On the Construction of a Silvered Glass Telescope / Fifteen and a half inches in aperture, and its use in celestial photography

by Henry Draper

§2. THE TELESCOPE MOUNTING.

Fig. 27.

Miss Herschel’s Telescope.

The telescope is mounted as an altitude and azimuth instrument, but in a manner that causes it to differ from the usual instrument of that kind. The essential feature is, that the eye-piece or place of the sensitive plate is stationary at all altitudes, the observer always looking straight forward, and never having to stoop or assume inconvenient and constrained positions.

The stationary eye-piece mounting was first used by Miss Caroline Herschel, who had a 27 inch Newtonian arranged on that plan. Fig. 27. (Smyth’s Celestial Cycle.)

Subsequently it was applied to a large telescope by Mr. Nasmyth, the eminent engineer, but no details of his construction have reached me. He used it for making drawings of the moon, which are said to be excellently executed.

When it became necessary to determine how my telescope should be mounted, I was strongly urged to make it an equatorial. But after reflecting on the fact that it was intended for photography, and that absolute freedom from tremor was essential, a condition not attained in the equatorial when driven by a clock, and in addition that in the case of the moon rotation upon a polar axis does not suffice to counteract the motion in declination, I was led to adopt the other form.

A great many modifications of the original idea have been made. For instance, instead of counterpoising the end of the tube containing the mirror by extending the tube to a distance beyond the altitude or horizontal axis, I introduced a system of counterpoise levers which allows the telescope to work in a space little more than its own focal length across. This construction permits both ends of the tube to be supported, the lower one on a wire rope, and gives the greatest freedom from tremor, the parts coming quickly to rest after a movement. In the use of the telescope for photography, as we shall see, the system of bringing the mass of the instrument to complete rest before exposing the sensitive plate, and only driving that plate itself by a clock, is always adopted.

The obvious disadvantage connected with the alt-azimuth mounting—the difficulty of finding some objects—has not been a source of embarrassment. In fact the instability of the optical axis in reflecting instruments, if the mirror is unconstrainedly supported, as it should be, renders them unsuitable for determinations of position. A little patience will enable an observer to find all necessary tests, or curious objects.

The mounting is divided into: a. The Tube; and b. The supporting frame.

a. The Tube.

The telescope tube is a sixteen sided prism of walnut wood, 18 inches in diameter, and 12 feet long. The staves are  3/8 of an inch thick, and are hooped together with four bands of brass, capable of being tightened by screws. Inside the tube are placed two rings of iron, half an inch thick, reducing the internal diameter to about 16 inches. At opposite sides of the upper end of the tube are screwed the perforated trunnions a, Fig. [28] (of which only one is shown), upon which it swings. Surrounding the other end is a wire rope b b′ b″, the ends of which go over the pulleys c (c′ not shown) on friction rollers, and terminate in disks of lead d d′. These counterpoises are fastened on the ends of levers e e′, which turn below on a fixed axle f.

By this arrangement as the tube assumes a horizontal position and becomes, so to speak, heavier, the counterpoises do the same, while when the tube becomes perpendicular, and most of its weight falls upon the trunnions, the counterpoises are carried mostly by their axle. A continual condition of equilibrium is thus reached, the tube being easily raised or depressed to any altitude desired. It is necessary, however, to constrain the wire rope b b′ b″, to move in the arc of the circle described by the end of the tube and ends of the levers and hence the twelve rollers or guide pulleys g g′ g″. Over some of the same pulleys a thin wire rope h h′ runs, but while its ends are fastened to the lower part of the tube at b, the central parts go twice around a roller connected with the winch i, near the eye-piece, thus enabling the observer to move the telescope in altitude, without taking the eye from the eye-piece.

Fig. 28.

Sectional View of Observatory.

The iron wire rope required to be carefully made, so as to avoid rigidity. It contains 2 1/3 miles of wire,  1/100 of an inch in diameter, and has 300 strands. Each single wire will support 7 pounds. It is, however, more flexible than a hempen rope of the same size, owing to its loose twisting.

Fig. 29.

The Mirror Support.

At the lower end of the tube, at the distance of a foot, and crossing it at right angles, held by three bars of iron i i′ i″, Fig. 29, is a circular table of oak e, which carries an India-rubber air sac d, and upon this the mirror f is placed. The edge support of the mirror is furnished by a semicircular band of tin-plate a, lined inside with cotton, and fastened at the ends by links of chain b, (b′ not seen) to two screws c c′; g and h are the wire ropes, marked b and h in Fig. [28].

