A motor focuser for £30
A small stepper motor, an Arduino Nano and a printed bracket now turn the focuser from the laptop. Getting the shafts in line was the hard part.
- Change
- Motor focuser fitted, replacing focusing by hand,
- Joined the rig
- Motor focuser
- telescopeyoke
- v0.2.1, the latest release on this date
- Cost
- £30.03 in partsnoted
Focusing by hand in the dark means touching the telescope, and every touch shakes it. So the focuser now has a motor.
Parts
| Part | Cost | Notes |
|---|---|---|
| 28BYJ-48 stepper motor, 5 V, with its ULN2003 driver board | £6.50 | A 12 V version looks identical, so read the label |
| Arduino Nano V3 clone | £5.23 | |
| Leads | £3.30 | Six jumper wires join the Nano to the driver board |
| Printed bracket, coupler and box | £15.00 | “Skywatcher 200P focuser” by AstroCosas, printed by a print service in black PETG |
| Total | £30.03 |
On top of that are a few M3 screws: two with nuts for the motor, two grub screws with nuts for the coupler, and four for the box.
The bracket was drawn for the 200P, but the 150P’s single-speed focuser takes it. It is held by the focuser’s own centre thumbscrew, with no new holes. The motor replaces one focus knob and turns the shaft through the printed coupler.
Wiring
| Nano | Driver board |
|---|---|
| D4 to D7 | IN1 to IN4 |
| 5V | + |
| GND | − |
The driver board has no pin marked GND, only a small minus sign beside the plus. The motor’s own plug only fits one way.
What went wrong, in order
- Squeezing it into the box. It fits, just.
- The Nano kept dropping off USB. With the box hanging on the telescope, the lead’s weight worked the plug loose in the Nano’s socket. The lead needs tying off.
- A dead port. One of the laptop’s two USB-C ports turned out not to work at all, which had me blaming good leads.
- It couldn’t turn the focuser. The motor made a half turn on the bench and fell well short on the telescope, with the camera on or off. Slowing the stepping from 2 ms to 10 ms a step helped a little.
- The real cause was the shafts being out of line. The rigid coupler binds if the motor’s shaft and the focuser’s are even slightly offset. Slackening the motor’s two screws and letting it settle where it wanted to be fixed it.
- Then a quarter turn of slack each time it changed direction, because those screws were now loose. Tightened in the new position, it runs freely.
Where it stands
The laptop can drive the drawtube in and out, at twenty seconds to the half turn. It has not yet seen a star. Still to measure are how far the drawtube moves per step and how much slack is left on reversing, and then the software can learn to focus by itself.
The firmware, the wiring and the commands are in telescopeyoke’s focuser notes.