Remote Focus Motor Project – Part 2 – Continuing Progress

Find out how I handled the mechanics of attaching a stepper motor to my Celestron Nexstar 4SE for the Remote Focus Motor Project!
The Mechanics, Software, and Version 2

Check out part one of the Remote Focus Motor Project!

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Focus Motor Mechanics

Stepper motor prototype bracket
Stepper motor prototype bracket

One of the challenges with adding a focus motor to the Celestron Nexstar 4SE (Amazon Affiliate link) is the lack of a dedicated mounting spot. Fortunately, since the 4SE has a built-in diagonal and a visual back, I used the visual back cap to hold the motor mount bracket to the telescope.

Making stepper motor bracket
Drilling holes in the mounting bracket

Recycling or reusing parts that would otherwise be discarded is something I enjoy. I crafted a bracket from the cover of an old microwave, and if you’ve ever replaced vacuum belts, you may recognize the gear on the stepper motor and the focus knob. The gear on the stepper came from the motor for brush rollers, and the gear on the focus shaft came from the ends of a brush roller.

Testing stepper motor and bracket fit
Testing stepper motor and bracket fit

I used two gears from a brush roller and glued them together for two reasons. First, the gears need shoulders to prevent the belt (also from the vacuum) from slipping off. Second, as the focuser on the telescope moves in and out, it exaggerates the belt slippage. In the image to the right, I was testing a method to keep the belt from coming off. It worked for a while, but eventually, the belt caught on the edges of the wood. Gluing two of the larger gears together provides more space to slip, and the lip on them prevents the belt from slipping off.

focuser closeup

I can’t do that for the gear on the stepper motor, so instead, I’m going to try melting some plastic to make a stopper for the smaller gear on the stepper motor. The “cap” will be tapered to keep the belt in place. There was some space in the end of the gear I added to the stepper motor, so I ended up filling it with paper and super glue and drilled a hole in that. I’ll use it to attach the plastic cap I make using a screw. Hopefully, that will solve the belt slipping issue.

Of course, I could always buy the appropriate belt drive and gears, which I may do when I upgrade to the NEMA 17 stepper motor, but where is the fun in that?!

Software

When I first got the software running for the stepper motor, it worked fine initially, but after a while, the commands started backing up. During the first few command cycles, the stepper motor responded nearly instantly over Wi-Fi. After a few minutes, it seemed like the commands were backing up. The stepper started lagging behind the commands sent. I racked my brain for a while and eventually figured it out, more or less. It still sometimes misses commands, but it works.

It doesn’t focus as fast as I would like, even at “full speed.” I suspect part of that is the limitations of the small motor and the gear ratio. The gears should be reversed, with the smaller gear on the focuser shaft and the larger one on the stepper. Doing it that way would have taken more effort, as the predrilled holes in the larger gears are bigger than the shaft on the stepper motor.

I may try rewriting the stepper controller software and add some buttons on the web interface to allow the motor to run longer than it currently does. This would allow me to change focus faster, for instance, if I wanted to change focus from something in the sky to bring tree branches into focus during moon/sunset, or before the moon/sunset clears the trees.

Future versions

I’m considering upgrading to a more powerful and faster stepper motor, a NEMA 17 (Amazon Affiliate Link). This should overcome the limits of the small stepper, though it may also be overkill and could cause problems if it overdrives the focuser and strips something harder to replace.

Another upgrade for version two is using a more powerful Arduino with built-in Wi-Fi/Bluetooth, such as the HiLetgo NodeMCU (Amazon Affiliate Link). Integrated Wi-Fi would eliminate the serial data connection that is currently being used between the ESP8266 and the Uno R3. This way, response time should be faster and may prevent lag after several minutes. It will also allow more inputs, so I may consider adding IR remote control, replacing the DHT11 with a sensor that can detect air pressure changes and has a more accurate humidity and temperature sensor.

I’m also considering adding a GPS module (Amazon Affiliate Link) and a larger screen. This would make it more convenient to get GPS coordinates when needed for telescope alignment when in the field. It would be a bonus if I could connect that to the telescope’s AUX port and automatically set the GPS coordinates for the scope.

Additionally, I’d like to add a bank of either 4 or 8 relays. This would allow me to power cycle various components remotely, such as the telescope, camera, and dew heater. I could also write the software to automatically switch the dew heater on and off based on weather conditions. One last feature I may add is remotely controlled LED lights. I’ll start drawing things out on paper to see how many I/Os the NodeMCU has and if it can handle all those tasks.

Stay tuned for updates!