From: atm{at}misterg.fsnet.co.uk (Andy Gray)
To: 
Cc: "Jerry B. Hillman" 
Reply-To: atm{at}misterg.fsnet.co.uk (Andy Gray)


On Mon, 16 Jun 2003 16:58:59 -0500,  "Jerry B. Hillman"
 wrote:

>Why not put the mask openings vertical?

Hi Jerry,

This is fine (as far as I can tell), but you must then put the knife edge
horizontally, and have it cutting the beam in a vertical movement - don't
see that you gain much! (There may be a subtle point about the effect of
any "potato chip??" distortion from the test stand having less
influence for a horizontal mask than a vertical one). I found that on an
f/6 mirror I had to get my eye so that my eyelashes were brushing the KE. I
completely re-jigged my Foucault tester to accommodate this.

I've been following your efforts, and have to admire your persistence!

I'm sure you've got more advice than you need at the moment, but if I could
offer some thoughts [qualifier - I'm a first mirror know-it-all]: The 15
minute steps between your Ronchi pictures seem awfully short - I would have
expected a much longer period to really get the lap working (90 minutes or
longer)AFAIK, you can't "overshoot" a sphere! My gut feel is that
you're not getting the results you want because the lap isn't really
getting going (see qualifier above!).

Just to add to your mountain of "good advice": When polishing my
7" f/6, I found it easiest to adopt a rather sloppy stroke where I
would push the tool (TOT) backwards & forwards 1/3 W COC without any
rotation for six strokes, then rotate the tool one way ~30degs and step the
other way. Think of gouging a big hole in the centre of the mirror as you
push. I found this generated a vaguely spherical shape with a turned *up*
edge (and I would lapse into it & keep going without concentrating).
Adding some side-side wWw stroke brought the middle spherical without
affecting the edge. I proceeded directly to parabolising from here (first
time) with a very wide wWw to knock the edge down. The ability of this
stroke to generate a TUE was repeated as I was polishing back to a sphere
for my second go. This may be peculiar to my circumstances, and in any case
I was using a full diameter lap.

FWIW, I found the Foucault test much more useful than the Ronchi for
anything quantitative, but it does take some practice to get definite about
where the null is - it *will* get easier. I found I could repeat
measurements on my (crude) rig to 0.01 mm after a while. I even surprised
myself with this.

Wishing you all the best - polish more, worry less.

Andy

(waiting like an expectant father for the first-born to return from the coaters)

Andy Gray, N. Wales, UK.

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