Sky-Watcher Esprit 100 ED PRO Triplet Motor Focuser

[Guest]

Hello,

On this bleak and windy day, I thought I would share with people a design and make project I did over the Christmas Break last year (2017).

One of my Telescopes is a Sky-Watcher Esprit 100ED Pro, the stock focuser is a 3” Rack & Pinion focuser with a 11:1 fine focuser and I find is a very good focuser.

I wanted to motorise the focuser, but after much searching around could not find a reasonably priced option, so decided to design and build my own. I did have to remove the single speed focuser Knob to measure the shaft, which on mine was Diameter 6mm.

I decided to purchase the Sky-Watcher Auto focuser along with the HitecAstro DC Focus control unit from FLO, all I then needed to do was come up with some way of attaching it to the Esprit Focuser. After some careful measuring and partial strip down of the focuser I drew up a plan and a 3d computer generated model.

The tools I used were:

• Vernier Calliper

• Steel Rule

• Engineers Square

• Hacksaw

• Metal work Scriber

• Metal File’s

• Hand Held Electric Drill

• Dia 3.2mm Drill for M4 tapped holes in Aluminium bracket

• Dia 5.0mm Drill

• M4x0.7mm Tap Set

The Materials Used were:

• 200mm length of Unequal Aluminium Angle from a company called “CLICKMETAL” – Cost £19.33

• Pulleys and Fixings from a company called “MOTIONCO”– Cost £33.49

• Sourcingmap 6mm to 8mm Bore Brass Robot Motor Wheel Coupling from Amazon, to connect to focuser shaft. – Cost £2.94

• HiTechAstro DC Focus control unit, from FLO – Cost £69.00

• Sky-Watcher Auto Focus Motor, from FLO – Cost £45.00

I had to remove the focuser Pinion Assembly from the focuser by removing the 4 outer most Cap screws, be careful though not to let the focuser draw tube slide out as this will end up with the ball bearings coming out of the race in the tube. I did not touch the 4 inner grub screws as these are a fine adjustment for the pinion meshing against the rack on the tube.

I removed the single speed Knob off the focuser as I wanted to drive off the main shaft, not the fine focus.

I then fixed the Brass Robot Motor Wheel Coupling to the focuser shaft, which I was able to tighten up the grub screw through the focuser Assembly casing.

I then bolted the Pinion Assembly back onto the tube with the new Aluminium bracket on top with 4 off 30mm long cap head screws with washers from the fixing kit, I tightened them down a bit at a time until fairly tight and then adjusted the 4 inner screws to ensure correct contact with the rack. This took a fair bit of adjusting a bit at a time until I was happy there was no free play or it was too tight.

I then attached the Skywatcher Auto focus Motor to the new bracket with 2 off 8mm cap head screws & washers from the fixing kit and added the smaller pulley to the motor shaft.

I then attached the 8mm Stainless steel shaft to the Brass coupler and slid the larger pulley onto it to line up with the smaller pulley, the shaft was then marked, then trimmed flush with the pulley.

With it all fixed in place I loosened the 2 off screws holding the motor to the new bracket, added the belt and pushed the motor up to tension the belt as the Motor bracket is slotted.

The only alteration I had to make was to cut a small slot in the Aluminium bracket for the focuser lock lever.

   

Although it is not a stepper motor, I find it is sufficient for focusing whilst in the Observatory from the comfort of the warm room & the HitecAstro controller does also allow override with the “IN & OUT” buttons if you are stood alongside the telescope. If I want to use the focuser without the motor I just loosen the 2 screws to the motor and slip off the belt.

The only thing I have to go careful with is that I don’t run the focuser to its stops as this won’t do the motor any good as I found out, although it only broke the bond on one of the motors internal gears to its main shaft, (easily fixed with superglue).

As I had all the tools, the only costs were the materials which focuser came to £169.76

If anyone wants to use my plans to make their own, please do so, but do check the measurements as I believe they can vary dependant on the age of your scope and you do so at your own risk as I cannot accept any responsibility if it goes wrong for you.

Alien Neil