Advice needed for slow motion control of Fullerscopes Mk3 mount

[adrianwb]

HI all, I've recently rediscovered a Fullerscopes 4" Newtonian reflector from my youth. Unfortunately, the telescope itself isn't in great shape (primary mirror seems to be corroded), but the mount seems to be usable (see attached photos). There is a motor drive on the RA axis, although I haven't checked yet to see if it still works, but I'd like be able to control the RA with a slow motion cable (or knob):

- what do I need in order to add slow motion control to the RA axis (and maybe the Dec axis as well)?

- where might I be able to source cables/knobs compatible with the Mk3 mount (I'm in the UK)?

- do I need to disengage the motor drive in order to move the axis manually (with or without slow motion)?

- is it worth thinking about a replacement mirror for the OTA, if so any recommendations?

Any help/advice links appreciated,

Thanks, Adrian

,

 

[Tomatobro]

Back in the 80's I made a driver for this kind of synchronous motor by push pulling dc across the windings of a 12 volt / 240 volt ac transformer and by controlling the frequency from about 45 hz to 55 hz gave a degree of adjustment over the speed controlled via  a potentiometer. This enabled the system to be driven from a 12 volt car battery.

[Peter Drew]

The Ra worm assembly incorporates a slip clutch in the aluminium connector between the motor and the worm shaft.  There is a nylon disc situated under the grub screw nearest the worm to form the slip clutch, they were designed to be useable without disengaging the drive but if you decide to operate it manually it would put less strain on the motor gearbox if you do disconnect it.  The worm shafts should be 5/16" diameter so will need a control rod that size.  4" mirrors are uncommon today and re-aluminising can be quite expensive, it might be more economic to find a used 4.5" OTA with mirrors in good condition.     🙂

[Rusted]

There is also the matter of having live "mains" electricity associated with the mounting in a [normally] damp environment.

https://fullerscopes.blogspot.com/2011/10/fullerscopes-drives.html?m=0

These days, telescope mountings tend to use much lower voltages and stepper motors, for vastly more sophisticated control of drive speeds.
 

[adrianwb]

On 22/02/2022 at 22:59, Peter Drew said:

The Ra worm assembly incorporates a slip clutch in the aluminium connector between the motor and the worm shaft.  There is a nylon disc situated under the grub screw nearest the worm to form the slip clutch, they were designed to be useable without disengaging the drive but if you decide to operate it manually it would put less strain on the motor gearbox if you do disconnect it.  The worm shafts should be 5/16" diameter so will need a control rod that size.  4" mirrors are uncommon today and re-aluminising can be quite expensive, it might be more economic to find a used 4.5" OTA with mirrors in good condition.     🙂

Thanks Peter, that's useful info. Now all I have to do is find somewhere that sells a slow motion knob/cable that fits the shaft (5/16" is pretty close to 8mm). I've looked on Astroboot but it only seems to have the SW 6mm ones, even if they posted to the UK 😞 (although I do have relatives just over the border in  Donegal). I'll be posting a wanted ad on astrobuysell and on cloudynights.

Has anybody got suggestions for other possible sources, or know of any more modern mounts that have 8mm shafts instead of 6mm? I've confirmed that the motor is working and turning the worm shaft, and I can disengage relatively easily it by loosening the two bolts which hold it onto the mount frame 🙂

 

[adrianwb]

On 23/02/2022 at 05:46, Rusted said:

There is also the matter of having live "mains" electricity associated with the mounting in a [normally] damp environment.

https://fullerscopes.blogspot.com/2011/10/fullerscopes-drives.html?m=0

These days, telescope mountings tend to use much lower voltages and stepper motors, for vastly more sophisticated control of drive speeds.
 

Indeed! Thanks for the link and the good advice - the plug isn't even earthed!