Re: [ATM] Null tests and astigmatism

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

This is why I explictly note that there are two locations which
are 90 degrees apart and 45 degrees from the vertical or
horizontal axis where the test will show the same results.  If
you look at the way that astig. shows up, you will see that these
two alignments will produce identiaal numbers yet there can be
several waves of astig. in the mirror surface!
Also, there is no need to have an "absolute" measurement of the
diatance as the center of the mirror is always the same on a
decent mirror (one that doesn't have serious problems in the
central area of any significance) which provides the zero
reference point for the shape of the surface.
I also mention the Ronchi test for the reason that it does an
excellent job of determining astig., more so than that of the
Foucault test with which one can only infer that there is astig.
of smaller amounts.
Please note that astigmatism is the saddle shaped surface error
where you can measure a different radius on one axis relative to
the other axis of the surface which is 90 degrees away from the
first axis.  "Higher orders" are nothing but ripples in the one
direction and they will be seen differently in either test.
Bob May

rmay at nethere.com
http: slash /nav.to slash bobmay
http: slash /bobmay dot astronomy.net


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