Re: [ATM] Ideal tool size for f/7 figuring

[Mike Lockwood <melockwo@uiuc.edu>]

Jeff,

jeff newsom wrote:
> Howdy folks--- Currently working on a 12" f/7.25 . Any thoughts on
> what might be the ideal tool size for figuring????Plate glass by
> the way.I'm thinking something on the order of 65% of the mirror
> diameter.-------Cheers----JN 

There's no "right" answer to that question.

At F/7.25, I recommend that you use whatever technique you are 
comfortable with, and whatever technique you believe you can produce 
the smoothest, most accurately corrected surface with.  A smooth 
F/7.25 mirror should produce exceptional planetary images, even for 
plate glass.

If you are good with a full-size lap, that will work just fine.

My personal preference is to use a 6" or 7" lap (a soft 8" lap can 
even do the job) and sneak up on the correction very slowly with a 
fine figuring compound.  If you have subdiameter lap experience, this 
is an excellent, controllable technique, especially with a softer 
glass like plate.

Pay attention to your testing techniques.  You may have to experiment 
to find the best width for the zones you are reading (assuming you are 
using Foucault testing).  Take the time to find the mask you can read 
most accurately, because the correction on such a mirror can be tough 
to read accurately.  Consider using a small telescope behind the knife 
edge to view the mirror under test.

	Mike Lockwood


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