H.W. and L.A. Cox's observatory

Mitcham, Surrey

Journal of the British Astronomical Association, 51 (2), 63, January 1941

This observatory, was, until the outbreak of war, operated at Mitcham, Surrey. The following description covers not only the observatory itself, but also the workshop and laboratory attached to it. Owing to conditions arising out of the war, all the equipment has been dismantled and removed to another address, where it is hoped it will be re-erected when conditions permit. The observatory is 11 feet in diameter, with a dome height of 11 feet. A wood floor is laid on a cement base, and from this floor rises a wooden wall 3 feet high, built of tongued and grooved planks. On the top of this wall is fixed a laminated circular wood track on which the dome itself rotates. The base of the dome also consists of a circular wood ring of laminated boards, screwed together to form a rigid assembly similar to that on the top of the wall. Large trolley wheels mounted on the dome ring permit the dome itself to rotate on the track beneath. It is prevented from moving sideways by horizontally mounted rollers on the dome ring which run against an iron band fitted all round the track. The dome consists of ash hoops fixed to the dome ring, and these are all interconnected with wood laths nailed in position. The whole structure is then covered in stout canvas. The shutters fitted are wide compared with the diameter of the dome, this being necessary because of the span of the telescopes and cameras used inside. These shutters are also made from ash hoops, covered with plywood and canvas, and are made to slide outwards on straight tracks by means of rollers on angle iron slides.

The whole observatory was given two coats of white lead paint, and the wall of the base painted dark green inside and out. The dome was finished dark grey inside, and given a coat of aluminium paint on the outside to assist the reflection of the heat of the Sun. Originally the telescope was erected in the open, and as the observatory was built around it, the fixed wall had to be kept down to 3 feet in order that the telescope could be used at low angles of elevation. This automatically restricted the height of the door to about 33 inches, but no real trouble has been experienced with a door as low as this.

The equipment in the observatory consists of the following. First of all there is a 12-inch reflecting telescope mounted on a heavy English type mount. This telescope is of the open lattice-work type in order to reduce circulating tube currents, and is fitted with a large rack-mount which takes a whole range of eyepieces or photographic plates. The largest field covered by the lowest power eyepiece is 1 degree, so a flat is fitted having sufficient area to give even illumination over the whole field. The polar axis of the mounting is massive and runs in large ball races, and is supported at each end on cement pillars. Spindles 1½ inches in diameter terminate the polar axis and rotate in the ball races. A very heavy cast box forms the centre of the polar axis, and also provides the housing for the declination spindle. This spindle is 1¾ inches in diameter, and also runs in ball races. The drive for the polar axis consists of a synchronous gramophone motor driving through a chain of Meccano gears until it finally turns the worm engaging with a 12-inch diameter gear wheel, which wheel can be made free or rigid to the telescope by means of a clutch. Instead of supplying the synchronous motor with power from the mains, a valve oscillator is made to supply the necessary current. By varying the frequency of the oscillator, with a switch at the end of a long flexible lead, the rate of the motor can be varied at will. This telescope drive has been described in a previous number of this Journal. The top end of the polar axis is fitted with an engraved circle, around which a pointer is driven by an ordinary alarm-clock move ment through an extra 2/1 ratio Meccano gear, so that R.A. can be read off directly. When starting observing, the pointer and circle are set from a known bright star. The box containing this clock movement can be seen in the photograph just above the R.A. circle. On one side of the polar axis is fitted the 12-inch reflector, and on the other side is mounted a 4-inch aperture refractor and an f/5 6½-inch aperture Schmidt camera. These two instruments also act as a counterweight to the larger telescope, permitting it to swing freely when the polar axis is unclamped. Other equipment in the observatory consists of a slave seconds clock, operated from a Synchronome master clock in the house, and a chronograph. With the exception of the clock and a few eyepieces and so on, all the equipment, including the optics, was made in the workshop attached to the observatory.

The main observing programme is centered around the study of long-period variable stars, of which 42 in the BAA list are observed. Besides these, a large number of other stars, suspected of variability, are kept under systematic observation. Besides the normal visual observations of variable stars, many experiments have been carried out, and apparatus constructed for carrying out photoelectric measurements of variable stars.

The photoelectric equipment consists of a photocell and valve amplifier in a metal container, and this is fitted on the rack mount of the 12-inch reflector so that the light from the mirror falls on the photocell when a shutter is opened. Opening the shutter causes the light to suddenly fall on the cell, and this, converting the light into an electric current, in turn amplified by the valve amplifier, gives a ballastic kick to the galvanometer, the amplitude of which is shown up on a suitable scale. A very sensitive galvanometer is used, in order that stars down to 9th magnitude may be comfortably measured. Very sensitive galvanometers are, however, badly affected by local vibration, and as it was found that the observatory itself was not sufficiently free from vibration, this instrument was mounted on a mechanically damped fixture which was screwed to a wall of the laboratory in the house. The system of operation is as follows. One observer, in the observatory, operates the telescope and the photometer shutter, and the other operator reads the deflection on the galvanometer scale. It was hoped that the two operators would eventually be able to talk to each other by means of a telephone.

The laboratory was a small cellar in the house which was used as a combined photographic dark-room and a place where photoelectric measurements could be made and apparatus tried out. This room was fitted out with all the usual photographic apparatus to be found in a dark-room, and also contained the necessary benches, light sources, meters, battery supplies, amplifiers and so on, that are required for research into photoelectric photometry. A well-equipped workshop has permitted the rapid construction of any apparatus that has to be made, and also allows quick repairs to be carried out when anything in the observatory goes wrong. An optical grinding machine and an array of optical testing equipment permit mirrors and lenses to be made with the minimum of effort. A large number of mirrors, lenses and flats have so far been made, including the optics for three Schmidt cameras. All the metal and wood work for the various pieces of equipment is done in this workshop, the only exception being metal casting. The wood patterns are made in the workshop and then sent to a local foundry for the actual castings. After the castings have been received from the foundry, they are machined in the workshop.