Re: [ATM] 12.5 inches BVC advice for parabolizing

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

Suiter is coreect but maybe not for the rason that you suspect although you
are showing the difference between two curves is essentially the same.
The thing is that you can see very nicely with the Ronchi test any changes
in the curvature of the mirror.  This sensitivity with the test will quickly
get the mirror into the correct shape where the ROC smoothly changes from
one length at the center to another ROC at the edge.  This is the strength
of the Ronchi test!  It's weakness tho is that you can't tell whether that
curvature is a parabola.  It could be a hyperbola of -4 for all that anybody
can see with any particular random mirror on the tester - this becomes a bit
different in a shop where many mirrors of the same dimensions are done as
you can put reference marks so that the grating lines can be measured and
referenced.
The Foucault test, on the other hand is uite lousy relively for looking at
the smooth curvature changs but will tell you very nicely whether the shape,
as measured by the different zones, is the right parabola shape.  It just
won't tell you if the zones you measured are just because the tilt of the
zone is part of that smooth surface ro something else.  Even a look at the
overall shape without a mask won't show the many small errors in shape if
the change is smooth.
Bob May
bobmay@nethere.com
http://nav.to/bobmay
http://bobmay.astronomy.net

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