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


Mark:

        I'm a stickler for edges and find that the Ronchi test can be difficult
to
read at the exact edge.  The grating causes diffraction of the light and
creates overlapping images of the mirror.  The Ronchi bands seen in these
overlapping images at the edge create a confusing image, looking like an
outward flare and hook.

        I suggest you take real close care in looking at the edge, and see if
this
overlapping image effect is what you're looking at.

        Varying the grating spacing may make it less of a problem.  I prefer
50lpi,
and careful observation.

        Also, it makes sense to use the KE to determine the hooking of the band
at
the edge. Put it a half inch inside focus and cut the shadow 1/4 the way
into the lightpath.  If you see the distortion at the edge, it's real.

        While at a sphere, try the star test using a bright pinhole or
underpowered
laser pointer with its collimating lens removed.  Use diffract or Suiter's
book on star testing to verify you have a sphere and a good edge.

        Using the Ronchi with a monochromatic source makes for interesting
ronchi
bands which I think are worth looking at.

Colin

--- BBBS/NT v4.00 MP