From: "Bill T." 
To: atm{at}shore.net
Reply-To: "Bill T." 



If the stig was created while polishing a surface that was spherical coming
out of grinding, you can probably polish it out.  But if the stig was
created during grinding, then only grinding will remove it in any
reasonable time.  What you polish-in, you can polish-out.  What you
grind-in, you can only grind-out.

If your mirror polished out unevenly, then you had a problem with your
grind.  If you had isolated "islands" on the mirror that polished
out faster or slower than the rest of the mirror, then you for sure had a
stiggy grind.
  If you had all-the-way-around circular zones that polished faster or
slower, then you probably didn't grind in stig, but the grind was not quite
spherical.

If you had the "isolated island" problem during polishing , then
maybe you have a bad stroke ratio on your machine.  If a stroke ratio is
too repetitive, you can come up with yin-yang looking effects that
typically have several noticeable radial artifacts. Put a marker on the
overarm and draw the actual polishing pattern on the table (or mirror
surface if you're brave).  If the pattern looks repetitive, then adjust the
gearing until the pattern spreads out over the entire surface within no
more than a few minutes.

Another effect sometimes confused with astigmatism is if the mirror is
ground or polished off-center.  Make sure the mirror is centered on the
turntable to better than 1/16".

I'm not sure how you're attaching the mirror to the table, but I would
advise against clamping it with bolts from the sides, or with pitch on the
bottom.  Both of these have too little give.  Best support for the bottom
of the mirror is something that can take up some unevenness in the table
surface, like felt, rug, bubblewrap or whatever.  Tape the sides of the
mirror to the sides of the table, with the tape bunched up to provide extra
give - it helps to make a wood adapter circle the same diameter as the
mirror it the table is too big.  Once in a while rotate the mirror is
relation to its support.

Bill T.


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