Open-tube Telescopes IN an article published in the American Journal 0/ Science Mr. David Todd discusses certain points In the construction of telescopes and suggests a plan for the building of an open-tube telescope presenting a number of novel interesting features. All telescopes are made up of three parts: Object glass, eye piece, and some form of rigid mechanical connection between the two. Usually this last is a round tube, but there is no reason why it should not take any shape that the exigencies of mechanics or engineering may demand. Many open-tube telescopes have been built. Huygens even built one with no tube at all. The objective was placed in a counter-balanced cell with universal joint, mounted at the top of a tall pole. He drew the axis of the objective into approximate line with the axis of the ocular by means of a cord or wire reaching down to the observer on the ground. No objective of modern optical excellence could ever be satisfactorily used in this way. The four-foot Melbourne reflector designed and built by Thomas Grubb has an open tube of spirally interlacing straps of steel. The optical advantages of great focal length have long been understood. To fully avail ones self of these, it becomes necessary, however, to satisfactorily deal with the mechanical problems of appropriately mounting the parts of the telescope. In the large instruments of the present day, this problem presents no mean difficulties, and Mr. Todd starts out with the idea that a suitably designed open-tube telescope would best satisfy all conditions. The principal points to be considered in setting to work on the solution of this problem are, first, to build the tube so that its flexure shall be negligible, and, secondly, to mount the weighted tube so that it can be pointed with ease to any part of the sky. Mr. Todd says: I have begun my projected telescope, not at the end, but in the middle, just as a bridge engineer builds a cantilever. (See the accompanying illustration. ) The basis Of its tube is a cubical section or compartment of steel plates, reinforced as in box-girder construction, so as to be absolutely rigid and unyielding. For a telescope 200 feet or 300 feet long, I would build this central cube about 20 feet square. On two opposite sides it is perforated to allow the cone of rays to pass through; and on two other sides of the cube, at right angles to the cone of rays, are attached the circular bed plates of the bearing pins. In other words, tube and axis are the same in construction as the ordinary type of transit instrument, with shortened axis and a minimum distance between the pivots Needless to say, this is the form which, in the evolution of transit and meridian circle, has been found to give minimum flexure. As we are per force restricted to standard commercial .forms of structural steel, the two halves of the tube must be built up, not as cones (the ideal form) but as square pyramids. As we have rotation about only one axis to deal with, the flexure of the great tube is easy to handle. It will be apparent that the alt-azimuth type of mounting follows as a necessary consequence of this evolution; and the altitude motion gives no trouble whatever in either design or practical construction; it is only when we reach the azimuth motion that mechanical and engineering difficulties arise, though they are not wholly insuperable ones. As we go downward from the telescope to the ground, our troubles increase; and they become a maximum when we reach the plane of junction between earth and mounting. Let us now consider the method of dealing with this problem. What we want is an absolutely smooth positive and yet exceedingly easy motion of a huge vertical shaft. In order to meet those requisites, and treating it as a shaft simply, we should extend its length downward, and confine its motion as a journal between two horizontal friction rings, slipping between roller bearings on vertical axes. These rings, with perhaps 100 roller bearings for each, 50 outside and 50 inside, should have horizontal wheels or rollers of large diameter, ground to perfect cylinders. These rings are intended to operate only when the telescope is in use in the wind. When the air 's still, a smoother azimuth motion would be possible if the friction rings are clampad motionless to the concrete well in which the vertical drum revolves. Then I would float the entire structure in either oil or water, leaving only a few hundred pounds of load on a central button or pintle, In this way a clock of minimum power would suffice to drive a maximum load with perfect smoothness. The expanded vertical axis, then, would be a little like the inner tank of a gasometer when at its lowest point. It should be not less than 50 feet in length, or vertical height; and this drum-shaped form would afford an easy interior construction, insuring absolute rigidity of the vertical towers, which might go down through it, as far as thought best. No wind could ever topple the structure over, and the open-air telescope would be safely usable on' all but the windiest occasions. By lowering the pintle underneath, and pumping out enough of the flotation to allow the drum