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


Well, thanks to all for their kind advice and offers of help. I changed
from the "Cerox" to some pink cerium oxide we use at work and
took the mirror back to a sphere. The improvement in smoothness (roughness)
was immediately noticeable.

I also changed the LED in the Foucault tester to a Maglite(TM) bulb - BOY
is that unforgiving!! Subtle shadows that only showed up on the digital
camera with the LED were suddenly glaring black blotches - I would
encourage people to at least try this - the bulb fits in the same hole as a
0.1" LED. (It also allows testing in daylight hours!) - I used the
frosted end of a microscope slide as a diffuser.

I overshot the parabolising on this attempt and spent several weeks in the
wilderness trying to recover the situation, returning to a sphere twice. I
decided to heed the advice that the lap was getting too thin (1/8")
and re-made the lap. This thin lap produced the most superb spheres with a
few hours polishing, but just didn't seem to react in a controlled manner
for anything else.

The new lap was cast in one piece with channels pressed in with a metal
ruler when the pitch was still hot. This is MUCH easier than the Texereau
method of stuck-on squares, but I don't think it would be as easy to keep
the channels open for any length of time. I also pressed a nylon net into
this lap. Initial attempts at parabolising with the new lap didn't respond
well, either, but I think it just needed time to "bed in".
Eventually, in about 4 steps I got to a good figure that measures as .99
Strehl. .68 transverse error and 1/18 P-V (average of 7 readings on 2
diameters if you believe the numbers! - worst individual result not
significantly different).

As I set up the test after the last step (once round on a raised zone at
60% radius) I was continually chanting to myself: "I'll stop now if I
haven't done any worse than last time.....Please don't let me have messed
it up...."

The primary ripple (from the lap) that was still evident in tests with the
original lap was absent in all tests with the second lap - I put this down
to texturing the pitch with the nylon mesh. It's one of those situations
where you don't know something's there until you see a test where it isn't.
(Sadly, the digital camera broke during this process, and I'm waiting for
it to be repaired before I can take any photos.)

The weather / seeing has been poor here since finishing the mirror (yep, my
fault) but I have had a chance to look at some stars with my undersized
secondary (30mm dia) and caught fleeting glimpses of the out of focus
diffraction patterns. In/out of focus look the same, but that's comparing
the six or so rings you're looking at with memories of how something looked
5 minutes ago.

So, flushed with success, I turned my attentions to the secondary  - see
separate post!

Thanks again to all.

Andy.


Andy Gray, Dyserth, N. Wales.

--- BBBS/NT v4.00 MP