Re: ATM Polishing out astigmatism

["Bill T." <twentiethwave@hotmail.com>]

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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