Large 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 slewing, tracking and
autoguiding functions. To
eliminate dependency on external manufacturers, I
am
avoiding using commercial telescope drive
equipment; the only exceptions are two
small 60:1 reduction
gears
(from an old Meade
492
motor kit)
and a Shoestring
Astronomy GPUSB guide port
interface adaptor; this will allow a
commercial autoguider program to control the GEM. I
am writing original computer code to control all
functions.
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.
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.
GEM Design
The GEM prototype was a full GoTo mount with original programming for GoTo, tracking, and autoguiding functions. I completed the prototype and began functional testing. Unfortunately my stepper motor control board does 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 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. Since I have most of the computer code for the stand alone autoguider and GoTo programs completed, I may still come back to these projects in the future.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. 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 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.
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.
Final GEM Design (This Section Under Construction)
This
section describes the final
GEM design. The final RA axis redesign involved only adding a
friction
clutch, otherwise it is as described in the Prototype GEM Section
(below left figure). The below right figure shows the M12
threaded rod and
bolts to fine adjust
the right ascension axis elevation.
The
declination axis required a
complete redesign to add a friction clutch. The final design is 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. I reused the prototype GEM declination worm mount,
but strengthened the worm mount attachment by replacing the steel
brackets with wood blocks (additional details are given in the
next paragraph). 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 prototype declination worm mount was attached with adjustable steel brackets. I replaced the steel brackets with a wood assembly because the OTA produced enough torque to flex the metal brackets. The final declination worm mount is shown below. The worm can be adjusted up-down because the vertical hardwood supports connect to the pine base via 2 cm long slits instead of bolt holes. The Autostar worm wheel and a temporary adjustment wheel are attached to both ends of the worm (the adjustment wheel will be removed once the stepper motors are installed).
The final GEM is shown below. The counterweights are only temporary until I shorten the counterweight shaft and then pour a new (lighter) cement counterweight. The next job is to install the stepper motors and then disassemble the declination axis and OTA mounting board for final sanding and painting. 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.
The prototype declination worm mount was attached with adjustable steel brackets. I replaced the steel brackets with a wood assembly because the OTA produced enough torque to flex the metal brackets. The final declination worm mount is shown below. The worm can be adjusted up-down because the vertical hardwood supports connect to the pine base via 2 cm long slits instead of bolt holes. The Autostar worm wheel and a temporary adjustment wheel are attached to both ends of the worm (the adjustment wheel will be removed once the stepper motors are installed).
The below photos show the
declination worm wheel with the worm mount installed. The declination
worm mount assembly attaches to the GEM with four 8 mm galvanized
bolts, giving a very sturdy assembly. The hardwood worm supports
(vertical supports) can be adjusted up-down for alignment with the worm
wheel. Because the worm wheel and friction clutch can slide along
the declination axis, it wasn't necessary to build a back-forth
adjustment into the declination worm assembly; the friction clutch
is just loosened, slid to align the worm with the worm wheel, and
tightened.
The final GEM is shown below. The counterweights are only temporary until I shorten the counterweight shaft and then pour a new (lighter) cement counterweight. The next job is to install the stepper motors and then disassemble the declination axis and OTA mounting board for final sanding and painting. 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 will be PC controlled and include slewing, tracking, and
autoguiding
functions. Because it will be a PC based system, it's easy to add lots
of different user input options and give flexibility to the
control
system. I will most likely have some screen buttons allowing user
specification of 3 or 4 slew speeds, then pressing the hand controller
speed button will cycle through the possible selections.
There
will most likely be a screen button to fine tune the tracking
speed
(increase/decrease the RA axis stepper motor speed).
This project stalled for several months because I couldn't get my programming language to read the digital inputs (the digital inputs were very unstable). I eventually purchased a regulated 5 volt mother board power supply and added several pull down resistors to stabilize the low (0 volt) input values; this solved the problem and the PC now registers the digital inputs. I have a prototype board completed that sends digital inputs into my PC. I have test programs completed that can register the digital inputs and control the stepper motors, now I just have to put everything together into a single control program. Autoguiding will not require any special program code to interpret and use the GPUSB output. I will add a few relays in parallel with the hand controller push buttons to input the GPUSB commands through the hand controller connection to the PC. I still need to mount the stepper motors onto the GEM, pour a new cement counterweight, and then do the final sanding and painting.
Feb. 2013 Status:
I rebuilt the declination worm mount to replace the steel brackets with wood and have begun writing the PC control system program code.
Current Status
Jan. 2013 Status:This project stalled for several months because I couldn't get my programming language to read the digital inputs (the digital inputs were very unstable). I eventually purchased a regulated 5 volt mother board power supply and added several pull down resistors to stabilize the low (0 volt) input values; this solved the problem and the PC now registers the digital inputs. I have a prototype board completed that sends digital inputs into my PC. I have test programs completed that can register the digital inputs and control the stepper motors, now I just have to put everything together into a single control program. Autoguiding will not require any special program code to interpret and use the GPUSB output. I will add a few relays in parallel with the hand controller push buttons to input the GPUSB commands through the hand controller connection to the PC. I still need to mount the stepper motors onto the GEM, pour a new cement counterweight, and then do the final sanding and painting.
Feb. 2013 Status:
I rebuilt the declination worm mount to replace the steel brackets with wood and have begun writing the PC control system program code.
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