Large GoTo German Equatorial Mount (GEM)

GEM Project

This project is construction of a large computer controlled German Equatorial Mount (GEM) for my 10" ultralight Newtonian telescope. I plan to use this mount for imaging, so it will include GoTo, tracking and autoguiding functions. To eliminate dependency on external manufacturers, I am avoiding using commercial telescope drive equipment; the only exception is a Shoestring Astronomy GPUSB guide port interface adaptor to allow a commercial autoguider program to control the GEM. I am writing original computer code to control GoTo functions.

GoTo alt-az mount was based on commercial telescope drive components, which have recently stopped functioning, and are no longer available from the manufacturer. Rather than basing this project on commercial equipment, which can require costly repair or be discontinued without manufacturer support, I am constructing the GEM drive system from ordinary components; this gives me greater control over all design aspects, reduces costs, and allows easy modification or repair.  

GEM Design

The first GEM prototype was a full GoTo mount based on a stepper motor control board that did not support microstepping, giving extremely slow slew speeds (approx. 25-30 min to slew 180 degrees). I decided that the slow slew speeds were unpractical for a GoTo system and decided to redesign the GEM so it could be pushed to the desired location ("push to") and then fine slewing and tracking functions automatically engage; this type of system requires some sort of clutch to disengage the drive motors during repositioning. Installing a friction clutch on the prototype GEM right ascension axis was easy, but I could not find a way to retrofit a clutch onto the existing declination axis; it was necessary to redesign the declination axis. I had recently purchased a GPUSB guide port interface adaptor for use with my Celestar 8 mount, and decided to integrate the GPUSB into the GEM tracking system; this allows commercial autoguiding software to control the GEM.

November 2014 update: I have started using microprocessors for several different projects (building a LED image cube and for my prototype direct drive motor control system).  Because the microprocessor can directly interface with breakout stepper control boards supporting microstepping, I now have a solution to increase the slew speed and build a full GoTo system. I am currently building a "push to" system with the necessary slew speed and microstepping (tracking) so that only a software modification is needed for full GoTo control. 
The friction clutch that I designed for the current "push to" system will be retained, since it's a nice option for manual positioning. I will start by programming a simple push to system with tracking, and autoguiding functions. The next step will be to modify the control program for full GoTo operation. I have added a few details and a picture of the microprocessor and stepper driver in the final webpage section (GEM Computer Control System). I have purchased smaller step angle motors and fabricated several 450 tooth worm wheels. The 450 tooth worm wheels and smaller step angle motors give an 80 deg./min. slew speed with 0.45 and 0.23 arcsec/step  at 1/16th and 1/32nd microstepping, respectively. 


The GEM uses 1" diameter galvanized pipe.  I wanted to use larger diameter pipe, but had to settle on 1" diameter pipe to keep the total costs within my project budget. The right ascension bearings are two 1-1/8" pillow block bearings (purchased for $13 each). The declination axis bearings are two 1-3/16" two bolt flanges (purchased on sale for $5.50 each); I used larger bore flanges for the declination axis because I found several on sale at a good price. I wrapped a thin strip of stainless steel around the 1" diameter galvanized pipe to give a snug fit inside the bearings. Note that declination axis flange bearings were purchased instead of pillow block bearings, because the prototype design incorporated the flange bearings inside the GEM central support ring. I reused the flange bearings on the final declination axis redesign, however pillow block bearings would have simplified the construction. A first test fit of the bearings is shown below.


Prototype GEM 

This section gives a very brief description of the first prototype GEM, shown in the below photos. The right ascension bearings (pillow block) sit on a wood box that fits around the top of the pier and pivots on a M12 threaded rod. There is a M12 threaded rod and bolts to adjust the right ascension axis elevation (metal plate under the bottom RA bearing, see the Final GEM Design section for a photo). The two declination bearings (flange) are recessed into a pine arc, which distributes the load evenly along the telescope central truss support. In the prototype design, the declination flange bearings are recessed inside the OTA central support ring.


