[ATM] The 15.5 Ultrathin Project, Astigmatism discussion

[vorblesnak@peak.org]

> David,

> Please describe the mirror support used for the fine grinding and
> polishing stages. Photos would be good.

No special support for grinding.  I ground with the mirror on the top of a
full sized tool.  Here is a picture of the mirror on the tool.  I think
this was 320 grit ..

http://www.atmlist.net/contrib/vorblesnak-at-peak-dot-org/15%20inch%20ultra%20thin/Mirror%20and%20Tool.jpg

> For example how do you know you have good contact when applying polishing
> or fine grinding pressure?

I just look for contact through the glass.  Polishing it was the same,
look for contact through the glass.  My initial polish was done a 12.5
inch lap mirror on top and I suspect I started the astigmatism them.  It
was kind of like pushing hard rubber over the lap.

> Also please describe your method so we can better understand. How often
> did you rotate the mirror and tool? Also how heavy is your tool relative
> to the mirror?

Hmmm.  Well the mirror is only 3.5 pounds so the sub-diameter tool is
about twice as heavy and the full sized tool is about 20 lbs.  With the
mirror on the top I rotate the glass continuously, and the tool every so
often, probably every hundred strokes or so.  With the mirror on the
bottom I rotate every stroke or every other stroke or every four strokes
depending on what I am trying to do.  Right now I am trying to 'hog out'
the middle and get rid of the hump, so every two or three strokes I rotate
the mirror.  I rotate the tool continuously.

The glass is now double stick foam taped to a stacked disk.  The support /
tape points are from the plop mirror cell design for a 54 point cell.  I
will see if I can get a picture and post it.  You can see all of the
pictures I have posted in the member contributions section of the atm web
site.  Look under vorblesnak at the bottom of the list.

David Davis
Toledo, OR 97391

> Thanks,

> Peter

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