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Having alloy plates instead of plastic alters the potential for
collimation screw fixings. The collimation 'pull' screws are vital
because they support the considerable weight of the objective lens.
Failure of the 'pull' screw threads in the relatively thin [4mm] and soft
aluminium would be catastrophic. The 'push' screws only align the lens axis
to ensure optical collimation. Failure of the 'push' screw threads would be
merely irritating due to loss of collimation.Needing a strong 'pull' screw thread suggests I use T-nuts instead of tapping threads directly in the aluminium. The anchor spikes on the T-nuts could still be useful to prevent unwanted rotation. Small holes could be drilled in the aluminium for these spikes and the spikes themselves filed to provide a more suitable anchor in harder materials like aluminium. The spikes could be reduced to fit in very small holes since they have no need to resist torque in a relatively soft, wood-based material.
Until now I had been assuming metal to metal contact for the 'bayonet' plates. Though even this is not strictly necessary as long as collimation is maintained between repeated removal and refitting of the 'bayonet' objective lens mount. So plastic standoffs in the form of washers or even plastic nuts could be fitted to the 'bayonet' screws. The 'bayonet' screw heads would still project and provide the secure location and retention as before. But now with a fixed gap between the two plates tightly controlled by the plastic packing material and the underside of the bayonet screw heads. The 'bayonet screws would be adjusted and then locked to provide the minimum of slop between the packing and underside of the screw heads.
I only suggest plastic spacers to avoid burring or wear on the mating aluminium OTA plate from metal fixings. There are further advantages to using spacers. The collimation 'pull' screws might project beyond the T-nuts and the T-nuts themselves will have some thickness. The 'bayonet' acceptance plate on the OTA must not suffer from any projections or it will not allow the lens cell to drop to the [lower] anchored position in the keyhole slots. Having spacers allows the 'bayonet' system to function cleanly while still maintaining the closely fixed distances between the lens [cell] and the OTA. It also moves the rear of the cell clear of the OTA location plate. Otherwise the collimation screws would have to push the 'bayonet' plate a full centimeter away to clear the OTA plate. Which again increases the loads on all the screws due to increased overhang. Not to mention lengthening the OTA to compensate for this increased overhang.
However tempting it might be to cut down the objective's bayonet plate this might reduce the quality of relocation. Widely spaced 'bayonet' screws make the most sense to avoid rocking once in place. With an astronomical telescope the OTA is normally pointing upwards so that the heavy lens rests naturally against its mounting plate. Thereby relieving loads on the 'bayonet' lens mounting system.
Difficulties might arise if the telescope was set horizontally for testing or collimation on a distant object. The heavy lens in its cell would then have a much greater tendency to sag forwards on the 'bayonet' mount. A smaller' footprint' of the bayonet screws would tend to suffer greater loads and magnify any play in the location system. The larger, full plate 'bayonet' footprint should be far more tolerant of 'bayonet' screw maladjustment. The footprint could be seen as the degree of leverage the lens can place on its location system. A large plate and well separated screws has mechanical disadvantage [in the physics of levers sense] rather than in the English sense.
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