Making a stepper motor focuser for a Celestron 8″ EdgeHD

DIY Stepper motor focuser

I have a Moonlite high resolution stepper focuser for my BK80ED refractor. I didn’t have an equivalent stepper focuser for my 8″ EdgeHD so I invented my own. I used a 12 Volt stepper motor which has a step angle of 5.624 and an internal 64:1 gearbox. This means that it’s output shaft makes 1 revolution for every 4,096 steps. I connected the stepper motor to the focuser knob on the 8″ EdgeHD using a MXL Type Timing Pulley 24 Teeth 5mm Bore and a 60MXL timing belt.

I bought the motor, gear and belt when you could still buy stuff from China. Today you would need to find local sources.

I made a wooden bracket that consists of a 3/4″ thick base plate and a 1/4″ motor bracket that also acts as a belt tensioner. The central hole in the 3/4″ thick base plate is 2 1/2″ which just fits over the neck of the visual back of the scope. 3/4″ is about as thick as you can use…anything thicker will interfere with tightening down the 0.7 focal reducer against the visual back. Don’t get to crazy with how big you make this base plate. It needs to clear the two clutch knobs.

I used my bandsaw to make the cuts but a jigsaw would also work. I have a small sanding drum that fits on a drill which helped me smooth and fit the 2 1/2″ hole. Just be sure to test fit a few times and don’t oversize the 2 1/2″ hole…creep up on this dimension. I also made a cut from the outside of the base plate to the inside of the circle. This is how I tighten the base plate onto the optical back of the scope. You should notice the machine screw that passes thru this cut. This screw allows me to tighten down the base plate.

The 1/4″ motor bracket started off with fitting the stepper motor and the gear. Once that was done I figured out how to connect the bracket to the base plate. The wood screw that holds the plastic cable support is not fully tightened which allows the motor bracket to pivot on this location. Next I figured out how to shape this bracket so that would not interfere with the optical back and was as small as possible.

Finally I found a spring and connect it to the base plate and motor bracket so that it could act as a belt tensioner. To stop the spring from flying off into the weeds I drilled small holes and screwed in small screws. Finally I cut off the heads of the screws with a hacksaw. The stubs that remain stop the spring from flying off into the weeds.

The cable is a DB9 serial cable. The DB9 is a male 9 pin connector where only 5 pins are connected. This wiring is for plugging into the moonlite focuser controller. It will also work for the Robofocus controller.

I figure I get roughly 4,500 pulses for every rotation of the focuser knob which is pretty high resolution. There is some mechanical backlash in this set-up so I autofocus using an antibacklash positioning scheme.

I have been using this arrangement for 3 to 4 years and it works fine for me.