32" F/4 Dewedging
Well, sometimes things just don't work out.
The photos of the 32" F/4 BVC mirror on my "Foucault Testing Hall of
Shame"
page are of a friend's mirror. Bob Holmes hunts supernovas and
minor
planets - he does real research - and his newly built 32" scope waited
for
its mirror for nearly a year and a half before we got the blank back,
wedged,
unfinished, scratched, and complete with tens of waves of astigmasitm.
The
blank had lots of wedge - 3/16"-1/4" or so. Therefore, the first
step
to finishing it was to dewedge it.
For this task, the large machine that John Pratte and Bob built was to
be
used to provide the turntable, with a borrowed diamond tool and a DC
motor
providing the grinding power to true up the blank. The motor and
tool
ride on a wooden beam that can move across the blank manually, and is
simply
clamped in place while the mirror rotates underneath. Here is a
photo
of Bob (left) and John (right) with the machine and the dewedging
apparatus.
(They
smiled when the work was done!) Also, below is a photo of the
motor
that turns the tool, hooked up to the speed control from my small
grinding
machine.
Below is a photo of the machine, covered in plastic, ready for the
dewedging
operation. On the right is a photo of the dewedging operation
after
a few passes. The left side of the mirror shows a darker area
where
the blank has been ground. The blue lines are ink that help show
where
the blank had been ground.
After quite a few hours of work, the back of the blank is now quite
flat,
and ready for regrinding and refiguring, which is my next task.
The
grinding was done slowly and with constant water lubrication to avoid
generating
dust or vapor containing glass particles. It was nice to see
ourselves producing the same sort of "generating marks" that we have
only seen on commercial blanks!