5.1 Drive System Mechanics

My original plan for the drive was to make a DC motor driven RA and a Tangent arm for Dec adjustment. When planning started for the telescope computer control was only something one dreamed of. We figured that obtaining large precision gears for this size of telescope would have been prohibitively expensive. Early on we recognized that the the polar disk could be driven. I needed a suitable gear box for the initial reduction for the RA drive. I built a tangent arm for the Dec axle. Several years later I was discussing the drive system with Kevin, he suggested that we should use a Celestron CompuStar and make the scope computer controlled. This was in the late 1980's when the CompuStar was the only computer controlled telescope. Kevin provided a CompuStar in kit form. I collected parts and stuffed the boards. We also decided to use belt drive on large disks on both axis.

A 24" diameter disk was found in the scrap yard for the Polar Axis and 22" disk was also found for the Declination Axis. The gearing for 22" telescope was made compatible with the CompuStar's requirements. The ratios for the drive are 720:1 on the RA axis and 432:1 on the Declination axis. There are several advantages in this arrangement. The disks are very inexpensive to make, they provide a great deal of torque and they provide a very smooth drive. The belts and gears for the drive were purchased from Berg.

Right Ascension Drive

A solid disk was machined into a ring that would fit on the polar axle just behind the polar disk. The drive disk is 24" in diameter and is driven with a 1" toothed belt. The gear reduction to the disk consists of a stepper motor that drives a 2:1 reduction via 2 belt driven pulleys. A 4 start worm drives a 96 tooth worm gear for a ratio of 24:1. The worm gear drives the final belt pulley. The last pulley drives the 24" disk via the 1" belt. The final drive ratio is 15:1. The total drive ratio is 720:1. The final pulley is quite small, 1.5", so, two rollers provide a guide to insure that the toothed belt remains in contact with the small pulley. One of these pulleys is adjustable for belt tension, Figure 5.1.1.

Figure 5.1.1. The RA drive assembly is pictured here. The total gear ratio of the belt drive is 720:1.

Declination Drive

The declination drive also uses a belt driven disk. For the declination drive uses a 22" diameter disk and the same type of 1" toothed belt used on the RA drive. The 22" disk was used as some sort of indexing system. The holes around the disk were in the disk when we found it. The gearing on the Dec drive consists of gear box made of a gear drive and a belt drive. The motor drives the belt reduction of 7.2:1. The shaft that the last pulley is on drives a spur gear reduction of 8:1. The final gear drives the toothed belt pulley and Dec disk with a final ration of 7.5:1, Figure 5.1.2.

a  b

Figure 5.1.2. The Dec drive is similar the RA drive in that they use a final belt to drive the axis. The total gear ratio is 432:1.