Saturday, February 10, 2018

DIY Electric Focuser For Celestron 127 SLT Maksutov/Cassegrain

It's been really cloudy. For eight weeks. So I've been working on scope projects - at some point, one day, I'll be able to see the night sky again. :-)

One of the tradeoffs with the mid-price Celestron 127SLT is that it shakes rather badly when you touch it, and takes several seconds to settle down. I'm focusing using a Bahtinov mask and Sharpcap's Bahtinov aid, which gives a numeric indication of the accuracy of your focus. It can be a slow process to focus it since you tweak, wait, and tweak again.

I decided that a remotely controlled electric focuser would allow me to do a much finer focus than I could achieve by hand, since it would not require touching the scope.

This design doesn't require removal of the focuser knob - only the rubber sleeve that covers it. This sleeve slips off by hand.

If you do pursue a different design that requires removal of the focuser knob, be warned that you must not tip the telescope down toward the table with the focuser knob removed - the knob is all that prevents the main mirror from falling forward and smacking into the corrector lens, which requires a teardown and cleaning to fix. This design does not risk that situation.

It took some head scratching to figure out how to mount the motor to the optical tube assembly (OTA). It finally dawned on me that I could remove the dovetail mount, and install a bracket under it. Aluminum stock which is 1" wide fits nicely in the gap that the dovetail leaves between it and the OTA. I then made a simple motor mount out of 1.5" x 1.5" right angle aluminum stock.

I cut some slots in the motor mount so that the stepper motor could be moved back and forth a bit, to set tension on the belt. I don't have a mill, so just drilled some holes next to each other and then filed the slot to open it up.

The focuser knob is 15 mm, and the stepper motor shaft was 5 mm. I used the following parts:

Aluminum 50 tooth MXL pulley with 15 mm bore
Aluminum 30 tooth MXL pulley with 5 mm bore

Based on some belt length calculators I found online, I first tried an 8" MXL closed loop belt, which worked, but had the motor run out to the end of it's range of adjustment. I wanted the motor closer to the OTA, so I found a 7.6" MXL belt from McMaster-Carr, part number 7887k78.

The 15 mm bore pulley was a perfect fit. The 5 mm bore pulley needed to be bored out to 5.2 mm to fit the shaft.

Here's a test run, with the original 8" belt.

The stepper motor can be controlled any number of ways, depending on what you want to do. There are a number of ASCOM compliant stepper controllers driven by microprocessors. My goal was to make my scope remotely controllable across a network, so I used a Raspberry Pi Zero W and an EasyDriver , along with a 5V regulator.

For a simple web browser interface, I used information from this excellent Flask tutorial to make a very simple web app to allow coarse, medium, and fine motor control. Source code will be in a separate article once it is a bit more polished.

Everything was buttoned up in a box with strain reliefs - it runs off the same 12V jump start battery I use to power the scope. Initial tests have been very positive - I can't wait for some clear skies to test it! It focuses very smoothly with no shaking - you can see clearly through the scope during the entire focusing process. Additionally, with the stepper motor, you can move the focuser in much smaller increments than by hand.


  1. I love your project, and thank you so much for sharing your knowledge with the rest of us. It is interesting, informative and educational, i.o.w very well done. Im considering buying a smaller telescope thats easier to bring around than my Celestron Evolution 9.25, and this is right up my alley. I hope we get to see a lot more of your ideas and awesome projects, and if you have the time, please write some good advice's and opinions about the different topics you touch upon.The sharing and the gathering of knowledge is what I really love about the internet, thank you for making the web a good place to be. :)

  2. Great project, thanks for sharing!

    Is there a particular model of stepper motor you'd recommend?