"The observatory," he states, "is 16 feet in diameter. Its side walls are 48 inches high. For a foundation we put down four six by six-inch chestnut posts, below the frost line. Next the post for the telescope was put in. This is a six-inch, steel well casing, set about six feet into concrete, and off center enough to allow for the overhang of the polar axis. The floor of the observatory was then built up on two-by-sixes, with the flooring laid diagonally for rigidity. Clearance was allowed around the post, to obviate vibration.

"On a 16-foot circle two-by-fours were placed at two foot intervals. These were braced plumb, and on them on the inside was put a two-inch angle iron with the leg down. Through this a lag screw was driven into each post, making the whole very rigid. All angles throughout the whole job were rolled on an eight-foot radius. This simplifies things a great deal.

"We then sided the structure with regular six-inch weather boarding, which had the lower inside edge rabbited out so that the siding would fit the strip beneath and prevent the ends from pulling up when laid on a circle. This is one theory which distinguished itself by coming up to expectations.

"For the lower track we used a two-inch angle iron with leg turned up on the outside, screwed to the top of the uprights and screws countersunk to give a smooth track. The upper track is a one and one half inch angle iron, with a three quarter inch angle spot-welded to its under, inner edge, to make a race for the balls.

"Steel balls apparently are very expensive, even if 'seconds' are used, so we had the balls turned out of maple and then soaked them in linseed oil for a few days. All were heavily coated with grease and then put into place. Between the balls are sticks of wood about two feet long, cut as segments of a 16-foot circle, to act as spacers.

"For the dome, we made the opening of one and one half inch angle iron, with a flat top. The lower end is bolted into the upper track. The material used was just light enough so that we could punch out the holes with a large hand punch. After the frame of the opening was in place we bolted in the ribs, which are three quarter inch angle iron, leg side down. The upper or flat side of the angle was then punched in three or four places and slats of wood screwed on it. To these slats the canvas covering was tacked.

"In putting on the canvas, we started at the opening, which has a lateral brace at the bottom, and stretched the canvas over the adjoining rib, holding the rib in place by putting a couple of sticks between it and the opening; and so on, around, until we reached the other side of the opening. The wooden braces were then knocked out and the dome held its shape. This makes a remarkably rigid dome, and not too heavy for one person to handle. The canvas was painted at once with about 50 pounds of white lead and four gallons of linseed oil. On the first painting the canvas takes up all the oil it will hold. This makes it a perfectly watertight, neat, light dome.

"Along the sides of the irons forming the opening, we bolted on a wooden coaming about three inches high, and then made a strip of canvas four inches wider than the opening, sewing a flexible wire into the sides of the canvas, and a heavy roller across the bottom. There is also an occasional light wooden strip tacked across its width, to act as a spreader and to help bear the weight of snow and ice in the winter. Then, with a rigging very similar to what is used on a roller awning, we roll up the canvas out of the way at the top of the dome; and when finished, roll it down very quickly, stretching it tight so that the sides of the canvas come down over the coaming, making it storm proof.

"The telescope itself has a 16-1/2 inch mirror of 125-1/2 inches focal length. The tube is of 16-gage steel, allowing a three quarter inch air space along the sides. The cell is of the conventional type. The finder is a two and five eighth inch doublet, 16-inch e.f.l., with cross hairs in the field.

"For the axes, Ford axles were used. The housing at the brake end was machined off, and a 194-tooth gear bolted into this, and against the gear rests a slip plate which acts as a clutch. Through this plate run three heavy adjusting studs over springs for regulating the pressure of the plate on the gear, and thus the amount of drag or slip between the ring and the axle. In this way, we swing easily to any position, and at once pick it up in slow motion. As the tube and fittings weigh several hundred pounds it is necessary to have this very accurately fitted for use with high powers.

"The declination axis has a rod running from the gear to an easy position at the eyepiece. The declination lock is another rod, parallel to the first, running through the brake-band housing and screwing against the part which was formerly covered by the brake lining.

"The polar axis has the same gear and plate movement, but the drive goes through another 12-to-1 worm and gear, giving reduction of about 4000-to-1. This makes the drive very steady and powerful. This gear is driven through a speedometer cable, on the end of which is a handle similar to a file handle, with the cable run through it lengthwise and having a small crank. With this arrangement, the drive can be used by the observer; or, once the tube is set in declination, the object may be held in the field by some one other than the observer, who makes a turn of the handle once in 23 seconds. Next to having a driving clock, this is very satisfactory when one's friends wish to look, as the only necessary admonition is to keep hands off the telescope. "The eyepiece is the regular rack and pinion type. We use negative oculars down to one fourth inch. This size gives a magnification of 500 diameters with this mirror. We are having 750x and 1000x made, but 500x is as high as we can generally use, and then only on very good nights.

"We expect to build a clock drive, although the flexible cable idea is highly satisfactory-in some ways perhaps preferable. The whole machine is surprisingly satisfactory, and for a first attempt, much better than we had expected."

Just as this issue goes to press we receive a letter from Mr. Steber, which reads as follows: "The telescope was built by H. A. Thurn and myself, Mr. Thurn being the mechanical genius and I supplying the ideas, most of which did not work. The telescope is most satisfactory and is arousing considerable local interest in astronomy."

Perhaps the statement we made in the April issue, that the cost of the spectrohelioscope was comparable to that of a fine radio set, was a bit optimistic. If the outfit is purchased, complete, it will cost about as much as a low-priced motor car. We have, however, seen too many examples of the ability of the ingenious amateur to get around high costs by doing most of a job himself, to do much worrying about this point. Those who plan to make spectrohelioscopes are requested to keep in touch with telescope editor, who quite naturally takes an interest in the work of other amateurs.

Don't miss the article on another page, describing Hamilton's large reflector. We have on hand several descriptions of rather unusual jobs, and these will be published in due course. It is thought that many other jobs have not been reported for publication. Don't hide your light under a bushel, fellow amateurs.

Amateurs who definitely plan to make Cassegrainians will receive information to their advantage on communicating with A. G. I., Tel. Ed.

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