At the age of your telescope, my guess would be that the baffle needs to be re-greased.
As others have explained, the mirror on these scopes is attached to a carrier plate and this plate has a hole in it which the primary baffle runs through. As you turn the focuser knob, the threaded rod pushes/pulls the mirror carrier along the baffle and it slides on a film of grease. This grease film takes up at least some of the gap between the mirror carrier and the baffle, and when it thins, the excess play causes excess tilt between the baffle and the carrier.
Now at this age, I would consider re-greasing with a very high viscosity temperature stable grease, but prior to doing that, I would see if it is possible to re-distribute the grease that is already on the baffle.
The easiest way to do that is to place the scope on a horizontal surface. Begin by turning the focuser knob fully counter-clockwise. Remove the focuser knob, remove the three screws that hold the focuser assembly in place, and with the screws removed, grasp the focuser assembly and pull it away form the tube as far as it will go. Now, push and pull the focuser rod through its full range of movement maybe 10-20 times.
If there is any grease left, this will force the mirror to travel outside of the normal focuser range over the area of the baffle where it has migrated to and re-distribute at least some of that grease back to the area of the baffle where the carrier normally travels when the scope is at infinity focus.
I would not use superlube or other thin grease compounds. These are fantastic lubricants, but they will not provide a durable grease film between the surfaces of the carrier and baffle. The grease has to be high viscosity and ultra-low out gassing. Probably something like NyeTorr 6200 which is a lab grease that is recommended for linear glides, where motion control (play) is important. I am not saying this is the right grease to use, but Nye can probably give you a recommendation if you carefully describe the application requirements.
Even with fresh grease, some movement might still be noticed during focusing, but anything more than an an couple of arc minutes probably means you need to re-grease.
A lot of greases outgass. It is vital to choose one that is formualted to prevent that. From the Nye 6200 description:
eliminating or minimizing
airborne molecular contamination or giving off vapors
that can fog optics in high-speed inspection systems
or even contaminate wafers.
Edited by Eddgie, 15 November 2019 - 10:39 AM.