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Benefits of a split-dob mount.

Wouldn't it be nice to have the tracking capability of an equatorial mount and the convenience of a Dobsonian, all in one package? Several years ago I built such mounts for my 12 1/2- and 17 1/2-inch reflectors, and last spring the concept really paid off. When Comet Hyakutake passed near the celestial pole, where equatorial motions are quite awkward, I simply switched to altazimuth mode!

Originally these telescopes had split-ring equatorial mounts, the design pioneered by Russell W. Porter and later modified for use on the 200-inch Palomar reflector. All I had to do was add a third axis - the azimuth base of a Dobsonian - to complete the transformation. The result is what I call a split-Dob mount.

I had already borrowed a Dobsonian idea for these mounts, using aluminum-rimmed wood disks riding on Teflon pads for the declination bearings. But even amazing Teflon has its limits. As the load increases the friction does too, and eventually the performance suffers. So for azimuth bearings on the base I used a strategy that some fellow telescope makers developed. The usual three outer pads of Teflon are employed, but part of the load they would normally carry is transferred to a central, slightly thicker Teflon pad. The load on this pad is quite severe, of course, but the leverage involved in turning the telescope overcomes this added friction easily. Meanwhile, the lightly loaded outer pads still provide smooth, stable motion.

Since the Dobsonian azimuth bearing was an afterthought, I retained the split-ring base as the primary support for ground contact. Whenever I want to switch to the Dob mode, I lock the split ring in place with the ring tips up (that is, the telescope is aimed along the meridian). For this purpose I use a central locking pin on the 17 1/2-inch and two slide latches on the 12 1/2. The declination axis, now horizontal, becomes the altitude axis.

Next, with the aid of a small stand and lever, I lift the south end of the base slightly and insert small blocks under the edge of the circular Dob ground board at azimuth 120 [degrees] (south-southeast) and 240 [degrees] (south-southwest). Then I lift the opposite end, at its center, and place a third block at azimuth 0 [degree] (due north). The alignment pin between the base and ground board can be removed at this stage, releasing the instrument for motion in azimuth. The mount is now a Dobsonian, with no front board to restrict altitude travel.

To revert to the split-ring mode I simply reverse this 30-second process. Alternatively, if I want to slew equatorially while the instrument is sitting on the Dob blocks (which works fine), I just replace the alignment pin and unlock the split ring. If I had planned these mounts as split Dobs from the start, I might have made a larger, square ground board as the primary ground contact, eliminating the need for blocks and making the changeover even quicker.

Plywood is the main construction material, except for the mirror cells and tube trusses. To encase the bottoms of the tubes I indulged in some needlessly fancy (but fun to make) aluminum covers.

The 17 1/2-inch reflector has a 36 3/4-inch-diameter ring and all-up weight of about 220 pounds. Its primary mirror rides on an 18-point flotation cell. The 12 1/2-inch, with 32-inch ring, is some 50 pounds lighter. Its mirror performs fine with a 9-point support. The diagonal cages on both instruments rotate, so I can always find a convenient orientation for the eyepiece while viewing any part of the sky.


While I came up with the split-Dob idea on my own, I have become aware that its prototype was, in fact, the Porter Garden Telescope of the early 1920s. Intrigued by Porter's creative concept and economy of design, I have tried to learn all I can about those fascinating instruments. My research has uncovered certain details that eluded even Berton C. Willard, author of Russell W. Porter: Arctic Explorer, Artist, Telescope Maker (available from Sky Publishing Corp.). A great read, this book was quite helpful in my quest.

In the January 1923 issue of Scientific American, a story titled "A Garden Telescope for the Amateur Astronomer" may be the first published description of Porter's invention. The instrument is described as "now being manufactured . . . to provide an entirely new and beautiful ornament which becomes a permanent fixture in the garden or on the lawn of its owner." The unsigned article goes on to explain that the instrument was a modified Newtonian reflector built of durable bronze. The 6-inch f/4 primary mirror "rests in a bowl of bronze lotus leaves from which rises a graceful blade or leaf carrying the prism at its farther end." Only the optics had to be brought indoors when not in use.

The garden telescope could even serve double duty as a sundial. The split-ring mount included setting circles and a slow-motion control in right ascension. Furthermore - and this is what is most relevant to the split-Dob concept - "the base supporting the bell has a horizontal circular track on which the mounting may be turned to any azimuth. This motion is found most convenient for terrestrial observation of objects near the horizon."

No. 53 is the highest surviving serial number, so at least that many garden telescopes were produced at the Jones & Lamson Machine Co. where Porter worked at the time. The total built is not known, as the records have been lost, but most estimates hover around 100. Another mystery is the production time frame. Some inkling can be gleaned from the fact that advertisements for garden telescopes ran in House Beautiful magazine from May 1923 through June 1924. Initially the instrument could be purchased for $250, the equivalent of about $2,300 today.

Exactly 10 years after the original article appeared, the January 1933 issue of Scientific American carried an ad that announced: "Now Available to the Public: The Porter Garden Telescope. . . . Made and Marketed by D. A. Patch, 38 Crescent Street, Springfield, Vermont." This was Donald Alden Patch, who had posed with a Springfield-type mount for some Porter sketches that were published in the March 1926 issue of Scientific American and in the first volume of Albert G. Ingalls's classic Amateur Telescope Making series, published the same year.

Although Patch continued to advertise telescope items and services through late 1938, I have found no further ads for the garden telescope. Perhaps his efforts to reintroduce it met with only limited success. Whatever the result, the fact that he tried to do so in the midst of the Great Depression is quite remarkable!

The split-Dob concept thus continues a time-honored tradition, and I have been very pleased at how my two-in-one mounts perform. They offer a nice alternative to the combination of a Dobsonian mount and equatorial tracking platform that other telescope makers have developed.

DAVID EDWARD MOERKE 3841 Lyceum Ave. Los Angeles, CA 90066

An aerospace sheet-metal mechanic by trade, David Moerke pursues astronomy, gardening, and geology in his spare time. He welcomes correspondence about the split-Dob concept.
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Author:Moerke, David Edward
Publication:Sky & Telescope
Date:Oct 1, 1996
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