Large German Equatorial Mount(GEM)

Large German Equatorial Mount (GEM)

GEM Project and Current Status

This project is construction of a large GoTo GEM for my 10" ultralight OTA. I plan to use this mount for imaging, so it will include full GoTo, tracking, and autoguiding functions (all original programming). To eliminate dependency on external manufacturers, I am not using any commercial telescope drive equipment; the only exceptions are two small 60:1 reducer gears (from an old Meade 492 motor kit). I am also writing all computer code for the GoTo, tracking, and autoguider systems.

My 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.

The current status is that the GEM and worm drive systems are complete (below photos). The stepper motors and stepper control board have been installed, it has been verified that my programming language can control the stepper motors, and programming is in progress. The program code is based on my digital setting circle (DSC) program, so most of the code is already written and only requires minor modifications to communicate with the stepper motor control board. This webpage is being continually updated during GEM construction (last update 4 May, 2012).

GEM Design

The prototype design is shown in the below left diagram. I will connect together 4 sections of pressure treated posts for the pier (dark brown). The right ascension bearings (red) sit on a wood box (light brown). The wood box fits around the top of the pier and pivots on a M12 threaded rod (orange circle). There is a M12 threaded rod and bolts (blue) to adjust the right ascension axis elevation. The right ascension, declination, and counterweight axes are galvanized pipe (grey), joined together with a galvanized "T" fitting (purple). The center of mass (telescope and counter weight) is located at the "T" fitting. To improve stability, the mount will be designed so that the "T" fitting is directly over the central pier when adjusted for my observation latitude. The two declination flange bearings (green) are on either side of a wood arc that attaches inside the telescope truss tube support ring. The below right diagram shows the plan for adding full GoTo and autoguiding functions (large gears and stepper motors) to the right ascension and declination axes (yellow and orange, respectively). The exact motor placement will be determined after the GEM is assembled; the below right diagram shows only a "best guess" as to where the motors may attach.

The below diagram shows the prototype pier design. The GEM must retrofit onto the concrete pad (black rectangle) in my observatory. I plan to mould a concrete pier (light grey) around the wood support post (brown) as I did for my C8 pier (see the C8 Telescope Pier Section). The concrete GEM base will slide over the M12 mounting bolts on the observatory concrete pad. The concrete GEM base will contain arced slits instead of bolt holes; this will allow the entire GEM to be rotated (for polar alignment) and then locked into place with large bolts (red) after alignment.

Materials

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. The 1" diameter galvanized pipe (declination axis) will be inserted inside scraps of stainless steel pipe (3.0 cm O.D. x 2.5 mm wall thickness) to give a snug fit inside the larger bore declination bearings. A first test fit of the bearings is shown below.

The RA and declination prototype bearings are shown in the below left and right photos, respectively. A pine arc is fitted between the two declination bearings (below right photo); this arc functions to distribute the load evenly along the telescope central truss support.

Below are two photos showing a first test fit of the declination bearings into the optical tube assembly.

Below left is a photo of the first test fit of the OTA truss tube support onto the GEM bearings. Note that the mount is only a temporary pedestal made from a steel table leg on top of an old chair base. This temporary mount is only for fabrication purposes and was replaced by a more solid, wood pedestal. The truss tube support is bolted onto the declination bearings with M10 threaded rod and lock washers. The M6 threaded rod with the clear polycarbonate knob (located directly above the declination bearing) is a locking mechanism that presses a nylon pad against the declination axis; this rod adjusts the resistance to moving the telescope in declination and can lock the OTA into any desired position. The below right photo shows the first test fit of the 10" OTA onto the GEM mount. I had to fabricate a heavier duty temporary mount (below right photo) than the table leg mount. I also replaced the M10 threaded rods on the truss tube support with larger M12 threaded rods and increased the distance between the pipe "T" fitting and the OTA. The counter weight is just what I had on hand in my workshop (two 5L jugs of water and antifreeze) and is only temporary. I eventually poured a large cylindrical cement counterweight.