Instead of the blanket support which Herschel found so advantageous, M. Foucault has suggested this use of an air sac. In his instrument there is a tube going up to the observer, by which he may adjust its degree of inflation. It requires that there should be three bearings c c′ c″, in front of the mirror, against which it may press when the sac behind is inflated, otherwise the optical axis is altogether too instable, and objects cannot be found. The arrangement certainly gives beautiful definition, bringing stars to a disk when the glass just floats, without touching its front bearings. The first sac that I made was composed of two circular sheets of India-rubber cloth, joined around the edges. But this could not be used while photographing, because the image was kept in a state of continuous oscillation if there was a breeze, and even under more favorable circumstances took a long time to come to rest. It was not advisable to blow the mirror hard up against its three front bearings, in order to avoid the instability, for then every point in of an object became triple. To the eye the oscillations were not offensive, because the swaying image was sharp.

Subsequently, however, an air chair cushion was procured, and as the surface was flat instead of convex the difficulty became so much less, that the blanket support was definitely abandoned. It is necessary that the mirror should have free play in the direction of the length of the tube when this kind of support is used, and that is the reason why the tin edge hoop must terminate in links of chain.

The interval, eight or ten inches, which separates the face of the mirror from the tube, is occupied by a curtain of black velvet, confined below by a drawing cord and tacked above to the tube. This permits access to the mirror to put a glass cover on it, and when shut down stops the current of air rushing up. When the instrument is not being used this curtain is left open, because the mirror and tube are in that case kept more uniform in temperature with the surrounding air.

In spite of such contrivances there is still sometimes a strong residual current in the tube. I have tried to overcome it by covering the mouth of the tube with a sheet of flat glass, but have been obliged to abandon that because the images were injured. At one time, too, when it was supposed that the current was partly from the observer’s body, heated streams of air going out around the tube, the aperture in the dome was closed by a conical bag of muslin, which fitted the mouth of the telescope tightly. The only advantages resulting were mere bodily comfort and a capability of perceiving fainter objects than before, because the sky-light was shut off.

Fig. 30.

Section of Azimuth Axis.

b. The Supporting Frame.

The frame which carries the preceding parts is of wood, and rests on a vertical axis a, Fig. 30, turning below in a gun-metal cup b, supported by a marble block resting on the solid rock. The upper end of the axis is sustained by two collars, one c c′ above, and the other below an intermediate triangular box e e′ from the sides of which three long beams f f f 12 × 3 inches diverge, gradually declining till they meet the solid rock at the limits of the excavation in which the observatory is placed. These beams are fastened together by cross-pieces g g g, Fig. 31, and go through the floor in spaces h h h, so contrived that the floor does not touch them. At the ends they are cased with a thick leaden sheathing, to deaden vibration and prevent the access of moisture.

Fig. 31.

Plan of Observatory (lower floor).

This tripod support in connection with the sustaining of the telescope by the wire rope, gives that steadiness which is so essential in photography. Only a slight amount of force, about two pounds, is required to move the instrument in azimuth, though it weighs almost a thousand pounds.

The plan of the frame centrally carried by the axis a is as follows: From the corners of a parallelogram i i (2 × 13 feet) of wooden beams, eight inches thick and three inches broad, perpendiculars n n′, Fig. [28], rise. At the top they are connected by lighter pieces to form a parallelogram, similar to that below, and just large enough to contain the tube of the telescope. At right angles to the parallelogram below, and close upon it, a braced bar o o′, Fig. [28], crosses. From its extremities four slanting braces as at p p′, Fig. [28], go to the corners of the upper parallelogram, and combine to give it lateral support. At the top of one close pair of the perpendiculars n′, Fig. [28], are bronze frames carrying friction rollers upon which the trunnions move, while similarly upon the other pair n are two pulleys, also on friction rollers, for the wire rope coming from the counterpoises.

Movement in altitude is very easily accomplished, and with the left hand upon the winch i, under high powers, both altitude and azimuth motions are controlled, and the right hand left free. The whole apparatus works so well, that in ordinary observation the want of a clock movement has not been felt. Of course for photography that is essential.