Re: [ATM] FW: Figuring flats

["Philip Lardner" <philiplardner@eircom.net>]

So far I have avoided having to work mirror-on-top as I was aware of the
danger of hand-warmth. I am experimenting with using a very short stroke
with little sideways movement to make the work more concave (from convex)
and it seems to be working. I'm also trying out a paper ring when pressing
to put the outer zone of the lap out of action and not work the turned
zone - that also seems to be working... touch wood. I guess I'm just
satisfying a curiosity doing this before I bite the bullet.

The print-through from the saddle-shaped back is beginning to make sense
now. I have noticed a *very* slight increase in resistance when working
perpendicular diameters (easy in the 12 to 6 o'clock direction, say, and
more resistance in the 3 to 9 o'clock direction) that can't be accounted for
by the CeO drying or thinning out. The difference is only just noticeable as
a slight 'catching' of the lap when centre over centre. I haven't started
grinding the backs yet... still hoping for a miracle - but I have been
experimenting with ways of making a fine-ground surface translucent without
having to polish it out. Turtle wax is not good - Vaseline is so-so but
slippery - a drop of oil works nicely (but is oily... duh!), floor wax is
reasonable, and clear cellulose lacquer scratches off too easily.

Watch this space...

Phil.

-----Original Message-----
From: atm-bounces@atmlist.net [mailto:atm-bounces@atmlist.net]On Behalf Of
Mike Lockwood
Sent: 14 March 2007 16:22
To: ATM List
Subject: Re: [ATM] FW: Figuring flats

Philip,

Philip Lardner wrote:
> I have observed that touching (even momentarily) the crack between
> the two plates under test (to check they are centred) produces a
> very noticeable local bulge / distortion in the fringe pattern
> requiring another 30 minutes or more to dissipate.

Mirror-on-top work will heat the mirror because you are touching it.
So, use something to insulate your hands (use gloves) from the glass.
  Working for short periods on thicker Pyrex without gloves is OK in
my experience.

> To date I have worked the three plates tool-on-top. My tool is a
> good 2" thick concrete on floor tiles. I never touch the plate
> itself after I start pressing it - except briefly to rotate it
> mid-session, and then only the back edge.

Tool-on-top will enhance the print-through of a non-figure of
revolution mirror back, because the back of the mirror is being pushed
against its support by the polishing pressure.  The increased pressure
implies a greater difference in pressure due to the shape of the back
of the mirror (as compared to that produced by the force of gravity
alone on the mirror), and that slight difference in pressure warps the
mirror in a non-figure of revolution way, causing astigmatism, etc. on
the front face.

The back of the mirror can be non-flat, but it should be a figure of
revolution.  So long as the support under the cushioning material is
also a figure of revolution (ideally nearly matching the curve of the
back of the mirror, or being flat) then I suspect this can be made to
work.

This is why we grind our mirror backs - it makes them a figure of
revolution.

Welcome to the fun of making flats.  You now understand why they have
traditionally been made of thicker Pyrex.  If you can finish your
plate glass flats, your future Pyrex "master flat" will be easy to make.

        Mike L.


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