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11 5/16 Inch Byers RA Drive Gear Set

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After 20 years of film astrophotography, the demise of Kodak Technical Pan film in 2003 caused me to drop out of astronomy as a hobby. As an old-time telescope maker and film photographer I could not afford or emotionally accept the move to digital imaging and commercial mountings. I disassembled and stored my 30 year old home made 12.5" reflector and moved on to other things.

In 2007 I decided I would "saddle up" again and build a shorter focal length instrument using parts from existing telescopes I had in storage. As luck would have it, a Takahashi Epsilon 210 astrograph fell into my lap at a great discount, thus solving the tube assembly issue. I wanted to reuse the heavy polar axis assembly from the 12", but it was too heavy to move about with only one person. Eventually I mounted it to a small snowmobile trailer, which worked great.

5 months of testing with the 30 year old 10" Mathis gear convinced me that the periodic error was just too great--digital imaging is much less forgiving than film imaging. The best I could do was about a peak to peak error of 16 arc seconds. Even with the short (630mm) focal length of the Epsilon, my guiding could not reliably "guide-out" this error.

As an old timer, my reaction was to turn the the most trusted name in telescope drives, Ed Byers. However, I had visited Byers shop in Barstow, CA about 10 years prior and he told me he was on the cusp of retirement, so I didn't know if I could find one of his gears. Fortunately, Telescope Warehouse still sells a smattering of Byers gear sets, and I ordered the 11 5/6" version for $1399. It included the big 11 5/16 wheel gear with clutch, the mounted worm, a spur gear drive system and the synchronous motor.

I received the gear promptly, bored to my 3" solid axle. It was a thing of golden beauty, a wonderful sight after those 5 months in the cold trying to get the old gear to perform. I did notice a pretty deep ding on one of the teeth on the worm gear, but figured that I could work around that, as it was not in the middle of the worm. I installed it on my old mount and set to testing. Initial testing revealed two problems--a 6 arc second wobble with a period of exactly 1 minute (rotation period of the motor) and a 5-10 arc second "spike" every 4 minutes, (rotation period of the worm gear).

It was immediately clear that the spike was caused by the ding on the worm gear, which I thought I had avoided. A minor adjustment did get me past this problem, but left the "1 minute wobble". The motor rotated at 1 RPM, so the problem was with the motor or the small spur gear. Examination of the small spur gear under a 30x microscope indicated that it was not damaged. It turned out that the motor shaft was bent by about 6/1000ths of an inch!

Of course I could have just packaged it up and sent it back, but at this point I was somewhat desperate, so I decided to try to unbend the shaft. I built a jig to do this and to my surprise it worked perfectly, leaving me with a shaft run-out of less that I could measure (<0.0005").

After reinstallation on the mount, testing showed fantastic tracking performance, plus or minus 2 arc seconds, as promised, about 80% of the time, with no big transients. Image 1 shows the tracking of the system over a single worm rotation. With the short focal length it is only barely possible to pick up the periodic error above the noise caused by scintillation. With my new STL11000 camera (3 arc seconds per pixel) and good polar alignment, it is barely necessary to guide over 5 minutes exposures. First result with the system can be seen at http://www.flickr.com/photos/43047947@N06/

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