In the prototype, I replaced the short M12 threaded rods on the bottom RA axis bearing with longer 40 cm M12 threaded rods (below left photo). These two rods extended into the pedestal and allowed fine adjustment of the RA axis in a very similar manner to the fine adjustment systems found on SCT wedges. On the final pedestal mount, these two rods were replaced with a single M12 rod that fit inside a slotted steel bracket (below right photo). There is a M12 nut and washer on each side of the slotted steel bracket; turning the nuts allows fine adjustment (elevation/depression) of the RA axis. The steel bracket is attached to the GEM mount with screws and a M8 lag bolt. The M12 threaded rod extends through the pedestal and is secured to the back side with a M12 bolt and washer.

I fabricated a 10 kg concrete counterweight from an old paint bucket and a scrap of stainless steel chair leg. I placed the stainless steel chair leg in the center of the paint bucket and siliconed the chair leg flange to the bottom of the bucket (below left photo). I coated the inner bucket surface with raps oil to function as a mould release agent and filled the bucket with concrete. The below right photo shows the GEM mount with the finished counterweight. The chair leg inner diameter exactly fits the counterweight shaft outer diameter and is held in place with band clamps. I plan on fabricating a small, second counterweight. The 10 kg cement counterweight position will be fixed, but the small counterweight position will be adjustable.

Gear Drive System: The Following Sections Are Under Construction

The following gear drive sections are under construction. I will be updating this web page during drive system fabrication. Once the drive system is complete, I will probably rewrite this section into a more structured webpage.

Below is a photo of the first test at hobbing (cutting) an aluminum worm wheel with a homemade hobbing machine. Since I had a thick gear blank, I decided to save on materials by hobbing two test cuttings into a single gear blank; this explains why the gear is cut so close to the top edge. The next test hobbing will cut into the unused material along the bottom edge. I tested this first worm wheel with a piece of threaded rod in my power drill and the worm wheel turned very smoothly, without slippage or apparent problems. The next step is to try polishing the worm wheel against the matching thread (to be used for the worm).

I plan to make several more test gears to improve the hobbing equipment and the technique. After I am satisfied with the final worm wheels, I will post details including how to build the hobbing equipment and step-by-step instructions.

After some redesign to the hobbing equipment, I produced an 800 tooth RA axis worm wheel. The below photos show a section of the RA axis worm wheel and a close-up on the hobbing (left and right, respectively). This gear still needs to be polished to remove cutting debris and match it to the worm. I improved the indexing procedure, resulting in no visible indexing lines after cutting to depth (unlike the test gear in the above photo).

The final 215 tooth RA axis worm wheel:

Drive System: Worm Wheels

The below left photo shows the RA axis drive on the GEM: a 215 tooth worm wheel and the worm. 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 reduction gear. I fabricated a simple hand crank (wood disk at far left) so I could turn the worm and everything seems to function very well. It is extremely easy to rotate the RA axis with both the 10" OTA and counterweight installed. I still need to mount a small 60:1 reducer gear between the worm and stepper motor, but I can't do this until I have purchased my stepper motors so that I can align the stepper motor axis to the 60:1 reducer gear axis. The below center photo shows the declination axis worm wheel; this 215 tooth worm wheel is attached to the declination bearing and rotates with the telescope. I installed a spacer (green wood disk and steel plate) because the worm wheel diameter was too small to mount directly onto the declination bearing bolts. The below right photo shows a first test fit of both worm wheels. I still need to do a more precise alignment of the worm wheel to the RA axis. Even with only a "rough" alignment of the drive components, I am very happy with the results.

Declination Drive System

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 worm rotates inside roller bearings (12 mm diameter bore) recessed into the wood disks and held together with M12 lock bolts. The below photos show the worm assembly (left) and the end bearings (right).

I attached a pine board onto the declination axis to form a ledge to mount the declination worm, 60:1 reduction gear, and stepper motor (below photo). The original plan was to create only a small ledge, but I later decided to extend this board along the counterweight shaft to strengthen the pipe "T" fitting connection and dampen any oscillations due to the counterweight. The board is attached to the pipe with standard pipe mounting brackets and 40 mm long M6 bolts.

