Re: [ATM] FW: Figuring flats

[Mike Lockwood <melockwo@uiuc.edu>]

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.


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