Telescope drive

After some earlier reports of some unexplained 'knocking' noises coming from the telescope when slewing in azimuth, in March the noise had increased and the telescope was seen to move with respect to the building during slewing. After doing some tests it was found that the problem was associated with one of the tachometers of the azimuth drive. The faulty tachometer was replaced with a new one and this solved the problem. Since no obvious damage could be seen to the old tachometer all the coils of its rotor were measured and found to be OK, i.e. giving consistent resistances. It was decided to try the old rotor with only the stator with its magnets of the new tachometer. This combination worked well, identifying the original problem to be the stator of the old tachometer, the rotor being undamaged. A calculation found that the frequency of the knocking, and related jumps in the azimuth motor currents, corresponded exactly with the interval of the ten separate stator magnets. A complete new tachometer and a separate stator are being purchased as spares.