G.A. Hole’s observatory

Patcham, Sussex

Journal of the British Astrononical Association, 51 (9), 329, October 1941

This is an open-air observatory with, up to a few months ago, a 9½-inch reflector as its principal instrument, ably seconded by one of 6 inches. Now, however, a 14-inch instrument has been finished and installed. This has an equatorial fork mounting of more or less standard design, its unusual features being: (1) it is constructed entirely of reinforced concrete; (2) the method of rotating the polar axis. The concrete fork was cast in a wooden mould, the various bearings, reinforcing, and so on, being in position before filling with concrete. The 8-inch diameter main bearing is a cast-iron angle ring of L section, with 1-inch faces placed as near the top of the fork stem (or polar axis) as possible. A second identical ring is placed 4 inches farther down the stem, to take the pressure exerted by the clamping screws of the right ascension slow motion. The two Y-shaped bearings for the declination axis, which form the top of each fork arm, were assembled out of 2 x ¼-inch section steel bar, and are well keyed into the concrete by rods threaded into them, and bent at all angles to prevent movement. Great care was taken to get the centre lines of the polar and declination axes precisely at right angles, and further adjustment can be carried out by using sheet steel shims, there being tapped holes in one of the declination bearings for fixing these if necessary. The main reinforcing is of 2-inch steam pipe running up the centre of each arm and down the polar axis. Angle braces were inserted across the corners of the fork, where most of the strain is taken, and plenty of smaller iron was wound and interlaced all round this main structure. On the bottom of the reinforcing rod running down the polar axis is located the bottom bearing, being 3 inches in diameter. The base of the fork is of 8 x 4-inch rectangular section, and the arms taper from this to 6 x 3 inches at the top. There is also a tapering rib up the side of each arm and under the fork base. The making of the mould was the biggest part of the job. This fork rotates in roller bearings set in a concrete cradle, cast with four lugs for bolting to the main foundation angle block, and carrying fixing studs for the top half of the main bearing, and the bridge that carries the R.A. slow motions. These are well seen in the photograph, and the hour circle is also visible. Rotation is controlled by a steel girder type arm carried on a ring which encircles the polar axis, and can be rigidly clamped to it at any point. This arm carries a sliding box, to compensate for changes in radial length, which is moved by a swivel nut along a

fixed screw, driven by hand with a Hookes joint. This screw is set at 14 inches from polar axis centre, and is thus equal to a worm-wheel of twice the size. It gives a controlled motion of 15–20 degrees, and thus allows objects to be followed for an hour or more before it is necessary to unclamp and reset. The hand control can be fixed to either end of the driving screw.

The tube is of timber, built up with four hexagonal formers (halved, glued and screwed at the angles) and six longitudinal members. It is solid round the mirror (there is of course a door) and open lattice-work from the balancing point. The flat and eyepiece mounts are carried at the top of a lattice-work cylinder which drops into the end of the main tube, and rotates in bearings at each end. This was made as long as possible (3 feet) in order to preserve adjustments when rotated, and I have found that it is quite satisfactory. The internal lattice cylinder accounts for the maze-like appearance of the tube top in the photograph. The large mirror, its cell and the tube bottom have been described in a recent paper [see below], the only point being that the cell and cover cap are now totally enclosed in a sheet-metal box with a removable weather-tight lid, forming a ‘cell within a cell’.

The declination slow motion arm is pivoted on a hollow cast-iron stud, 3 inches diameter, in line with the centre of declination axis, and is counterpoised. It can be clamped solid with the fork arm and further adjustment obtained with an extending screw, working through a swivel nut in the end of the girder work arm.

The whole of this declination gear is counterpoised by a weight fixed to the other fork arm. The clamping screw, slow motion screw and declination circle are well seen in the photograph. All adjustments are within easy reach of the observer at the eyepiece, except the R.A. clamp, and it is hoped to alter this soon. The fork is large enough to allow the tube to swing clear through it, and, with an eye to the future, the whole telescope tube and mount has been made large enough to take an 18-inch mirror. It is, in fact, an 18-inch telescope working with a 14-inch mirror.

In conclusion, the use of concrete enables the worker to eliminate expensive metal castings, the points to remember being these. As concrete is not as strong as iron, there must be more of it to do the same job (all sections must be larger). Adequate reinforcing must be provided to any point taking strain, and properly prepared bearings must be incorporated wherever motion is to take place. These last must be well keyed into the parent mass, and this can only be done if they are in position in the mould before pouring in the concrete.

The construction of a cell for a 14-inch telescope mirror

Journal of the British Astrononical Association, 51 (3), 95, Februsry 1941

On completing this mirror of 14 inches aperture, 2 inches thickness, and 98 inches focal length, I found that, owing to the war, metallic castings for the cell were unobtainable. It was therefore necessary to find an effective substitute. In view of the weight of the mirror – some 27 pounds – the cell construction advocated by the late Canon Ellison, calling for a stout metal rim, was not used. Instead, a base board of 3/8-inch plywood was marked with a circle an inch larger than the mirror, and enough arc pieces were cut out of ½-inch mahogany to form a rim 2½ inches deep, 1 inch wide, and 15 inches internal diameter. These pieces were assembled on the baseboard, and glued and screwed together, taking care to cross the grain. When dry it was detached, and cleaned up on the inside with hand tools and glass-paper. Two arc pieces, carefully fitted to the curve of the mirror edge, and of a thickness enough to centralize it, were permanently fixed to the inside edge of rim, at 120 degrees and 240 degrees. They are lined with velvet and are 8 inches long. The third arc piece, of the same size as the others, was made adjustable. It was made of brass bent to curve, and had two bolts threaded into it, one at each end. These were bent or ‘set’ until they were parallel, and slipped through two brass-lined holes in the rim. They carry nuts and two short springs countersunk into the thickness of the rim, which return the adjustable arc. Flush cover plates are fitted over the ends of the bolts, and are all of the mechanism that is visible. Two pointed bolts, working in tapped inserts in the rim, serve to advance the arc until it positions the mirror. The arc is also velvet lined. By these means the mirror is located, and held immovable, yet without nipping it. The rim was fixed to the baseboard with glue, and this was built up to 2 inches thickness by the cellular method advocated by Professor Ritchey for wooden polishers. Nine long bolts right through rim and base finished the fixing. The outside was cleaned up, and a ring of cork, ¼-inch thick, was glued to the inside for the mirror to rest on. Three retaining clips, sliding in sheet-metal guides, were fixed to the inner edge of the rim, and held by bolts threaded into them through slots cut in the rim. Pieces of cork are cemented to the clips where

they contact the optical surface. The whole cell rests on a triangular frame to provide for ‘remote control’ squaring on, as in Mr Hargreaves’ paper in the Journal of March 1939, and can be moved out of centre on this to provide further adjustment. In use, I have been unable to detect any signs of flexure or movement in the mirror, and knife-edge tests show no deformation of figure in any position where I have been able to apply them, although I suppose that theoretically it must be present.