R.F.T. Granger's observatory

Bramcote, Nottinghamshire

Journal of the British Astronomical Association, 51 (6), 207, July 1941

The dome is built on the lines suggested by the late Canon Ellison, but has a plywood covering under the ruberoid, which gives great rigidity. The cylinder on which it rests is a light frame covered with plywood only, all the edges being protected with doped cotton tapes. Well painted yearly, it has withstood the weather for six years without appreciable deterioration, The whole building rotates on rollers mounted on the floor, so that the doorway may be used for low altitude observations. The shutter is of canvas on wooden laths, and is operated on the venetian blind principle by cords inside the dome. The building, of 9 feet diameter, was designed to house a 6-inch reflector, and when it was decided to install a 10-inch of 80 inches focal length, a very compact mounting had to be designed to make it possible to operate an instrument of this size in such a confined space. A design of the German type was decided upon, and the principle was adopted that to minimise vibration not only must the mounting be adequately massive, but moments of inertia must be kept low; i.e., parts at a distance from the axes of rotation must be kept as light as possible. The tube, rebuilt from one of Canon Ellison’s, is of wooden laths in cast-aluminium angle rings. The mirror cell is of wood and weighs 5 lbs., and below the eyepiece is a rack for star charts, which is found a great convenience. The supports for the finders are turned from aluminium castings. The body rotates in a tube made of sheet aluminium stiffened at the end with rigid steel angle rings turned off a couple of 13-inch brake drums. Brass cheese-headed bolts in the wooden laths engage the inner surface of these rings and form the bearing surface on which the body of the telescope rotates. This outer tube is bolted to a cast- aluminium cradle which is pegged on to the 2½-inch declination shaft. The lower end of the 3-inch polar shaft turns on a steel centre point, while the upper end runs in a roller bearing housed in the heavy iron base casting, and is driven into a casting which carries the declination shaft and bearing. The bronze worm-wheel was hobbed on a lathe in the manner described in Amateur Telescope Making, and drives the polar shaft through a clutch made from a 7-inch brake drum. This drum is slotted into four sectors which engage a drum on the upper end of the polar shaft through four aluminium shoes, which were turned up in situ after the brake drum had been bolted to the worm-wheel, so as to ensure perfect centring. Adjustment is made by tightening a steel band round the slotted drum by means of a hand screw. Further reduction gearing is through meccano gears bushed and bored out to take shafts of adequate diameter. It was intended to drive through a belt from a motor under the floor, but the outbreak of war prevented further progress. Although the writer has built other mountings before, this is his first experiment involving castings and the construction of patterns. No

particular difficulties were met with, however, and practically the whole of the machining was done on a lathe of 5½-inch clearance in the gap. The work of the observatory has been the observation of variable stars and occultations, but this has had to be suspended during the writer’s absence.

Richard F.T. Granger (1900–1948) joined the BAA in 1937. He was a Director of R. Granger & Sons, lace manufacturers of Long Eaton, and was an astronomer, meteorologist and aviator. In 1930 he built and flew a tailless aircraft, and was later a test pilot at the Royal Aircraft Establishment, Farnborough. He died from injuries after a crash.