Re: [ATM] Ronchi testing

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

The number of lines is determined by the position of the Ronchi Grating, the
curve of those lines is the eccentctircy of the surface.  Yes, -1.0 is a
parabola and a -0.32 is about 1/3 of a parabola.
A problem that a lot of  people have when doing the Foucault test with a
Couder mask is that the typical mask that is made has all of the zones on it
and that makes it hard to look at the right pair of zones.  I keep sets of
masks for several common sizes of mirrors with the odd zones on one mask and
the even zones on the other mask of the set.  This allows me to see the
zones of interest with a lot less trouble plus I get to see equally above
and below the centerline of the surface.  For some mirrors that I've done,
I've done a seperate mask for each zone so that there isn't any dispute as
to where the shadows are.

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