Restoration of 15” Reflector
Drive Mechanism
A view of the scope inside its steel dome. There is plenty of room inside to work around the scope and mains power and lighting were all working properly. The inside of the dome would benefit from re-painting eventually.
The bottom of the scope carries a large number of cast weights (ex dumbbells) and several drums containing lead. This enables the relatively heavy plywood tube to be balanced at a point quite close to the mirror. This means the length of the fork arms can be kept shorter than would otherwise be the case.
A close inspection and photographic record was made. This obviously helped to produce this document but also allowed us to ensure that any parts removed were replaced in exactly the same position. It also meant we could study the details of drives and electrics at a later date without necessarily removing them.
The DEC drive wheel and worm assembly. This motor was working in both directions although the clutch adjuster was a bit rusted up.
The fork mount is very substantial. A large casting supports the polar shaft thrust bearing at the bottom as well as two large rollers at the top end. Here a large drum distributes the radial load. There is a large worm/wheel set and motor providing constant sidereal RA drive. Above this on the same shaft is a smaller worm/wheel set with its own motor which enables slewing in RA. The sidereal drive was running but the clutch was slipping. The slewing drive was not working at all. The hand controller was clearly broken but a wiring problem elsewhere was suspected as well.
The polar shaft. Note the clutch at the bottom, the larger sidereal drive, the smaller RA slewing drive (tucked in between the larger drive and the drum) and the large drum running on its rollers.
The RA clutch (part of the adjuster removed). The upper steel plate is pressed against the alloy flange at the top providing friction drive. The adjuster causes the two steel plates to rotate slightly in opposite directions. This forces four ball bearings, captive in sloping grooves between the plates, to force the steel plates apart, driving the clutch parts together.
Whilst the clutch was being cleaned up and the adjustment mechanism freed by Mark, Ian set about repairing the controller. The original controller had been replaced once before (Ian was involved in this job too!) and a new box was being built. As the internals were ok, it was decided to just replace broken switches and reassemble the parts.
Once all the bits were back together a multi meter was used to test the connections throughout the system. A poorly conducting connector was identified close to the fork mounting and some delicate cleaning and re-wiring was carried out. Once this was completed the motors all ran properly. At this stage the mirror was away being re-coated so no checks could be made on tracking accuracy but the movements of the scope (balanced up with extra weights to replace the very heavy mirror) were smooth and precise. Everything seemed very promising!
