2012 stellafane convention

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.

and many months pass...

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!

what next?

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.

new lap

The photo below shows my new lap, poured from straight Gugholz 64. My lack of experience caused me to let it cool a bit too much before I attempted to channel it, so I was in an awful hurry to get that done before the pitch got too hard. Hence, perhaps, the uneven facet sizes, although I hope that's not a problem. The lap is also thicker than necessary, and the channels only go down about half the pitch thickness. I may deepen them before I attempt again to parabolize.

A couple of sessions of 1/3 center-over-center strokes have not made a noticeable difference in the ring zones visible in the photos in the last post. They have, however, produced some overall roughness. I guess, barring contrary advice, I'll just go ahead and do some more of the same. The same stroke, with constant around-the-barrel walking and mirror rotation, produced a nice smooth sphere prior to my first attempt to parabolize.

negative progress, positive learning

So, above is the FigureXP graph of the surface error of my 6" F4 mirror when I put it aside to fiddle with my tester (see post below). Stellafane Mirror Class was last Saturday, so I took the mirror with me in this condition. Dave Kelley, one of the resident testing and figuring gurus, took a look at it. His beautifully machined aluminum testing apparatus is of course vastly more precise and sophisticated than my makeshift plywood rig. For starters, his test bench includes an exqusite (better than 1/20 wave PV, I've been told) 16" flat for autocollimation testing. More importantly, he has a professional career’s worth of experience interpreting what he sees at the knife edge, and an apparently intuitive knowledge of what to do strokewise in response.

Through the day and with frequent testing, Dave had me work with various strokes and various amounts of suppression of the middle of the lap—which was accomplished by pressing with a square of paper between the mirror and the lap and later by actually shaving away at the lap in the middle with a razor blade. I should have made better notes; I don't remember precisely all the variations in strokes used. At first I was completely mystified because I assumed that the goal was to move the mirror toward a paraboloid. But after a while I understood that Dave wanted me to go back to a sphere and start over with the parabolizing.

I still wasn't sure why he was having me do what I was doing. When I got home it occurred to me to have Figure XP display the graph again, but this time in reference to a sphere rather than a paraboloid. This is easy—the conic constant of a paraboloid is -1; that of a sphere is 0, and this is something you enter when setting up the file, so all I had to go was go back to the setup page and change that one field. Below is the result, showing the mirror’s departure from sphericity. It's easy to see why he wanted me to work the outer zones but not the middle, and I think the strokes he prescribed were chosen to avoid turning down the edge.

The result at the end of the day was very near a sphere, but with annular rings of error showing in the foucault test, as you can see below. I can see them a little in the Ronchi test, and more so in the Foucault, but they're pretty small errors.

The kicker, though, was that Dave thinks many of my problems are due to the lap, which he deems too hard and insufficiently off-center. So my next project is to make a new lap. I hope to find time to do this well before the next (and last for this year) class, so I can work the mirror a bit before then, first to try to smooth out those rings, and then to start parabolizing.

Note: I had to exaggerate the contrast in photoshop to make those rings visible onscreen. They were somewhat more obvious to my eye at the KE. Also, those black blotches are artifacts of dust in the camera optics.

And a final note: At the meeting after the class, I once again was voted into (associate) membership in the Springfield Telescope Makers. I can't begin to express how great it feels to be once more, after so many years away from the venerable group, welcomed into the fold. To any members who happen upon this site: THANK YOU VERY MUCH.