From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


I have read accounts of using a very flat pressing block to press the lap.
The pressing block you use might be causing the problem, as it is not flat.
Could you use the 4" flat you have to press the lap?

Colin

-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Andy
Gray
Sent: Monday, April 21, 2003 8:33 PM To: atm{at}shore.net
Subject: ATM polishing secondary flat - TDE problem



I'm trying to make a Newtonian secondary mirror using the method in
Texereau of polishing an oversize blank on an oversize lap. I need a
diagonal ~50mm x 36mm (~ 2" x 1.5"), so have started with a 85mm
dia (~3.5") disk of 10mm float glass.

I'm polishing it on a 100mm (~4") diameter pitch lap approx 3mm
(1/8") thick, but the problem I'm getting is that although the central
~50% of the diameter is wonderfully flat, the outer 25% on each side rolls
off by 3-4 fringes. (I'm fortunate enough to have a 4" optical
flat!!).

This doesn't quite leave me with a big enough area to cut the secondary from.

D/3 stroke with D/3 side-side MOT just seems to maintain this situation.
D/3 C over C MOT generates a consistent 2-3 fringe depression in the centre
with ~2 fringes roll-off on the outer 20mm. Chordal strokes MOT dig a HUGE
hole in the centre very quickly that takes hours to recover from! 0.75D
strokes with D/3 side-side seem to make the whole surface convex (i.e bring
the roll-off in towards the centre).

TOT sends the edge diving for cover - really accentuates the roll-off.

I press the tool on another piece of glass that's flat to about 3 fringes
between polishing sessions, and  I've tried polishing without applying
pressure to the glass, but the edge seems to be the edge.

Any ideas how I can expand that flat spot in the centre just another few mm?

Thanks,

Andy

Andy Gray, Dyserth, N. Wales.

--- BBBS/NT v4.00 MP