Completed CGE Pro Conversion
Stephen J McIntyre
I have now a completed and operational CGE Pro converted to OnStep.
The setup is:
Stepper motors ROB-10846
Nema 17, 400 step, 1.7A/phase, torque 48N-cm
158 tooth belt drive with 15-60 teeth pulleys
Supports limit sense, home sense and PEC
Controller Connections, Wiring and Packaging
I reused the CGE Pro cables and connectors, including the ribbon cable and 9 pin circuit board connectors. So i fashioned a separate cable harness board. The motor connections use the 4 terminal screw down blocks. For the other sensors i just soldered wires from the MiniPCB2 to the cable harness board.
I stuffed this into a relatively compact case. The ribbon cables are a bit stiff and i could probably have done with a larger enclosure. But the compact case works well when attached to the scope.
The case has aluminum end plates. Cutting the openings for the USB, RJ45, power and switch was a bit tedious, but a file works well on aluminum to tidy up the edges. I printed faceplate covers on an overhead transparency sheet (saved from ages ago when we used that sort of thing).
The finished controller box attaches nicely to the side of the pier where the original RA and Dec ports were. I used a piece of orange cutting board to provide a secure base for the box.
Motors and Mounting
The motors just barely fit where the original motors were and the original motor enclosures and covers work without modification.
The stepper motors require an adaptor plate to connect the 4 NEMA 17 screws to the 3 screw used by the Pitman motors. Also, the motor has to be set back from the motor frame to allow it clear a ledge at the bottom of the frame. Otherwise the short 158 tooth belt was about 1mm too short. The next longer belt pushed the motor passed the enclosure cover.
A larger hole was driller through the motor frame to allow the motor shaft to be located in the centre. (The Pitman motors have the shaft off-centre.) Also, the original 3 screw holes were elongated a bit to allow the tension in the belt to be adjusted.
The picture above is the Dec motor assembly. My mount is an early version which only has a spring tension arrangement on the Dec motor mount. I still assumed, incorrectly as it turned out, that the position of the new stepper motors relative to the motor frame faceplate would be the same for RA and Dec. However, when i did the "final" assembly i found the cover for the Dec enclosure was touching the motor and did not fit. So the motor needed to be lowered. I had to drill out more on the motor frame faceplate hence the odd shape of that opening.
The 60 tooth pulley fits where the original spur gear went without any modifications to the mount. However, the motor mount assembly has to be removed from the mount in order to push the pulley onto the worm gear shaft. The belt can slip on after assembly only because the pulleys i used have a low shoulder.
Sourcing a pulley for the 6mm worm gear shaft was also a challenge. 5mm is common and i originally ordered that and drilled it out to 6mm. That proved to be a bad idea and the pulley wobbled noticeably. Fortunately someone on this group found a supplier in Greece for the 60 tooth pulley with a 6mm shaft diam.
In order to get the Dec motor to fit with the short belt, i had to grind off about a millimeter of the Dec motor "spring plate". I also replaced one of the button screws with a flat screw.
Motor Enclosure Wiring and Connectors
Like the controller board, i fashioned a wiring harness to reuse the existing external cables and connector and the internal ribbon cable and connector. The cable harness attaches to the bottom of the motor enclosure for both RA and Dec.
The simpler motor mount for the RA allows for more space to the side of the motor. So the wiring harness does not have to go under the motor.
The RA motor enclosure is a bit of a tight fit with all the wires stuffed into it.
The Dec motor mount takes up more room than the RA mount, so the wiring harness board has to be located under the motor. After lowering the motor a bit to allow it to clear the cover, there is only amount a 2mm clearance from the top of the pin connectors to the motor and about the same between the motor and the cover. I'll admit to a bit of luck that it all fit.
With the wiring harness located under the motor, there is more room for the ribbon cable.
The mount appears to be working. I have managed just one short visual observing run and the mount appeared to track well. I did have the Dec motor direction backwards during that session so i could not select an object and slew to it. It took me awhile to figure out the problem was the Dec motor direction. With the configuration corrected, a daylight test confirms that it now points in the correct general direction and i can select and slew to an object using a Stellarium.
The next steps are to properly align the mount and test pointing accuracy and tracking. Limit and Home sense work. I will eventually test PEC. I will also do a proper measurement of the tracking accuracy and periodic error using a camera and guiding app.
For now i am just happy that thanks to the OnStep developers and members, my completely non-functional boat anchor of a mount is now usable again!
Thanks for sharing.
Added to the showcase page.