato chip" while using
> your modified stroke for start of parabolizing?

The full-sized lap method should work fine.  While it might potato 
chip a little bit during each stroke, you have to trust that it will 
all average out as you rotate the blank in your hands and walk around 
the barrel, provided you work consistently and gently.

Since the mirror is light weight, soft pitch is best for the lap.  You 
will need to be gentle, letting the weight of the mirror do the work 
(don't apply downward pressure), and taking ~2 seconds per full stroke 
(forward and back movement each take 1s).  To grip the mirror without 
applying downward pressure, I usually hold the mirror at the back 
bevel.  That is, the thumb and pointer finger of each hand contact the 
mirror on the bevel of the back side of the mirror.  (The hands 
typically are at the left and right edges of the mirror, rather than 
being at the leading and trailing edge.)  Holding the mirror in this 
minimal way will also help reduce heating effects.

Drag should be consistent on every stroke, indicating good contact. 
This gentle approach means correction might appear slowly, but it will 
appear.  You'll want to test after 15 minutes or so and see where the 
correction is appearing, and the modify your stroke if necessary.

> At the onset of this project I cast a 50% dental stone lap
> substrate. I'm tempted to use it, but this is only my third mirror
> and I've never used a sub-diameter lap.

That would work too, but you don't need to use a subdiameter lap at 
F/5.5.  A full lap will keep the mirror smoother and less zony.

I will say that for me, a subdiameter lap is the standard technique on
most mirrors over 8" in size.  It is a powerful technique, but I would 
learn it on a thicker Pyrex mirror first (try a 10" F/4 next) before 
you try it on a thin mirror.

> I would like to get your advice on this. If you feel others could 
> benefit from your reply please just cc to the ATM list. Thank you
> for your time.

In response to your later message about a specially carved 
parabolizing lap, all I can say is it can definitely work, but you'd 
be off on your own working out your own procedures, which is not a bad 
thing, it's just not easy.  I've never used this type of lap on a 
parabola, but I have used "formatted" laps or petal laps on Cassegrain 
secondaries.

There are many techniques that will result in a finished mirror. 
However, if you want help, and because this is a thin mirror, I 
recommend the full lap and patience.  Keep us updated.

Mike Lockwood

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