Re: [ATM] fishing for a lil polishing advice!

[Mike Lockwood <melockwo@uiuc.edu>]

Hi,

Ken Hunter wrote:
> NO !!! The normal strokes will make the central portion of the disk
>  deeper making the edge to ALWAYS appear turned down until the
> curve reaches the outer edge.

True, but if the mirror is a good sphere after fine grinding, then the 
edge should start to polish within an hour, and should quickly display 
a uniform diffraction ring if the edge is good.

Why not start polishing TOT?  Or just mix TOT and MOT.  Either of 
those will avoid the problem, unless there's a rolled edge after 
grinding, in which case the most efficient thing to do is to return to 
fine grinding for a short time.

> Once you are fully polished - FULLY POLISHED - you can then start
> wondering about the APPARENT or REAL turned edge. They are
> different things and should be ignored until the time is right to
> worry about them. That is in the figuring stage NOT in the
> polishing stage.

The time to worry about an error in polishing technique is as early as 
you can figure out what you're doing wrong, in my opinion.  It causes 
less time to be wasted, and can save many hours of frustration and 
wasted energy.

I know, I know, beginners don't understand testing and what is really 
a serious problem versus an insignificant one that will go away.

Perhaps what is lacking here is a set of photographs or a tutorial 
showing what Foucault testing of a mirror that is not completely 
polished should/could look like.  Then such tests would be less likely 
to be misinterpereted.

A simple check of the edge condition and the maximum deviation of the 
zones from a sphere would be very good information.  If the zonal 
readings depart from each other by more than say 0.050" (moving source 
tester) then that might be cause for some investigation.  If they vary 
by 0.1" or more, then it's time to correct something.  If zonal 
departures are less than 0.050" and the edge is good, keep polishing 
with confidence.

I like the idea of getting acquainted with testing as early as 
possible, so long as the results are only used to correct significant 
flaws in polishing technique, and the beginner mirror makier is not 
trying (pointlessly) to figure an unpolished mirror.

	Mike Lockwood

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