Since 1926, amateur telescope makers and astronomers have gathered at Stellafane in Springfield, Vermont, to compare notes, compete for awards for the best home-built 'scopes, and enjoy rural Vermont's dark skies. This year the convention will be held July 25-27, and registration is now open; you can download a brochure here.
AMATEUR TELESCOPE MAKING
I made my first mirror and telescope in my teens, and then did other things for close to fifty years. Now I'm pushing glass again. This is just a convenient place for me to keep track of the process. For a wealth of information about amateur optics and telescope making, visit the Stellafane web site.
Yesterday was mostly overcast, and I vacillated until the last minute before finally deciding to gamble on a weather website's prediction of an appropriately timed gap in the cloud cover at Stellafane. I drove to Burlington and picked up my son Jake, and we made the two-hour-plus trip to Breezy Hill, finding a nice crowd of other astronomical optimists. A patch of blue sky opened up as people set up their scopes, but the clouds moved back in at the beginning of the transit, so we missed first and second contacts. But then things opened up again, and our optimism was amply rewarded. There were a dozen or more STM members present, including some who drove farther than I did (that's real optimism!), and several interested newcomers found their way to Stellafane. It was a lovely time—we were both very glad we decided to go. After all, neither of us is likely be around for the next transit of Venus, in 2117.
Above, an image from a member's 10" Meade Newtonian 'scope, made by aiming my low-end point-and-shoot camera through the eyepiece. Venus at the top, sunspots visible below.
Part of the group and some of the 'scopes. The Porter Turret telescope is on the right.
More of the group, Stellafane clubhouse in background.
Solar image projected in Porter Turret Telescope.
Waiting for a gap in the clouds.
Image from a member's Televue refractor, through thin clouds, again made with point-and-shoot camera.
The Springfield Telescope Makers will host the 77th annual Stellafane Convention August 16-19 on Breezy Hill in Springfield, Vermont. Amateur astronomers or telescope makers everywhere owe it to themselves to make at least one pilgrimage to Stellafane, a National Historic Landmark and the birthplace of amateur telescope making in America. This year's convention will feature keynote speakers The Meteorite Men, a special emphasis on solar observation, a broad program of workshops and talks, as well as observing opportunities using the on-site observatories and the scores of 'scopes brought by attendees (bring yours!). For more information, you can download a brochure here (pdf), and by all means visit the STM website here.
Well, spring and summer brought many chores arround the house, a constant scramble after enough business to keep marginally afloat financially, and a bazillion other distractions. Time available for atm-related activity was scarce and pretty much entirely taken up with Stellafane Convention. For the first time I pitched my tent a few days early and helped with setup and preparation for the big event. That was a lot of fun. It really is remarkable how the STM crew works. People just see something that needs doing, and do it. Somehow, without much overt organization, the whole immense project comes together.
Finally, last week, I got back to the mirror. When I left off after the last post below, I had been advised continue to do a narrow W to both deepen the correction in the central zone and spread it toward the edge, so, after several hours of pressing to get the lap back into contact, I did that for fifteen minutes or so. Then on Saturday I took the mirror to Stellafane to work on it where I could get guidance from experts. A standard Foucault test showed about 75% of the total needed correction (in terms of knife-edge motion to move a null from the center zone to the edge zone), so once again we did a short session of narrow W strokes (I say "we" because Dave Kelley, one of Stellafane's mirror-making gurus, did some of this work both to show me exactly what he meant and because, lacking a cleat setup to hold it, we were having trouble getting the tool to stay put on the table—so I held the damn thing with extended fingers while he worked the strokes. This brought the total correction right to the correct value, but when we looked at the mirror using autocollimation, we could see that the edge zone was still somewhat discontinuous from the desired paraboloid.
Autocollimation testing involves a reference flat, and as I've noted below, Dave has a spectacularly good one. The light reflects twice from the tested mirror—originating behind a central hole in the flat, traveling to the tested mirror (at focal length, not radius), reflecting back to the flat, and then from the flat back to the mirror, and finally back through the central hole in the flat to the knife edge. The result is that with a true paraboloid you see an evenly illuminated field, which "winks out" evenly and abruptly when the knife edge intervenes—a null test. Any zonal shadows represent departures from the paraboloid. It's a qualitative test, with no easy way to measure errors, but very sensitive because any error affects the rays twice.
On my mirror we could see a brighter zone on the right edge and a darker one on the left—an outer zone on the mirror that was focusing longer than ideal. Dave prescribed a short session of chordal strokes, with varying overhang around 1" or less and a little thumb pressure on the overhanging side. I did three rotations of the mirror like this, maybe ten minutes, and then cleaned it off and put it back on the tester. Dave took a look and said “you bagged it” (it took me a second to process that—at first I thought he meant something had gone terribly wrong). But what I saw when I looked at it was beautiful. The knife edge cut off the light almost like flipping a switch, and the surface looked velvety smooth. It seems I have a very good mirror, considerably better than the first one. Can't wait to get it into a scope!
Above: foucaultgrams, at radius of central, 70%, and edge zones.
Prior to Saturday's STM mirror class I worked the mirror with center-over-center strokes to try to get rid of the rings you can see in the images in the March 11 post below. It took almost 2 hours of polishing away, but all that was left by way of departure from a sphere when I arrived at class was a slight mound in the middle 40% of the mirror. Dave K. suggested I start with a narrow parabolizing W stroke, which added correction to the center, and then with a slight chordal stroke, with extra thumb pressure on the overhang, which pushed the correction toward the edge. When I got home I decided to do some more of the narrow W—about 10 minutes of that brought the mirror to the state depicted above. So now my question is: what next? My guess is work on the edge some more, perhaps with the same slight chordal overhang and thumb pressure. But I'm unsure and wonder how much overhang to use. Below are the zonal knife-edge measurements, made with a 5-zone couder mask.