Large GoTo German Equatorial Mount (GEM)

GEM Project

This project is construction of a large GoTo German Equatorial Mount (GEM) for my 10" ultra light Newtonian telescope. The basic design theme for this project was to inexpensively construct a large computer controlled GEM using only basic tools and ordinary materials. Most materials are available at any hardware store; the only exceptions are the stepper motors, flexible shaft couplers, and stepper motor control boards (these are available on-line). The worm gear drive system is also homemade. I developed a method to fabricate large worm wheels from strips of aluminum, using only basic tools: an ordinary electric drill, a router, a small hobby metal saw, and a file (see the Large Aluminum Worm Wheels Webpage). To eliminate dependency on external manufacturers, I avoided using commercial telescope drive equipment; the only exception was a Shoestring Astronomy GPUSB guide port interface adaptor,  allowing a commercial autoguider program to control the GEM. I am writing original computer code to control GoTo and tracking functions. 

GEM Design Requirements

GEM Design requirements:
  1. Construct the GEM from ordinary materials available at any hardware store 
  2. Develop a simple method to produce a large homemade worm drive with common tools and equipment (see the Large Aluminum Worm Wheels Webpage)
  3. The drive system should slew at 80-90 deg./min. (full step) and track at 0.25-0.45 arcsec/step (micro stepping)
  4. Both telescope axes must include a friction clutch for manual positioning
  5. Isolate the drive mechanics from the GEM with a Sorbothane vibration dampening material
  6. A fully adjustable spring mounted worm drive for easy alignment with the worm wheel
  7. Construct the motor control system from common electronic components and breakout boards (keep the motor controller cost at $50)
  8. Connect a Shoestring Astronomy guide port interface adaptor to the GEM hand controller, allowing a commercial autoguider program to control the GEM
  9. Write original program code for GoTo and tracking functions


  1. The GEM axes are 1" diameter galvanized pipe.  
  2. The right ascension bearings are two 1-1/8" pillow block bearings (purchased for $13 each). 
  3. The declination axis bearings are two 1-3/16" two bolt flanges (purchased on sale for $5.50 each); I used slightly larger bore flanges for the declination axis because I found several on sale at a good price. 
  4. Most of the GEM is constructed from 9.2 x 4.4 cm pine 
  5. All hardware is galvanized
  6. Components purchased on-line:
    1. Shoestring Astronomy GPUSB guide port interface adaptor
    2. 5 mm x 12 mm flexible shaft couplers
    3. Stepper motors (0.9 deg. step angle)
    4. Microprocessor, misc. electronic components, and breakout stepper control boards
    5. Sorbothane vibration dampening elastomer sheet

GEM Axis Design

The pier is only temporary, and constructed from two 9.2 x 4.4 cm pine boards. The right ascension bearings (pillow blocks) sit on a wood box that fits around the top of the pier and pivots on a M12 threaded rod. The declination axis was just a duplication of the RA axis design with flange bearings instead of pillow block bearings. I used flange bearings on the declination axis because I had them from another project.  A 2x4 declination board is attached perpendicular to the RA axis with a galvanized pipe flange. M8 bolts extend through the 2x4 and RA axis pipe, locking the 2x4 in place so it can't slip in the pipe flange. The 2x4 sits on a wood spacer disk to prevent the mounting hardware from restricting the RA axis motion. I bolted on 4 wood blocks to attach the declination flanges to the 2x4. I purposely made the 2x4 as long as possible to stabilize the declination axis and dampen any oscillations of the counterweight rod.  The declination axis extends through the OTA attachment 2x4 and attaches to a pipe flange; this OTA attachment 2x4 connects the GEM to the telescope (bolts inside the OTA). A M8 rod runs vertically through the OTA attachment 2x4 and declination axis to prevent the galvanized pipe from rotating in the pipe flange. I added two sheets of Sorbothane vibration dampening material to either side of the OTA attachment 2x4. I also placed Sorbothane pads under the M12 washers on the rods attaching the OTA to the mount, and on both stepper motor mounts.

