To: atm{at}shore.net
From: mdholm{at}telerama.com
Reply-To: mdholm{at}telerama.com


Scott is correct, 8 - 10 zones is a bit much for a 4.25 inch mirror.  If I
were doing it, I would make two masks with three zones each as follows.
(Assumes clear aperture is 4.20 inch and secondary diameter is 1 inch.)

Main mask zones
Zone    R inner    R outer    R center

  0      0.500      1.279      0.931
  1      1.279      1.739      1.518
  2      1.739      2.100      1.924


Intermediate mask zones
Zone    R inner    R outer    R center

  0      0.000      0.853      0.522
  1      0.853      1.488      1.192
  2      1.488      1.924      1.713

These dimensions are in inches.  Rinner and Router are the cutting
dimensions for the mask. Rcenter is the number for the analysis.  These
zones stagger above
and below the centerline of the mask as shown in Texerau.

This test won't quantify the central hole, but that is probably immaterial
if the mirror is intended for a Newtonian.  The central hole will be
shadowed.

When entering these zones into the analysis, the readings would alternate: I0
M0
I1
M1
I2
M2
because that is the order of increasing Rcenter.

These six zones will capture a fair amount of the undulations in that mirror and
 give a fairly good idea of the true profile.  The very edge might not be well
represented.  It may be possible to use a very narrow edge zone mask. for
that, I would try a third mask:

Zone    R inner    R outer    R center

  0      1.900      2.100      2.001

This still doesn't get the measurement all the way to the edge of course,
but, unless that edge is really rolling hard, a sample at 1.924 plus a
sample at 2.001 should allow Sixtests to extrapolate the edge with
reasonable accuracy.


Others have used a small telescope behind the knife edge to make viewing
the Foucault test on a long focus mirror easier.  This might be especially
helpful when trying to read that narrow edge zone.  One atm suggested the
"GolfScope" sold at Radio Shack as being well adapted for this
purpose.  The finder from a cheap department store telescope might actually
be useful in this application, though if one wears glasses, the horrible
eyepiece usually found on these may not give enough eye relief to get the
glasses in.  Recently I got a "special offer" with one of my
credit card statements that included as one of the "free" options
a 3x12 prism monocular.  "Shipping and Handling" came to $4.50
each.  I ordered two, figuring that one might be useful in the Foucault
test application,
and maybe one might be useful as a finder on a low power scope.  The
picture of the monocular shows that the prism introduces a 45 degree bend. 
That might take
some of the neck strain out of my current Foucault test setup.

Mark Holm
mdholm{at}telerama.com

--- BBBS/NT v4.00 MP