Motorized focuser for reflecting telescope

This is an Arduino NANO-based motor driven focuser. I've installed this on an 8" Meade Starfinder as well as a homemade 6" reflector based on an 8" Sonotube. The design include a mounting saddle and backing for both telescopes. Also, each telescope has a different focuser design; I've included files for the saddle, backing plate, and GT2 pulleys for both scopes. These may or may not work for your project.

The controller includes two presets. This is useful for changing between two non-parfocal eyepieces. Press "SET1" or "SET2" to remember the focus position for each eyepiece, then "RCL1" or "RCL2" to go back to the stored position.

Since the motor consumes a lot of power, the program has a timer which powers down the motor if no buttons are pushed for 5 minutes.

The motor is disabled if the control pad is disconnected. When disabled, the focuser can be adjusted manually.

The controller is based on the following designs:

Arduino-based focuser controller: https://oshwlab.com/charlesodonnell/gp-mount-motor-control_copy Keypad: https://oshwlab.com/charlesodonnell/motor-control-keypad

Bills of Materials:

Focus controller:

• PCA: https://oshwlab.com/charlesodonnell/gp-mount-motor-control_copy
• Arduino NANO. https://a.co/d/faer9xY
• DRV8825 stepper motor controller. https://a.co/d/9mnK7W8
• NEMA-17 1A stepper motor. https://a.co/d/2xVPGKI
• 12V power socket 5.5x2.1 mm. https://a.co/d/3yB7RVw
• RJ45 connector. https://a.co/d/aUa2zUw
• 2-pin terminal block (2 req'd). https://a.co/d/e4GhavR
• 4-pin terminal block (2 req'd). https://a.co/d/e4GhavR
• 8-pin terminal block. https://a.co/d/dpvWNWE Sockets (optional). https://a.co/d/iob9XRY

Keypad:

• Printed circuit board. https://oshwlab.com/charlesodonnell/motor-control-keypad
• Pushbutton switch, MC32829 (6 req'd). https://a.co/d/hdghlzt
• 8-pin terminal block. https://a.co/d/dpvWNWE
• RJ45 connector. https://a.co/d/aUa2zUw

Misc. parts:

• GT2 timing belt, 2mm pitch, 162mm long. https://a.co/d/eOuzwsJ
• 8-conductor coiled cable, RJ45 connectors. https://a.co/d/cQN9WJt
• M3 screws & nuts
• No. 4 x 3/8" sheet metal screws
• Sockets (optional). https://a.co/d/iob9XRY

Printed parts:

• Focuser PCB housing. Enclosure for the control PCA, NEMA-17 motor, power and RJ45 connectors.
• Focuser Control PCB Mount. Cover for the enclosure. The control PCA mounts to this cover.
• Focuser control housing. Provides mounting location for the control panel keyboard.
• Focuser control panel. Cover with legends for the control panel.
• Focuser control key (6 req'd). Pushbuttons for the controller.
• GT2 Pulley Stepper Motor 24T. Pulley for the timing belt. Mounts to the stepper motor with M3 hardware.
• Focuser Pulley. Replaces the knob on the focuser. There are two versions, one for my 6" scope and one for my 8" scope. These may or may not work on your focuser.
• Focuser Control Saddle. This is a curved piece between the scope OTA and the control enclosure. There are two versions for my 6" and 8". These may not fit your scope precisely.
• Focuser Control Backing. This goes inside the scope. It holds the M3 nuts that go through the enclosure and saddle. There are two versions for my 6" and 8". These may not fit your scope precisely.

Assembly notes:

• I recommend sockets for the NANO and DRV8825s, but you may be comfortable soldering them directly to the PCB.
• The DRV8825 must have its current limits set. I set the limit to 1A. You can find the procedure here: https://www.makerguides.com/drv8825-stepper-motor-driver-arduino-tutorial/
• The 47uF 25V electrolytic is not optional. This is to protect the driver.
• You must compile and load the sketch to the Arduino. The sketch is available at the link in the BOM above.
• The 8-pin terminal block on the keypad PCA is soldered to the underside of the board, opposite the pushbutton switches. The connections to the terminal block are shown in the schematic at the link in the BOM. Use a zip tie to secure the cable to the PCA through the two holes provided.
• PCAs are mounted with No. 4x3/8" sheet metal screws. The enclosure covers are also mounted to the cases with No. 4x3/8" sheet metal screws.
• When printing the panel with legends, edit the gcode file to insert an M600 command (filament change) at Layer 10 (assuming 0.2mm vertical resolution). Start the print with white PLA. When the print pauses, change to black PLA to print the legends.
• Pay attention to the connections to the 8-pin terminal block on the controller and keypad PCAs. It's easy to get these reversed, or to stake the wires to the wrong legend on the RJ45 connectors.
• I highly recommend heat-shrink tubing for all soldered connections to connectors and the switch. I use 3/32" tubing, shrinkable to 3/64".
• All parts are printed in PLA, although I printed the keypad keys in translucent PETG. All parts can be printed at 0.2mm resolution.
• Positioning the control enclosure on the scope is crucial. Here's how I did it:
  • Install the stepper motor in the enclosure. Attach the stepper pulley.
  • Replace the rear-facing focuser knob with the GT2 focuser pulley.
  • Attach the enclosure to the saddle with double-sided tape, aligning the mounting holes with the holes. Apply a strip of double-sided tape to the bottom of the saddle.
  • Position the enclosure & saddle with the two pulleys aligned. With the 162mm timing belt between the pulleys, position the enclosure so that the belt is fairly taut. Press the enclosure/saddle in place.
  • Drill four holes with a 1/8" bit using the enclosure as a guide.
  • Pull everything apart and remove the double-sided tape.
  • Press four M3 nuts into the backing. Secure the enclosure & saddle to the backing inside the tube using M3 screws.

Category: Hobby