GEM Conversion to SiTech



I acquired a C-14 on a Celestron CI-700 GEM in October 2008.  It had a couple of rinky-dink stepper motors with a gear drive system on the two worm shafts that were for tracking and moderate speed centering.  It had a digital setting circles setup using a 4,000 tick/rev optical encoder on each axis.  Pointing was push-to.

Naturally, I wanted a Sideral Technology go-to system so I ordered two Pittman servo motors and a controller from Mel Bartels and set out to figure out how to mount them to drive the worms.  After removing the stepper motors and gear train, I saw just how easy it would be.

The worms each have a 3/8 diameter shaft so I ordered notched pulleys from Sterling Instruments and a notched belt to match.  Because of some obstructions in the way, the pulleys for the worms could be no larger than 1 3/8 inch in diameter.  I ordered suitable pulleys with 21 teeth and a 3/8-in bore.

For the motor end I ordered the smallest pulleys I could get to get the maximum gear ratio and that turned out to be pulleys with 10 teeth and a 3/16-in bore to fit the motor shaft.  Thus, I  achieved a gear ratio of 2.1:1.  I wanted more and I may have to make an adjustment later.   With the 2.1:1 ratio, I calculate a total ticks/rev on the servo motors of 2.1x180x9.98693181818*2000  = 7,550,120.







To make brackets to hold the motors I used a hacksaw to cut three sides off a 1-in square aluminum tube far enough down to accomodate the motors, drilled a clearance hole for the shaft and  holes for two of the three mounting screws.  The upper image at the left shows the part with a motor attached.  NOTE:  I got worried about the sideways stress on the motor bearings in the original configuration so I modified it as shown at the right.  Once again, I used a 1-in square tube (this time in steel) and installed two bearings from SDP-SI with a shaft to connect the motor.  I also took advantage of the effort to put a 30 tooth wheel on the (extended) worm shaft to get a better gearing ratio (the longer shaft moved the pulley out to where there were no obstructions from the OTA.)

To mount the motors on the telescope I used C-clamps as shown in the next two images to the right.  The bigger pulley is mounted to the worm shaft and it is possible to tension the belt manually before putting the squeeze on with the C-clamps.




Once again, the new configuration for the RA drive on the right versus the old configuration on the left.


The last image is a view of me with the telescope in the Skypod dome.
                                                                                                                                                                                                                                           
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