Re: ATM polishing

["Richard Schwartz" <richas@earthlink.net>]

Failure to complete fine grinding in the edge zone of a mirror is a common
problem with beginners.   The problem seems to be more common when the
grinding tool is solid glass.   This problem is made worse by the urban
myth that 5 micron is bad and causes scratches.   A grinding tool with
channels, such as a tile tool, does a better job all over the mirror.    If
poor grinding is the problem, you will see the sun become a red giant and
engulf the earth before that glass is polished.

There are some simple tests you can do on your mirror surface before you
start polishing.  These tests will tell you if your final fine grind is OK.
Of course, these tests do not appear in any book on telescope making and
were never published in any authoritative magazine that I know of, so they
must be invalid and should be avoided as heresy.  I think I will delete
them from my web page to protect the faithful.

. . . Richard
Death to the Unbelievers!

> [Original Message]
> From: Mel Bartels <mbartels@bbastrodesigns.com>
> To: <atm@shore.net>
> Date: 1/1/2004 8:32:11 AM
> Subject: Re: ATM polishing
>
>
> >>>
> Hi , Im working on my first 8 inch mirror, and have spent about 10 hrs
> polishing with a pitch lap and CeO. A four millimetre ring around the edge
> of the mirror does not seem to be polishing out, . Could anyone suggest a
> strategy to remedy this, without of course going back to fine grinding.
> <<<
>
> Does it show any signs of polishing?  Sometimes a narrow ring persists at
> the edge during polishing, particularly if the lap is too small and or the
> polishing strokes too short or not wide enough.  By the way, 10 hrs is a
> long time for a 8 inch mirror.  Did the mirror mostly polish out in
several
> hours, or are you being cautious?  You can really ramp down during
> polishing - the goal is to remove the damage caused by the pits in fine
> grinding.
>
> Mel Bartels
>