Russ Jocoy's 13.1" F/4.5 Dob
Somehow over the years I've ended up corresponding with Russ Jocoy,
glass disk cutter extraordinaire. I like making mirrors, and he
has glass to cut. So, after some shady deals and many phone
calls, I ended up fine grinding, polishing, and figuring a 13.1" x
3/4"-thick plate glass mirror (NOT slumped) from an old pregenerated
blank he had obtained through other shady dealings (kidding). I
polished the heck out of it with my machine and then hand figured it to
a smooth parabola with an excellent wavefront rating.
Relating to the recent discussion on the ATM list regarding thin-mirror
testing, my strategy on this mirror was to first flatten the back very
carefully, taking the grinding up through 5u, and then properly support
it throughout the TOT grinding, polishing and figuring. Testing
was done with a contoured bottom support lined with pile-type
weatherstripping. This supported the mirror with the least
distortion of the various methods that I tried. The mirror was
tilted back very slightly.
Anyway, figuring proceeded nicely (with an 8" lap, by hand). I
could see slight astigmatism in the Foucault shadows, but rotating the
mirror produced the same shadows. Having taken care to avoid
generating astigmatism with any grinding or polishing operations, I
attributed the astigmatism to the stand, and simply TESTED THROUGH
IT. (Interestingly a 13.1"x7/8" F/4.5 Pyrex mirror that I made in
parallel with Russ's plate mirror showed no visible signs of flexing on
the test stand.)
After figuring was done, we star-tested the mirror. It provided
excellent images, a beautiful star test, and perfectly ROUND star
images. No astigmatism to be found. So, my conclusion is
that with thin mirrors (but not SUPER-thin) one can get fairly accurate
testing results with the mirror supported gently vertically.
Russ has finally completed his latest masterpiece, a neat-looking Dob,
and has achieved first and second light and maybe a few more.
Below are some photos he sent me of the completed scope. As of
early February 2006, Russ is tweaking his collimation skills and
enjoying some fine views with a mirror that equilibrates very
quickly. He also has a 9" F/4.8 that I figured for him a while
back, and I hope to post some photos of them here in the future.
Now I need to get to work on the my 13.1" F/4.5 scope! Here are
the photos.
