Saturday, September 29, 2001
Meeting of the S*T*A*R Astronomy ATM SIG (Amateur Telescope Making Special Interest Group)
Four of us got together Saturday afternoon to work on various projects. In attendance were the host, Michael Lindner, along with Andy Zangle, Gordon Waite, and Rob Teeter. Rob was home from college and had a chance to drop in and help test one of my mirrors. I had a very old 6" mirror that I acquired in the early 70's from a friend. I had never checked out the focal length or condition of the mirror. I knew it had been polished, but nothing more. Mike put the mirror on his test stand and ran a Foucault test, and determined that the little mirror was parabolized. Michael posted, "I ran some confidence error bars and the results are the mirror is nominally 1/14 wave, with an 80% confidence of better than 1/12 wave." That's far better than I had any right to expect! Now I feel obligated to build a telescope to put that mirror to work. Anybody have a good design?
Andy and Michael helped me pour a plaster and tile tool to use for grinding the 8" f/6 mirror. Here are some of the highlights... click on the thumbnails to see full-size images, mostly 1024x768 and around 135K each. And above all, don't miss clicking on the movies!
|
We started by getting together the materials. An orange Halloween bucket was used to mix the dental plaster. Michael had a collar to go around the mirror to support the plaster. We used hex tiles. |
 |
First we covered the face of the mirror with plastic wrap. |
 |
Then we wrapped the collar around the mirror, and smeared the face with some vegetable shortening to give the tiles something to hold on to, and to keep the plaster from sticking to the plastic wrap. |
 |
We mixed the plaster to the texture of heavy cream, and poured it into our mold. |
 |
We had a small bucket, so we mixed two batches and poured one after the other. Michael's recently-turned-four son Alex stopped in to help! |
 |
Andy finishes up the last batch, and wonders how to get the pumpkin cleaned out... |
Here's an mpg movie of Andy pouring the plaster into the tool mold. (Download size is about 1.4MB!)
 |
After letting the tool dry for about an hour, we stripped off the collar. Michael poked it to make sure it was dead. |
 |
This is the face of the tool before the cleanup operation. Lots of plaster hanging around, and the white areas are still covered with shortening. We used a tiny screwdriver to clean some of the surplus plaster away from the channels between the tiles. Michael quoted John Dobson, something on the order of, "the diseases of the tool are not reflected in the finished mirror" or some such condolence. In other words, it looks ugly, but it will work just fine! |
After we cleaned up the tool, Michael actually got out some #80 grit, and we set to work on the mirror to see how the tool behaved. Each of us took a few turns around the barrel, and the new tool did just fine. We managed to scratch off the old aluminum coating, and put some fresh pits in the surface!
We also took care of a couple of other little projects. I had also brought along another 6" mirror that was partially ground, along with the matching tool. We determined that the little mirror was at about f/4.5. We used a carborundum stone to turn the edge on both pieces of glass, in anticipation of another grinding project.
Michael's grinding machine is under construction. He has a temporary turntable in place (made of two layers of 3/4" plywood) and it is connected to a 1/4 hp DC motor. Michael has a massive aluminum plate that he's going to mount in place of the wooden table. The platter turns at 72 rpm's, and is a little scary when it is running full blast with a big mirror, a cast-iron tool and 25 pounds of weights on top. Here's a movie (1.3MB) that shows Michael taming the beast! And no, Andy's not crouching in fear behind Michael... he's just checking out the incredible grinding action!
We finished our meeting a little after 5:00 pm, and decided to have dinner together. We collected up the various wives and children and met at The Bistro in downtown Red Bank, New Jersey. The nine of us feasted on some great food--my beef tenderloin was terrific. The stuffing ended around 8:00, and a good time was had by all!