Re: ATM ronchi-confused

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

The images with the Ds in them are more for reference as to what the image
looks like that close to the center.  Better to go further from the ROC with
the grating and see what the lines look like as they curve.  The problem
with the Ronchi Test is that you need to be very careful with the
positioning of the grating and in reading the lines or you won't be seeing a
Paraboloid but rather a Hyperboloid or Elipsoid curvature.  The Ronchi Test
is best done as a check on the smoothness and evenness of the curvature
rather than the qualititive test as to exactly what figure that you are
looking at.  The Foucault Test is excellent for finding out if the various
zones are the correct ROC but relatively poor for finding out whether the
curvature changes are correct.  The two tests actually compliment each other
and my process is to get the zones right and then use the Ronchi Test to
insure that the shape of the mirror is correct.  Just using the Foucault
Test, especially when not enough zones are done, can produce a very zoney
mirror while the Ronchi Test has a very difficult job saying when you have
the correct Paraboloid curvature.
Usually when testing, I use zone 1 and 5 of a 5 zone mask to tell if the
mirror meets the Paraboloid change for the ROC and then look at the overall
shape of the mirror for whether it is a smooth and correct shape.