Re: [ATM] HCF help!

[Jim Burrows <burrjaw@earthlink.net>]

At 2005-02-10 14:19 +0000, Matheus wrote:

>some time ago, I had tried to use HCF to polish a mirror of 160mm.
>The ripples produced by the HCF disappeared after
>softer polishing, using very little rouge and slow movements, soft and
>without pressure.  The only bad result of this experiment was the appearance
>of an edge extremely raised (a zone of several centimeters).  This zone
>affected only 5mm end of the mirror.
>I would like to know if somebody has experience with this technique or if
>somebody knows what it can have happened for the edge of the mirror to be so
>high.

I've used HCF on the primary and secondary of a 10" RC cass.  It's powerful 
stuff and perfect for figuring (using HCF "wax dolls") a mirror like the 
f/2.6 primary, which couldn't be done with any strokes on a full pitch lap; 
small tools would do it, but they're awfully slow.  In your case, an HCF 
ring around the edge would dispatch the high edge in a few strokes - be 
CAREFUL.  Probably the high edge came from the HCF lap not going all the 
way to the edge, or the edge of the HCF substrate was depressed so you 
didn't have contact.

The trouble with HCF is the ripple.  Set your mirror up for a whole-mirror 
Foucault test and look along the shadow boundary - the ripple will be 
obvious.  The resulting roughness isn't a killer, but its RMS has to be 
included with any other surface errors.  If you have a copy of Suiter, look 
at roughness star tests on p. 244-5.  The out-of-focus images look 
horrible, but the 13.8 nm surface RMS itself is passing for in-focus images 
(cf. Tex, p. 62,  "The combined defects...would pass quite unnoticed when 
observing a star at the focus.")

         -- Jim Burrows
         -- mailto://burrjaw@earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84) 

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