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.

Bob and John Generating motor

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.

Covered machine Mid-grinding

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!

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