From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


        It only takes a few seconds to trim the lap channels. Cold lap, cold
running water, sharp razor blade, quick shaving strokes like sharpening a pencil.

I use strips of pitch and channel only one way.  If your squares fill in,
why not clean the channels in just one direction and let the others close
in.  Half the work or less.  I think the ability to quickly conform the lap
by cold/warm pressing is a plus, especially when figuring. Channels insure
this.  Allows slurry to circulate better, I think.

Sounds like your pitch is about right, closing in after 4 hours. I polish
out 8"-10" mirrors in 6 hrs using Barnesite or cerium, trimming
twice or so. I like about 20lbs weight and get a liquid smooth surface, so
I suggest using more weight/pressure and see if the sleeks/pits disappear.

Polish out first, then test.  You will adopt a longer stroke, almost a
parabolizing stroke to go to a sphere, then to the asphere.  Notice the
oblate spheroid in the test looks the inverse of the paraboloid, and you
can kid yourself that the relief is reversed, like those lunar craters that
look like bumps.

Fun, Huh?

Colin

-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Scott
Berfield
Sent: Monday, March 31, 2003 12:21 PM To: atm{at}shore.net
Subject: ATM three polishing questions




I am at a total of 8 hours polishing and judging from the excruciatingly
slow change I am seeing (can you say asymptotic approach?) in the surface
quality, I am guessing I have at least another 6-8 hours to go.

I have seen some discussion here (and I searched the archives) about
channel-less laps pro and con. I am using a lap made with a nice rubber
mold from Kevin McCarthy - which made it VERY easy and clean to make and
which resulted in a very easy to conform lap. The pitch I was using was way
to soft, so I re-made the lap with a harder mixture. It has been working
well, but I was getting used to it changing only very slowly. We had a warm
weekend and I did an extended polish on Saturday and Sunday - putting in
about 4 hours total - and now the channels have mostly closed up. I am
getting really good fit, and the surface is looking very smooth. Do I need
to re-open the channels or can I stick with micro-facets only going
forward?

Also I checked the Ronchi patterns on Saturday after the mirror had
stabilized and the curve appears to be more oblate than before. Any
suggestions on strokes, MOT vs. TOT, etc... To get closer to a sphere? Or
does it really matter?

Finally, the Sunday session appears to have created numerous small VERY
find short scratches (sleeks?) about 3/8 - 1/2" long and hard to see
except with a light at a sharp grazing angle. I don't think there is any
contamination - did I just go too long on the wet? Any suggestions on
avoiding these? They seem very faint/shallow - can I assume they'll polish
out?

Thanks in advance.

-scott

--- BBBS/NT v4.00 MP