Renovation

I am renovating an old Astro Equipment of Luton Class C mount in my garage, have been for two years now. The purpose of this is to place the mount in the observatory as the big load-bearing mount to replace the Vixen GP/DDX that is there currently. What's been taking the time is working out how to drive it. It came to me with a seized synchronous motor in RA for tracking only and a chunky dec motor with double worm gearing. I took these off - I wanted goto!

This mount comprises a heavy machined C-casting which pivots in a shoe to form the adjustable polar mount block, a cast DEC axle mount and two pairs of PTFE-lined bearings for the RA and DEC blocks. The axles are 1.5" Stainless steel and the clutches for the worms on each axle are friction driven using springs for constant pressure, with thrust bearings between the spring and the pillow block to provide the rotating pressure surface. The RA worm wheel is cast bronze with spokes, 10" in diameter with 360 teeth. The DEC worm is solid bronze 6" in diameter by 1/4" thick, again in bronze with 196 teeth with a twin-start worm. Both axles have worm mounts using bronze busings to hold the shaft which carries the worm. The worm is spaced using spacers at either end and uses PTFE washers to maintain accurate spacing and take up any play. The worms are 3/4" diameter steel which are locked on to the the shaft using two grub screws.

The jobs I needed to do on this mount include :

• De-rust and re-spray all parts - done
• Polish and paint the DEC and RA setting circles - done
• Clean and wear-in the RA and DEC worm wheels - done
• Build or procure a RA and DEC drive to replace the original synchronous and DC motors to allow GoTo control - underway ( See PIC Microstepper driver & PIC GoTo drive board) - underway
• Fabricate a worm wheel cover for the RA and DEC Worm wheels. - beginning
• Install a digital encoder on the RA and DEC axles to measure 'absolute' periodic error. (The pointing position is already handled using incremental encoders on the worm drives). This will allow me to measure and correct for Periodic Error on the worms without guiding.

Starting Point

This is what it looked like before I started and while dis-assembling the parts.

Painted

After dis-assembly, stripping the paint, clearing the grease and rust and then undercoating and whitepainting in Hammerite, I replaced the springs with with new springs (bright zinc plated), BSW bolts with stainless steel bolts and most other bright parts. I also polished the dials, removed the dial pointers and ditched the existing motors.

And this is what it looks like after:

The OTA

While restoring this telescope I have added a moonlight Crayford focuser (there wasn't a focuser fitted when I received it) and fabricated a curved secondary holder so that the secondary moves up and down with the focuser. The original secondary mount comprised a heavy brass stalk jutting from the moving focuser plate. The plate itself rides the skeleton tubes and clamps wherever needed.

I re-balanced the skeleton tube once I took it all apart and got it moving easily in the tube-frames. Moving the tube to balance point now requires a gentle tapping on the centre frames until balanced then tighten the griub screws in the frame holding the tubes.

I built a finder mounting from aluminium plate. It's a simple pair of tube clamps machined as a pair and bolted to a carrier plate with a vixen finder shoe mount. This can move up and down the skeleton tube frame as required and then tightened.

The motors

I was originally going to take my existing Vixen Skysensor motors off my GP/DX and put them on this, fitting replacement iOptrons on the GPDX as that would become my portable mount. But I could never find a way within the confines of the space I had on the mount that would let me mount them. Eventually I found a mounting method that would fit, bought another Skysensor, made some mount brackets and have finally got the motors mounted. The mounting of the motors meant removing the motors from the SS2K enclosures they come in, machining the worm brackets on the telescope to provide clearances for the motor and mounting the motor on brackets parallel to the worm. The brackets are retained by a single socket head screw and positioned on the cheeks of the worm bracket and using the worm shaft itself to retain registration.

The decision to use belts and timing pulleys is due to the space requirement - there was no way I could get gears large enough to mesh at minimum spacing without fouling the worm wheel or the dec axis on swing-through. Pulley wheels could be smaller and have any separation distance I chose. I went for 10mm by 2.5mm pitch pulleys which use 18teeth at any one time in mesh. Backlash seems tiny compared to the worm backlash. The pulleys are 2.5mm pitch, 10mm wide on 38mm diameter bosses, using 160mm belts.

Carrying other telescopes

I made a 'semi-universal' telescope carrier plate to replace the dedicated hardware that mounts the 10" tube to the dec axle. This is intended to be a standard bolt-on hub fitting (e.g. Fenner 1625 ). Doing this means I get a good alignment with the shaft using industry standard parts and I can build/buy more easily. The first one will carry the Vixen Visac 8" bolted to a plate bolted to the hub. To do this I turned off the back of the hub to a flat surface, while keeping the hub compressor the same size. This is needed to keep the bolt seats wchich push the compressor into the hub to seize the shaft. The carrier plate is then just drilled through to match the tapped holes in the hub rim.

I found I also had to make a shaft clamp to hold teh shaft in teh same position as it was with the original OTA carrier due to relative location of hardware. I made a two-part clamp from Alloy on the lathe, slit it and it just bolts around the shaft.

Re-assembly

I ground in the the worms for both RA and Dec axis by using lapping paste for engine valves to run between the worm and the wheel while I ran the motor backwards and forwards for a good while by disconnecting the encoder wires and running the motor from a battery at full pelt.

I have been trying to mould styrene and acetate to make gear wheel guards. Reports from MArtin Mobberley's use of one of these mounts is of good performance when the telescope balance is biased against the worm to take up backlash but suffers from dirt in the worm. Hence I need to keep dirt out. To do this I draped styrene sheet over a male mould in the oven at 170 ° fr 20 minutes. for the small worm this was almost OK but for the large, the sheet rolled itself up into a scroll. Alternative methods to investigate include.. Making a female mould for GFRP or bending plastic angle strip to a circle plate and gluing it there.

I made the clutch spring tensioners from 38mm plumbing brass connectors with their lock nuts. Turning off the top of the lock nut on the lathe gives a greater screw range. The unit sits between the clutch springs and a shaft clamp which is then adjusted by turning the thread/nuts to get up to 1.5 &inch; spring compression, The springs ride on a thrust bearing which takes the load against the worm wheel to press it onto the clutch plate which is attached to the shaftand provides the turning force on the shaft.

The final shape of the mount with a demo scope looks like this :

I will post tracking error output graphs here when I get them.

Other AE telescopes

There are other AE telescopes out there, in varying states of repair.

I'll post some links here soon!