to settle down to stationary beds, the drum could be rigidly clamped near the top and bottom, so as to withstand securely all stormy weathers when not in use. In case of a severe gale, the .telescope would be pointed quartering to the wind, observing carriage and tailpiece unshipped, and tube directed to the nadir. Here the objective would be double-housed against the storm, and the cell clamped firmly. This would secure both bearings and open-air pyramids against harmful stress due to excessive wind-thrust, as the gale would strike the structure edgewise. I have not intended in this paper to deal with the solution of any but the most general problems of this proposed telescope. All details have, however, been critically worked out, so thd an approximate estimate of cost is available. A 5-foot objective would cost about $125,000, and the entire instrument would represent about double that amount. For instance, the weather protection of objective and eyepieces, of altitude bearings and of altitude clamp, quki: motion and slow motion; al.so electric motors for operating the same in both coordinates, and for the requisite variable clock-motion in both altitude and azimuth, as weli as for driving the rotary tail-piece ' on a ball-race. With this arrangement, and an adaptation of Prof. Ritchey's double-slide plateholder with independent rotary motion, micrometric work and even precision photography might be quite practicable. All the clamps and motions and clocks may be controlled by electric motors operated from triplicate switchboards in (1) the observing box, (2) the altitude house (not shown on the upright piers), and (3) from the azimuth room concealed beneath the basal drum. In lieu of observing-chair or rising-floor, observer and assistant ride in a light carriage on the eye-end, swinging on a horizontal ball-bearing axis which passes through the focal plane. This may be wholly inclosed for weather protection, and it can readily be warmed electrically in winter. Should such an instrument ever be erected on a high mountain, as for instance, Fujiyama in Japan, 12,400 feet elevation, where a saddlo within the crater provides an ideal location such that wind would rarely shake the telescope, it would be wholly feasible, in fact very easy, to supply the observing box with air artificially compressed to sea-level tension. Thereby we should avoid the disastrous effect of mountain sickness, which not only interferes greatly with one's comfort at elevations much in excess of 10,000 feet, but in consequence of the quickened pulse, tends to shorten the life span of anyone who persists in tarrying long at great elevations, without frequent return to safe and lower altitudes. An Astronomical Method of Finding an Aeronaut's Position. WHEN terrestrial objects are invisible, an aeronaut is compelled to employ indirect methods of finding his position. Several such methods have been invented during recent years. Berlingmaier and Marcuse have employed magnetic measurements for this purpose and successful methods of informing an aeronaut of his position by means of wireless telegraphy have been devised. Finally, astronomical methods have been proposed, in which the tedious reduction of the observations is effected by means of special apparatus. An instrument of this character, invented by Voigt, is shown in Fig. 1, and the method of its use is thus Fig. I.—Instrument for finding aeronaut's position. explained in the Deutsche Zeitschrift fuer Luftschif-fahrt. The principle involved is illustrated by Fig. 2 in which A and B represent two church steeples rising from a level plain. If the angle of elevation of the steeple A, as viewed from various points of the plain, is measured, we know that all points for which this angle has a certain value lie on a circle a of definite radius, the center of which is the base of the steeple. Similarly, we know that every point at which the angular elevation of the steeple B is equal to f3 must lie on the circle b. Hence a point at which the elevation of A is ex and that of B is f3 must be situated at an intersection of the two circles, i. e., either at I or at II. The objects observed from the balloon are not steeples, but stars, and the variation of their apparent altitudes with the balloon's position is due not to absolute distance; but to the curvature of the earth. The altitudes of a number of reference stars, as seen at various hours from a definite point of the earth's surface, are first calculated. The point selected (50 deg. N. lat. and 10 deg. E. long.) forms the center and pivot of the revolving map shown in Fig. 1. With the aid of an accurate sidereal clock it is possible to draw the two circles on the map which determine the place of observation by their intersection. The drawing of the arcs is greatly facilitated by attaching to the map a curve-ruler which can be set accurately to the curvature corresponding to any altitude. This method, of course, can be employed only at night in clear weather. The principle was employed in 1909 by Dr. A. Brill, who devised an instrument similar to Voigts
This article was originally published with the title "Abstracts from Current Periodicals"