Zytra Tech. EQ-Z

EQ-Z is the name of the German Equatorial Moun (GEM) I designed and built for my 17″ CDK. Although it’s intended to be used with this telescope it would most likely be able to track instruments up to 24″.


  • Construction: CNC machined Aluminum and anodized
  • RA Drive System: 15.25″ 485-tooth gear w/ diamond lapped Stainless Steel worm
  • DEC Drive System: 12″ 360-tooth gear w/ Stainless Steel worm
  • Motors: Maxon RE25 (#339150)
  • Gearboxes: Maxon GP32A 576:25 Reduction (#166161)
  • Encoders: Maxon MR 256 CPR (#225773)
  • Servo Motor to Worm Reduction Ratio: 48:1
  • RA Motor Ticks per Gear Revolution: 23,838,720 (or 18.39 ticks per arcsec)
  • DEC Motor Ticks per Gear Revolution: 17,694,720 (or 13.65 ticks per arcsec)
  • Shaft Encoder: RA only
  • RA Shaft Encoder Ticks per Shaft Revolution: 6,297,600 (or 4.86 ticks per arcsec)
  • RA Shaft Encoder Read Head: Renishaw RGH20H (50nm resolution or x400 interpolation factor)
  • RA Shaft Encoder Ring: Renishaw RESR20USA100 (20 um pitch or 15,744 LPR)
  • Mount Controller: SiTech Servo II
  • Counterweight bar: AP 1.875″ x 20″
  • Saddle: Custom Dovetail
  • Mount Weight (without counterweights): ~ 250 lbs

Build log:


  • Design: Started August 2012
  • Fabrication: Started January 2013
  • Final Assembly: Started June 2014
  • Installed: September 2014


I started working on EQ-Z shortly after finalizing the design of my 17″ CDK. Back then, it wasn’t obvious if I could even afford the optics for it, so I figured it might be a good idea to design a mount to save a bit of money. Commercial Equatorial Mounts of this capacity capable of tracking for long exposure Astro Imaging typically cost anywhere from $12K to more than $20K.

I realized that most of the mechanics was something I could handle, so I started by listing all of the items I couldn’t make and that I would need to purchase to make it happen: the gears and worms, the servo motors, the RA encoder and the controller.

Servo motors and encoders were pretty easy to select: Maxon makes very good quality motors and even offers to build motor/gearbox/encoder combinations. A quick search for High Resolutions encoders confirmed that optical encoders are the only cost effective way to get 5 Millions + resolution. Renishaw has a wide selection of rings and read heads.

Gears and worms… I had heard of Ed. Byers’ famous gears but I also thought retired several years ago. But one day I ended up finding some of his products on sale on Ebay. It turned out Ed was selling the rest of his inventory. After a few emails he offered to stop by his shop in Barstow, California to talk about my project. Ed gave me precious advises some of which I implemented in my design. He also showed me some of the products he still had in stock. Among them were a 485 teeth 15.25″ gear with diamond lapped worm and a 360 teeth 12″ gear with worm, which I bought from him a few weeks later. He also got to work for me by making custom aluminum spur gear for my motors / worm assemblies with the exact reduction ratio I wanted. I am still in contact with Ed occasionally updating him on the project.

I initially designed the mount around my Gemini 2 controller used by my G11 mount. There was a number of features (especially upcoming features) that I really wanted: the most important one was the High Resolution encoder support. Being able to save some money by using a controller I owned was just ideal. 15 months later as I was finally putting everything together, that specific feature was still on the to-do list. While I could have decided to use the mount without the Renishaw encoder on RA, I decided to shop for a new controller instead. I ultimately went with the Sideral Technologies ServoII controller which fully supports high resolution encoders.

My design differs from typical GEM’s mainly by the positiion of the RA gear. It sits in between the two large RA bearings. My RA clutch is also fairly unusual design; it uses a “captain wheel” accessible from a window built-in the gear cover which is a direct consequence of having the gear in between the two bearings. For practical reasons (minimizing the lathe work) the DEC shaft housing is made of milled Aluminum plates. It also kind of matches the rest of the design.

Other than that, it’s pretty much just a big mount. Without any counterweight, it weights over 250 lbs. I don’t know its Astro Imaging capacity but I don’t expect my 17″ CDK to be anywhere near the maximum load.

Build thread (on the Cloudynights.com forums): 17″ CDK and Equatorial Mounting

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