Re: [ATM] Ain't got no shperometer

["Bob May" <rmay@nethere.com>]

That is indeed a big DUH!
I've been there and done that myself with some small lenses which
is more fatal than with a mirror as the thickness of the glass is
also important.
For that mistake, just do the Sharpie test or the pencil test
until they show that the two surfacces are back in agreement.  As
long as the hole is there, the center will strongly show that
you're not all ground back to a sphere again.  You will see a
sharp delination between the two radii and that will persist
until you get down to a fairly close agreement on the radius
desired.
One thing to note on this thread is that the reason for the need
of the test wasn't mentioned right away so various people made
different decisions of how spherical you wanted to be in seeing
how spherical the surface is.
One of my favorite tests is the flashlight test where you setup
similar to the Foucault test only you use a flashlight  next to
your head and find the return lightt by bobbing your head about.
When you get to the approximate ROC of the mirror (which is
wetted_, the light will blink rather than travel over the mirror
in one direction or the other.  Inside of the ROC, the light will
follow the head motion and outside, it will go in the opposite
direction.   Large changes of radius like you have will show up
as either the ring or the center doing the blink while the other
part of the mirror will not.  A simple test that can be done
anywhere that there is enough room to do it.
Bob May
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