From: atm{at}misterg.fsnet.co.uk (Andy Gray)
To: atm{at}shore.net
Reply-To: atm{at}misterg.fsnet.co.uk (Andy Gray)


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