I built a gear hobbing machine out of scrap 2x4 pine (less than $20 total cost) and used an ordinary power drill to hob two 215 tooth aluminum worm wheels (below photos). I also produced several 800 tooth test worm wheels (not shown). Additional gear hobbing details, including a description of the homemade gear hobbing machine, are posted on my Large Aluminum Worm Wheels Webpage.


The below left photo shows the RA axis drive on the prototype GEM: a 215 tooth worm wheel and 60:1 reduction gear (13200:1 gear reduction with a 200 step per revolution stepper motor, giving about 0.5 arcsec per step). The worm is supported by ball bearings inside the wood disks on either side of the frame. I installed the worm bearings inside the wood disks because they could be moved to precisely align the worm, and then locked (screwed) into place. The worm wheel is located near the rear RA bearing so that there is plenty of wood frame for mounting the stepper motor and 60:1 reduction gear. The below center and right photos show the declination worm assembly and attachment. The declination axis worm assembly is a section of M12 threaded rod inserted into a 30 cm O.D. x 2 cm wall thickness aluminum rod (a scrap of the rod used for the ultra light telescope truss tubes). The 215 tooth declination worm wheel is attached to the declination bearing mounting holes, and rotates the bearing flanges and OTA about the declination axis. The worm mount is attached to a wood board on the declination axis with adjustable angle brackets. The worm rotates inside roller bearings (12 mm diameter bore) recessed into the wood disks and held together with M12 lock bolts. The worm is easily aligned to the worm wheel by sliding the worm mount in the adjustable bracket slits, then locking it into place. 

The stepper motor mounts are constructed from the same slotted brackets as the declination worm mount. The slotted brackets allow the stepper motors to be adjusted in all dimensions for alignment with the 60:1 gear reducers. The stepper motors are attached with M3 machine bolts and homemade aluminum washers cut from scrap material. Because the M3 bolts are much smaller than the slots, the stepper motors can also be adjusted sideways (as well as up and down). The slotted brackets are screwed into the GEM frame with M8 wood bolts, and allow the motor mount to also be adjusted forward and backward. The prototype RA and Dec stepper motor mounts are shown in the below left and right photos, respectively. I still have to isolate the stepper motor from the GEM frame with a vibration dampening material (Sorbothane sheet).


Friction Clutch

The final 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 RA worm wheel and tube friction clutch. The steel tube is a table leg with a flange that attaches to the worm wheel. The band clamp provides enough friction for the worm wheel to rotate the RA axis (galvanized pipe), but pushing the telescope causes the RA 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.

Current GEM Design (This Section Under Construction)

This section describes the current GEM design. The RA axis redesign involved only adding a friction clutch, otherwise it is as described in the Prototype GEM Section. The declination axis required a complete redesign to add a friction clutch, but the final result was just a duplication of the RA axis design. This is much simpler and more accessible than the prototype design, but unfortunately it is more bulky and less attractive. The below left photo shows the final declination axis design. A 2x4 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 (above left photo) 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 below right photo shows the OTA attachment system, which is similar to the prototype system. The declination axis extends through a 2x4 and attaches to a pipe flange. A M8 rod runs vertically through the 2x4 and declination axis to prevent the galvanized pipe from rotating in the pipe flange (not shown). I added two sheets of Sorbothane vibration dampening material to either side of the OTA attachment 2x4. I will also place Sorbothane pads under the M12 washers on the rods attaching the OTA to the mount, and on both stepper motor mounts.


The current GEM is shown below (the counterweights are only temporary). The next jobs are: replace both 215 tooth worm wheels with 450 tooth worm wheels, move the RA axis worm wheel outside the wood box around the pier, fabricate worm mounts, and install the stepper motors. The 450 tooth worm wheels and smaller step angle motors give a 80 deg./min. slew speed with 0.45 and 0.23 arcsec/step tracking at 1/16th and 1/32nd microstepping, respectively. 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.


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 prototypes were 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 microstepping value (full, half, quarter, eighth, sixteenth, or thirty second). As mentioned in the GEM Design section, I may abandon the PC based control system for a microprocessor based system supporting microstepping. If I construct a "push to" system with the appropriate slew speed and microstepping (tracking), then only a software modification is needed for full GoTo control.

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 microstepping 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 microstepping 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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