I added an elevation fine adjustment on the RA axis to elevate or depress the wood box and RA bearings. The RA fine adjuster is a galvanized M10 turnbuckle connected between the RA axis and the pier. The turnbuckle had a hook on one end that required modification. Starting with a wood disk containing a 38 mm diameter hole, I chiseled a depression to hold the turnbuckle hook and a M12 washer (below far left photo). It is difficult to visualize in the below left photo, but the flat turnbuckle hook completely fills the 38 mm hole. A second disk screws over the turnbuckle to hold it in place (below 2nd from left photo). I bored a 12 mm hole through both disks. The completed assembly slides on a 12 mm threaded rod and the disks prevent the hook from slipping free. The disk-turnbuckle assembly fits inside the GEM RA wood box and pivots to accommodate changes in elevation angle (below 2nd from right photo). The eyebolt end pivots on a M12 rod attached to the pier (below far right photo). Since both ends of the turnbuckle pivot on rods, the turnbuckle doesn't seize up as the RA elevation angle changes. 


Friction Clutch

The GEM includes a friction clutch on each axis. The friction clutch allows the motors to track constantly, but pushing the telescope causes the rotational axes to slip from the gear train (much like the drag setting on a fishing reel). The below left expanded diagram shows a common type of friction clutch consisting of metal plates.  The rotational axis passes through several metal plates (red, tan, and green). The motor rotates only the red plate, because the motor shaft just passes through holes in the tan and green plates. The telescope rotation axis is attached to the left green plate. When the nuts on either side of the green plates are loose, only the red plate rotates with the motor (the motor shaft slips in the holes through the tan and green plates). Tightening the nuts compresses the plates together, and frictional contact causes all plates and the telescope to rotate with the motor. The nuts are tightened to supply enough pressure for the motor to rotate the telescope, but slip free if the telescope is pushed.

I designed a very simple friction clutch that uses friction between nested tubes instead of plates (below right diagram). The worm wheel attaches to a stainless steel tube (red). I used an angle grinder to cut three wide slits along the steel tube, forming flexible metal strips. The rotational axis (white) is wrapped with strong, smooth plastic (grey). The plastic is the same type of plastic vapor barrier that I used around the base of my observatory. The rotational axis rotates inside the plastic lined steel tube attached to the worm wheel. A band clamp supplies the friction. Tightening the band clamp compresses the steel strips and plastic against the rotational axis, giving enough friction for the worm wheel to rotate the telescope. Pushing the telescope causes the rotational axis to slip free of the worm gear.


Below is a photo of the worm wheel and tube friction clutch. The steel tube is a table leg with a flange that attaches inside the worm wheel. The band clamp provides enough friction for the worm wheel to rotate the GEM axis (galvanized pipe), but pushing the telescope causes the axis to slip inside the plastic lined steel tube. This system holds my 10" Newtonian telescope in place, but disengages with only slight pressure on the telescope.

The GEM functions as designed. The friction clutch holds the OTA in place without slippage, but only slight pressure on the OTA disengages the clutch and the OTA moves. The friction clutch movement is very smooth and I am completely satisfied with the new design. 

GEM Drive System

The RA and declination drives are constructed identically. The below left and right photos show the assembled and partially disassembled declination worm mount, respectively. Two recessed ball bearings (not shown) hold the M12 worm. The worm is mounted on a 2 mm steel plate that pivots around a screw (lower right); I later added a layer of plywood on top of the steel plate to reduce flexing. The small spring provides the force that pivots the plate around the screw, pushing the worm against the worm wheel. Adjusting the M6 bolt (lower left) compresses the spring, increasing the force on the metal plate. The small angle iron (upper left side of the plate) is backed with smooth plastic and holds the steel plate firmly against the wood mount.

