From: Mark Holm 
To: lafey{at}netcabo.pt, atm{at}shore.net
Reply-To: Mark Holm 


Filipe Silva wrote:


> This introduces a question I have:
> Since my mirror isn't perfectly circular (due also to the above)=
>  -
> how can I calculate (in PLOP for example) the mirror cell=
>  supporting
> points? [I know I'll have to use a 9 point support since the=
>  mirror
> is a bit thin (15mm)]
>
> Thoughts?
>


I think this introduces a difficulty with Plop.  You want to optimize a
mirror of smaller optical size than it's physical size, and non circular as
well.  I don't think Plop can handle this without modification (probably a
rather difficult modification).  The reason is that Plop takes into account
both a mirror's physical and optical dimensions.  The good news is, you
probably don't really need to worry about it.  A 6 inch mirror isn't that
difficult for Plop, and the gain from optimizing for the slightly smaller
size would not be very great anyhow.  Just run the 6 inch case.  BTW, I am
certain you won't need a 9 point cell.  Three is enough.  Put them at a
radius of 1.185 inch. (See the 3-Point Cell section of
http://www.telerama.com/~mdholm/atm/cells/index.html) You could go down to
12.5 mm thickness and still be good with three point support.

If your chips are not too numerous, you might consider just painting them
with black paint after the mirror gets back from coating.  One caution, if
your chips
are serious. and occurred after fine grinding, there might be some distortion of
the glass surface around them.  You would see this in a Foucault test.  In
that case, you need to paint out or mask off the distorted part as well.

There is no need for the outer edge of a mirror to be perfectly smooth.  A
few chips don't really add much diffraction.  What you do want to avoid is
light reflecting from inside the chips back toward the focus.  That can be
done with the black paint.

Mark Holm
mdholm{at}telerama.com

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