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


Wow!

Thanks for all the replies - I've tended to go with the more cautious
types, and try for a smoother, better sphere before deliberately trying to
para[...]ise (insert variants on bol|boloid|boloidal according to personal
preference).

I'm pleased that my (undisclosed) interpretation agreed your collective
diagnoses as regards the shape of the mirror (centre hollow + turned up
edge). As I see it, taking down the edge of this shape should be achieved
by long(ish) centre over centre strokes MOT (as per parabolising but
without the side-side movement) and therefore should be
"practice" for parabolising.

To address specific points raised:

The tester is moving source using a green LED directly behind the ink jet
grating (a LOT of trouble) - no slit. Photos were taken with a hand-held
digicam, but it was tricky to get it lined up well enough to see the
patterns;

The laser pointer test: I find it very difficult to get the glass clean
enough to judge whether the polishing is finished with the mirror dry. I
have monitored this through polishing, and seen the surface improve (using
laser and maglite reflected from the surface and also through the edge of
the glass). I reached a stage where I was picking up "sparkles"
on the surface, but they would move around when the mirror was wiped (I am
also mindful of grease hiding a short finish - the old optical shop trick).
I polished mostly TOT, so the better surface moved in from the edge, and
the centre was last to clear. (Focal length got ~2 inches longer during
polishing, which surprised me.) Under water, the "sparkles" can
still be seen, but float away when disturbed. Under running water, I can
get the surface to be "invisible" to a laser pointer. I take the
point about there being less of a refractive index difference between water
and glass (this is SK5 - nd 1.589), but on balance, I'm pretty sure it's
polished out.

Thanks again to all who replied - I'll let you know how it turns out.

Andy

Andy Gray, N.Wales, UK

--- BBBS/NT v4.00 MP