BART


(Bart and Ron Wood in Checotah)

    BART (Big Astronomical Reflecting Telescope) is a 24-inch, f/5, equatorial, fork mounted, Newtonian telescope. It is integrally mounted on its own two wheeled trailer to be towed on the highway. It was built in 1992 near Checotah, Oklahoma by several members of the Tuva Astronomy Organization under the direction of Byron Melland. It is the second of two such scopes built by Byron; the first, built in 1982, is named Mira and was a project of the Pomona Valley Amateur Astronomers club. The building of Mira is extensively described in Volume #17 of Amateur Telescope Making and is also pictured on page 82 of the August 1982 issue of Sky and Telescope magazine.



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Mira

     Like Mira, BART has homemade components and is largely made of salvaged materials. In fact at the 1994 Texas Star Party BART was the award winner in the best-use-of-materials category, the same award won by Mira in the 80's at the Riverside Telescope Makers Convention in California.

    The axel, wheels and torsion bar were salvaged from the rear of a Renault Alliance, the spindle and bearings from an eighteen wheeler became the polar axis and the two rear legs which provide stable support when the trailer is jacked up for use were cut from an automobile drive shaft.. The fork and trailer superstructure were fabricated from heavy rectangular steel tubing and the mirror box from sheet steel. The serrier truss assembly is made of one-inch aluminum tubing which was one of the more expensive material costs. The homemade reflex finder operates on the same principle as the common commercial Telrad and works equally well. The focuser, also homemade, is a modified Crayford design with a 3-inch diameter copper drawtube found at the plumbing supply. Focusing is extremely smooth and precise. The spider and the 18 point flotation mirror cell are also homemade, simple and effective.

    Bart has an unusual drive which is patented by Byron. It is very accurate and easy to construct requiring only one precision part which by a clever technique was fabricated using only hand tools and a machinistís dial indicator. Furthermore it is a friction drive eliminating the need to fumble in the dark trying to tighten and loosen clutches such as are used on most commercial scopes.

    The drive uses a surplus one rpm synchronous motor costing six dollars at a surplus outlet. Gearing down the motor was easily accomplished using only a few pieces from a large $50 box of assorted gears. The output shaft was fitted with a chain sprocket driving a Walls #35 chain which fits like a saddle around the smooth edge of a 1/4 inch thick steel disk mounted on the polar axis. When the telescope is slewed the disk slips inside the chain but otherwise the friction between chain and disk is sufficient to drive the scope. The diameter of the steel disk was chosen so that it perfectly matched the output speed of the driving chain sprocket to give the desired one revolution per day final drive rate. Note that this approach allows great flexibility in choosing gears for the drive train.

    The steel disk was rough cut by a commercial shop to within .1 inch of the desired radius. It was then mounted on the scope and the fork arms without the mirror box were turned by one person while another ground the disk to the desired radius which was monitored by repeated measurements of the circumference. A temporary bracket helped steady the hand grinder and also held a machinistís dial indicator showing the variation in the radius as the disk was turned and ground. By this means it was possible to slowly reduce the radius to the desired value and achieve a variation of only .003 inches. John Dobson who is not known for liking powered scopes, inspected Mira at Riverside and pronounced it the best clock driven scope he had seen.

    The paraboloidal mirror for BART is 24-inches in diameter, 2 inches thick, weighs 62 lbs. and has a focal length of 120 inches. It was produced by Paul Jones of Star Instruments in Flagstaff Arizona and was recently given an enhanced aluminum coating by Galaxy Optics in Buena Vista, Colorado. It is supported by an 18 point flotation mirror cell and is held in place by an aluminum plug fitted through a hole in the center of the mirror. The plug is wrapped with Teflon tape and has a flange on top to prevent the mirror from falling forward when the scope is in a horizontal position. The mirror remains in position when traveling and rarely needs collimation.


(Maura Wood and Bart ready to travel)