To: atm{at}shore.net
From: "Matthew L. Brown" 
Reply-To: "Matthew L. Brown" 



>This hobby teaches patience.

I found that an hour of differential pressure using CeO definitely showed
up in my testing numbers but not in the appearance -- the width of the edge
remained about the same, but its focal length definitely increased.  Note,
however that although you are applying pressure over the edge, the pitch
spreads out the effect to the adjacent zones too, so it is the difference
in pressure between the zones that is working for you.  This means it will
be a measured pace, but will also be less likely to generate uneven zones.

With smaller tools it is easy to put in local gouges.  Work lightly in very
short intervals, and do everything you can to evenly distribute the wear.
Test frequently.  It's very easy to end up with a non-smooth surface.

On my little 4.25" f9 it took about 90 total minutes of polishing, in
10 minute intervals using rouge, to get to a sphere, then another 25
minutes to go to the parabola, but it all depends on how much glass you
need to remove, and that depends on the difference in radii of curvatures
and the width of your edge, size of the mirror, etc.  I would say that my
edge was about the same order of magnitude as yours, but it's easy for me
to misjudge a ronchigram.  My initial ROC difference was about 12mm on a
ROC about 1950mm.

Taking measurements between sessions helps show that you are actually
getting somewhere, and, for me at least, helps extend the patience.

I also learned to do a couple of sets of measurements, to get an idea of
how repeatable my numbers were.  Then I'd make sure I did enough polishing
to change the mirror surface by an amount greater than my measurement
variation, so I could really tell where I was going.  In the past, normal
measurement variation fooled me into thinking the shape was changing in one
direction when it was actually going the other way.   In my case, I
couldn't accurately measure the effect of a 5 minute polishing spell, but I
could measure the effect of a 10 minute spell.  Near the end, of course, I
was down to 2 minute spells, and making a lot more measurements each time
to improve the quality of the measurement.

=Matt



>Thanks for the advice
>What I have tried so far is what Texeraux suggests: tool on top and apply
>pressure on the
>edge of the tool. I havent seen much of a progress with this methode. I Have
>polished
>about 60 min in total using this techniqe but don realy se any big progress.
>Maybe I am impatiant and need to stick too it a bit longer.
>By the way, how long did you have to work on the center to get it to the
>same ROC
>as the edge ?
>I also have a swedish ATM book that suggests local retuching with a  small
>halfmoon
>shaped tool on the edge to get rid of the problem. Any input on this ? Is
>sounds like
>its very easy to ruin the edge this way.
>
>Magnus

--- BBBS/NT v4.00 MP