The worm mount attaches to the GEM declination axis with M12 threaded rods (below left photo). I added a small tan line to the below left photo to make the two wood parts easier to visualize. Adjusting the bolts on the M12 threaded rods moves the worm mount perpendicular to the GEM axis (for alignment with the worm wheel, below right photo). Since the worm gear and friction clutch can slide along the GEM axis, the drive system is easy to align: everything just slides into place and is locked by tightening the M12 bolts and friction clutch band clamp.

The 450 tooth worm wheels and 0.9 deg. step angle motors should give an 80 deg./min. slew speed (full step) with 0.45 and 0.23 arcsec/step tracking at 1/16th and 1/32nd micro stepping, respectively.

Stepper Motor Mounts

The below photos show the declination axis stepper motor mount. The design idea was to make this mount as simple as possible, but still allow fine adjustment of the motors in all 3 axes for precise alignment with the worm. I purchased several 5 mm to 12 mm flexible shaft couplers to attach the stepper motors to the worm shafts. In order to mount the stepper motors, I extended the worm mount by adding a longer (25 cm) steel backing plate. To reduce flexing where the steel plate overhangs the wood board,  I added a layer of plywood on top of the steel plate. The stepper motors slide in grooves on small right angle supports. Because I used 6 mm bolts inside the 8.5 mm wide right angle support groves, there are several mm of back and forth adjustment possible; this makes the motors adjustable in all 3 axes for precise alignment with the worm shaft. The below right photo shows how the stepper motor can be adjusted in all directions: left-right (yellow arrows), up-down (purple arrows), and forward-backward (red arrows). 


 I placed several pieces of Sorbothane between the steel plate and stepper motor mount to dampen vibrations (below photos). 


The below left photo shows the right ascension and declination axes worm mounts (top and bottom, respectively). The below right photo shows the declination axis and worm mount before attaching the worm wheel.


Hand Controller

The GEM hand controller (below photos) has 5 buttons: four buttons for the different slewing directions and one button to adjust the speed. Pressing the speed button will cycle through different slewing speeds. Instructions to make the clear polycarbonate controller and wire it for compatibility with a Celestar 8 mount can be found on the Gem and Celestar 8 Hand Controller Page. 


GEM Computer Control System (This Section Under Construction)

The GEM prototype was developed with the intention of using a PC based control system. I chose a relatively inexpensive ($ 75 USD) commercial stepper control board that controlled two stepper motors and included several digital I/O lines. This was a "plug and play" type product that connected to the PC via a USB line and included a dll file supporting Visual Basic or Visual C programs. This control board only supported full motor stepping, which gave problems with my intended application (a GoTo system with fast slewing and < 0.5 arcsec per step tracking). A less expensive option is to use a microprocessor and a breakout stepper control board (small prototype boards used in electrical projects). The microprocessor sends a 5V pulse to the stepper control board, causing the stepper control board to move the motor one step. The microprocessor also sends signals to specify the motor direction and the micro stepping value (full, half, quarter, eighth, sixteenth, or thirty second).

Below is a photo of my first stepper motor test using a microprocessor and a
breakout stepper control board. The red PCB card is the microprocessor programmer that connects the microprocessor to the PC for programming the microprocessor. I modified the programmer to plug directly into the prototype board. The microprocessor and breakout stepper control board are located on the top and bottom of the prototype board, respectively. I added a red button to speed up the stepping rate. I tested the micro stepping and direction functions by connecting the appropriate stepper control pins directly to the 5V and ground buses. This was a pretty simple test program: the microprocessor sent 5V pulses to the control board, causing the motor to move. Pressing the red button decreased the time between pulses and sped up the motor. This system is less expensive than a commercial "plug and play" type stepper control board: $12 USD for the microprocessor programmer, $8 USD for the microprocessor, and $12 USD for the breakout stepper control board. To control a 2 axis telescope mount I will add an additional stepper control board and an external crystal to improve timing accuracy ($1 USD). This gives a 2 motor micro stepping control system with an additional 20 I/O lines costing approx. $45 USD (not inclusive of minor parts). Depending upon how I structure the final control program, I may add a LCD ($15 USD) and a keypad ($8 USD) for data display/entry.

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