From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


Wow, lotsa issues!
Right, better to gauge smoothness and sphericity if you've got very few
lines, i.e. the surfaces nearly match. See Texereau's pictures in his book
showing fringe testing of a Cass secondary.  The fringes are nearly
straight except for the asphericity, which shows dramatically.

Maybe you could map points on a photo, but how to correlate?  Might be
easier math than quantitative Ronchi. Ya got me on that one.

Colin

-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Mike
and Sara
Sent: Thursday, December 12, 2002 8:09 PM To: atm{at}shore.net
Subject: ATM D-K Secondary Fringe Testing



Hello!

    I am wondering if fringe testing can be used to determine whether a
convex secondary is smooth, regardless of the figure.  I am also wondering
if fringe testing can be used to determine if a convex secondary is
spherical if its ROC differs slightly from the reference sphere.

    My logic might be flawed, so I'll go over my understanding of fringe
testing.

    It would seem to be that the shape of the fringes should be an excellent
guide to tell whether one has an edge or zone problem.  The eye is very
sensitive to shape changes, so if the secondary is laid on top of the
reference sphere without shims, a true bull's eye pattern would indicate a
smooth figure, though I don't think it would be possible to tell whether
the shape is spherical, ellipsoid, paraboloid, or hyperboloid, since the
eye is not good at determining the subtle difference in fringe spacing
needed to make such a determination.

    In my case, if the fringes look symmetrical, rather than trying to get
the ROCs exact to determine sphericity, I think I prefer to star test the
system as a whole once the primary is made.  I may end up with a true
cassegrain, D-K or something in between.  Regardless, with a smooth
secondary, the performance of the system should be able to be optimized by
the star test.

    Would this be a valid use of fringe testing, or could the figure of the
secondary still have zone or edge problems which would not show up with the
fringe testing?

    Finally, it would be nice to tell if the secondary was truly spherical,
though of differnt ROC, than the reference sphere, rather tha somem other
shape, such as a hyperbola.  Since I don't care whether the secondary has
exactly the same ROC as the reference sphere, I would appreciate an easier
alternative.

    Since each fringe reflects a differnce between the surfaces of exactly
lamba/2, could a picture be taken of the pattern, blown up to say 8x10, and
the spacing between fringes measured?  They could be plotted to see if the
pattern is spherical, paraboloid, ellipsoid, or hyperboloid?  In this way,
each fringe would act like the shadow of a coude mask.  Can Tex be modified
in some way to handle this situation or would the spacing need to be
plotted by hand and the curve calculated by hand?

    Thanks for your help.  Best regards.

Mike

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