Re: [ATM] polish time for 13 inch

["Mel Bartels" <mbartels@bbastrodesigns.com>]

> You maybe know there are discussions in books, etc about using
> different types of pitch/lap for faster mirrors. Now that you're
> working somewhat faster mirrors (F/3 in your case), I get the
> idea you're thinking this is really not different than more
> conventional F/ratios, like F/8, etc. In your experience thus
> far, would you want to have thinner lap, or different consistency?


I'm about 75% parabolized on the f/3 after once an evening polishing session
of 30 minutes each for 5 days.  It's been a smooth steady progression with
no backtracking. 

The f/3 acts just like any other mirror (f/4 and slower).  The pitch is a
bit more than 1/4 inch thick.  I add more facets every other session.

So far, the modestly oversized lap (14.5 inches dia compared to the 13 inch
mirror) that's chipped into a parabolizing pattern (gradually less pitch in
contact towards the edge of the lap) along with very long strokes of minimal
side swing has been the ticket.  

A crucial test is upcoming: that's the ability to hold 100% parabolization
while making very small changes in the figure so as to properly distribute
the parabolization across the zones.  I'm fond of the star test at extremely
high powers along with what I call the 'exaggerated Ronchi' test.

The 'exaggerated Ronchi' test is where you warm the mirror face (warm water
bath) above the room temperature then test immediately.  Even small zonal
problems stand out clearly amidst the churning figure.  After the mirror
cools, these zonal problems are extremely challenging to see.  This test is
a good one for bringing out very small zones and zig-zags or abrupt changes
in the mirror slope's derivative.

Mel Bartels


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