Converting a 12″ Dobsonian to OnStep

I have been bitten by the astronomy bug and quickly learned to love two things: aperture and GoTo telescopes. My first experience with a computerized GoTo scope came from a Celestron NexStar 6SE, which is a very nice scope and will open up a TON of viewing for many people. The GoTo feature is wonderful for those of us that have less than ideal viewing conditions such as light pollution.

I also had a chance to look through a friend’s 12″ Dobsonian telescope, a Zhumell Z12, and was amazed by how much brighter the views were. I quickly found myself wanting one of these wonderful scopes as well, but couldn’t find myself paying for a full goto 12″ or 14″ scope like the Skywatcher 300. As of February 2022 when I started this journey, a scope like this would cost around $2500 with full computer control. If you choose to go the Cassegrain approach, a new Celestron CPC1100 will cost ~$4000 at today’s pricing.

I ended up purchasing a 12″ Zhumell Z12 of my own, and decided to convert it to a computer controlled GoTo Scope myself. Given that a new Zhumell Z12 goes for ~$1300, that leaves about $1200 to break-even on a control system. I’m estimating I can get similar features in a $400 budget, which should be a nice savings. Especially considering that I’ll probably copy that system over to my friend’s Z12 as well.

I decided to use the OnStep software to control it because it looks like there is a robust user community supporting this, and it will give me the long-term ability to upgrade/repair any electronic failures without being held hostage by a shortage of parts from Orion or Celestron.

I’m going to have this as a primary post to document the process of the build, and hopefully link to the steps along the way. I’ll start with a list of my questions, and answer each one in a post to give more documentation.

How to select the motors/drivers to use for this?

Learning to Program the Controller

How to mechanically adapt to the telescope? Many files will be uploaded to:

AZ Motor Mount Bracket

AZ motor Mount and Wheel

100 Tooth Gear for Alt Axis

How much will this cost?

4 thoughts on “Converting a 12″ Dobsonian to OnStep

  1. joey matheson here, can you please where i can have the ( rack and pinion ) made or gearing as you call it. It is for my dob also. Please let me know ASAP. thank you.

  2. Again can you please tell me where you had your gear made? I would like one for my scope. please get back to me. joey.

  3. Hi Bryan, I saw your YouTube video ( and was wondering where you had your gears made? Or did you do it yourself? I have been trying to modify one of my scopes, but can’t find anyone that can make the parts for a reasonable price. Please email me with any helpful pointers you might have. Thank you.

    • Hi Joey, I designed and 3D printed the gears myself. I had the same issue – it’s hard to find custom parts for things like this. What scope/mount are you looking to modify?

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