A design requirement for both drive axes was adjustability to simplify changing worm wheels (to increase/decrease the gear ratio as needed). The RA axis can easily accept a smaller worm wheel without significant modifications. Adding a larger RA worm wheel will require relocating the worm wheel to the end of the RA axis (outside the pillow block bearings) and remounting the worm (probably only about an hours work). For the declination axis, I decided to make the worm fully adjustable (can be raised/lowered and moved toward/away from the worm wheel); I did this by mounting the worm assembly to slide in steel channels. M8 bolts secure the worm assembly onto two scraps of oak dowel that slide in channels on small angle brackets (below left photo). The angle brackets mount onto the pine board attached to the declination axis. There are also channels cut in the angle brackets on the side contacting the pine board (below right photo). These channels allow the entire worm assembly to be moved vertically or horizontally for alignment with the worm wheel. After alignment, the bolts are tightened to lock the worm assembly into place. This gives a high degree of adjustability and minimizes rebuild time if I ever need to modify the drive system to accommodate a larger diameter worm wheel, etc. The below right photo shows the declination worm contacting the declination worm wheel.

60:1 Reduction Gear

The 60:1 gear reducer comes from an old Meade 492 motor kit and is the only commercial astronomical drive component used in this project. The 60:1 reduction gear is placed between the stepper motor and worm gears to further reduce the 215:1 worm drive reduction to 13200:1. With a 200 step per revolution stepper motor, the 13200:1 gear reduction should give about 0.5 arcsec per step. The design of this system gives a lot of options if I ever decide that I need less than 0.5 arcsec per step and possible options are:

Replace the 60:1 reduction gear with a larger homemade worm wheel system. This is where having the capability to hob my own worm wheels gives incredible design flexability.
Replace the 215 tooth RA worm wheel with a larger worm wheel.
Purchase an alternative drive board supporting microstepping.

The below left photo shows the first test fit of the 60:1 reducer gear onto the RA drive worm. The white spur gear will eventually be removed and replaced with a shaft coupler to connect the brass worm to the stepper motor. The below right photo shows a first test fit of the RA axis stepper motor to the worm gears. The 60:1 gear reducer was attached to the frame supporting the RA bearings with a plywood spacer (not shown). I fabricated a simple shaft connector from 8 mm diameter tube (1 mm wall thickness).

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 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).

Completed GEM

All parts were disassembled, sanded, given several coats of exterior oil paint, and reassembled. After reassembly, I precisely aligned the worm wheels on the drive axes and gave them a final light hobbing while mounted to the GEM; this ensured all teeth were perfectly oriented about the drive axes (cut to identical depth). I removed the worms, inserted the cutting hob into the worm mounts, attached a power drill, and actually hobbed the gears while attached to the GEM. A small amount of aluminum was removed during the first few passes. After several passes with no aluminum cuttings, I discontinued the hobbing and reinserted the worms. The last job was to polish the worms against the worm wheels. I mixed my own polishing compound: a paste of very fine brick dust in high quality acid free oil. This homemade polishing compound worked exceptionally well.

The GEM has very smooth motion about both rotational axes. The drive system is very easy to turn and the worm wheels are self locking (require no holding torque); the RA axis worm drive can even hold the 10" OTA stationary without the counter weight installed! The completed GEM is shown in the below photo. The only remaining jobs are to install the stepper motors and 60:1 gear reducers, and to pour a concrete base around the pedestal. Modification of my digital setting circle (DSC) program to control the GEM GoTo, tracking, and autoguider functions is currently in progress.

Hand Controller

The GEM hand controller has 5 buttons: four buttons for the different slewing directions and one button to adjust the speed. Pressing the speed button will cycle through four different slewing speeds and LEDs will visually show which speed is selected. I made two different prototypes: a clear polycarbonate hand controller and a modified Meade 492 motor controller. Instructions to make the clear polycarbonate controller (below left photo) can be found on the Gem and Celestar 8 Hand Controller Page. To modify the Meade 492 motor controller: I removed the printed circuit board, filled in the membrane button holes with polycarbonate plastic, and installed 5 normally open momentary pushbutton switches. I removed the small original LEDs because they weren't rated for more than a few volts. I drilled out the LED holes and installed larger diffuse, red LEDs with built in resistors; these LEDs are rated for up to 12 volts, which is the output voltage from my control board. The below center and right photos show a front and inside view of the modified Meade hand controller. I still need to connect the LEDs, install a cord and then fine sand and finish spray paint the hand controller.