Re: [ATM] my ongoing war story (polishing and figuring)

[Mike Lockwood <melockwo@uiuc.edu>]

Rodney,

Rodney Mayes wrote:
> My apologies.  It was late when I posted.
> 10 inch diameter, f/7.55, 7/8 inch plate glass

OK.  That tells me two things - first, the focal ratio is quite 
manageable, and with patience you will produce a very good mirror. 
Second, this mirror will change shape on the test stand for up to an 
hour after you work on it, and possibly more.  It will also be 
sensitive to how you support the edge during testing.

The possible astigmatism you saw could have been caused by poor edge 
support of the mirror on the stand - the only way to confirm this is 
to then rotate the mirror in small increments (30 degrees or less) and 
see if the pattern changes consistently.  The mirror should ideally 
sit on the edge supports exactly the same way after each rotation.  If 
the edge of the mirror is unever or not square to the rest of the 
blank, this can cause the mirror to bend.  Of course, after touching 
the mirror to rotate it, you must allow some time for it to cool.

Best to support the bottom 30-45 degrees of the mirror with some sort 
of cushion.  A radiused wooden block and a soft piece of foam may 
help.  Tall shag carpeting may be good, too.  While the mirror is 
cooling it will also be slowly sinking into the cushion, so testing 
will be inaccurate for that reason, too.

I wrote:
>> I can't comment on the central hill because we don't know the cause 
>> yet. But I will say that the corrections for the outer zones are far 
>> more important at this point.  With proper technique, a central hill 
>> can be removed in minutes and is far less worrisome than a rolled 
>> outer zone or severely undercorrected outer zone.

> Since I began the project, I have been very, very, cautious with regards 
> to the edge (TDE).  So much so, that I have ended up with a TUE that 
> caused me to have to polish a lot longer because I did not want to 
> return to fine grinding if at all possible.  The edges look good as far 
> as I can tell. Ronchi bands look good and I have not, at least visually, 
> noticed any edge problems. Best regards.

What I was referring to were the outer zones, say the outer 1" of the 
mirror.  The edge is the last 1/16"-1/32" of the mirror.  During early 
figuring, I encourage people to concentrate on getting the correction 
of the outer zones right.  That's what my figuring article is geared 
toward.

As I mentioned before, and as Vladimir also argued, it would be good 
to learn Foucault testing.  We will likely be able to help you out 
more easily with the figuring.

Last request - please tell us how many hours it took for the channels 
in your pitch lap to close when you were doing NORMAL polishing, and 
how wide the channels were at the start.  This will help us judge the 
hardness of the pitch.

	Mike Lockwood

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