Re: ATM Another Astigmatism Question??

[Anthony Stillman <atmer@flash.net>]

>...a 16" X 3/4" blank ground and completely polished out
>to about to the last 3".
>...ronchi test...good...ROC...central bulge...TDE
>...rotate...mirror...different shape of lines

Ron,

First, assuming you mean a 3 inch wide annulus, then you have about 40
percent of the mirror's surface polished out.  Assuming on average the
remaining portion is half polished out you are not quite three fourths of
the way there.  Polishing out a 16 is a lot of work and personally I'm not
looking forward to doing mine, for the second time.

Second, you may or may not have a turned edge.  Trying to tell with an
unpolished edge is tricky.  It can be done by image processing.  Better to
spend the time polishing I think.

Third, you haven't mentioned your focal length (sagitta).  But with a
thickness ratio beyond 21 to 1  mounting induced stig is a real
possibility.  Of course polishing in stig is also a real possibility.  Try
rotating the ronchi grating rather than the glass.  Then rotate the glass
and repeat.  If the change tracks with the glass rotation its in the glass,
if it doesn't it may be in the mounting.  Putting a fadutial mark on the
mirror with a Sharpie helps with tracking.

Thin glass is easy to distort while its being worked.  Supporting it well
and rotating it often is very important.  I use a green plastic fake grass
door mat from Home Despot to support my thin blanks now.  Also I turn every
thing.  Mostly the mirror and tool but also the board, door mat, and me.

If it seems the stig is in the glass and you have been turning everything,
a lot, then you should check for stress.  Also I don't like foam as a glass
support, but that's based on only one bad experience.

Good luck

Anthony