C11 History

The telescope was purchased in 1981 from Celestron International at a cost of $2,850.00. This instrument was one of the first telescopes to roll off Celestron's production line after its release to the general public. These telescopes were orange in color until 1986 when Celestron changed the color of their telescope line to black. The telescope remained in stock condition until new drive electronics were added, and was primarily used for deep sky photography and photometry.

In 1989, the telescope was shipped to Epoch Instruments in California where the mount and optical tube assembly were aligned using a laser collimator. This collimation of the mount insured that the optical axis of the tube assembly was at perfect right angles to the R.A. and DEC. shafts. After the collimation of the mount was completed, a main gear and worm gear were installed on the right fork arm as seen in the picture. These gears allowed the DEC. axis to be moved through 360 degrees. A counter weight was placed on the left fork arm for balance. When the telescope returned it was mated with two DC servo motors and two 512 cycle per rev. optical encoders. These motors were used to both track and slew the telescope. The next step was to design and build the hardware which would replace the original telescope's drive system. Two constant velocity circuits were built that could drive the two motors at speeds between 15 arc seconds per second and 3.5 degrees per second. The system used closed-looped technology and had a pointing resolution of 10 arc seconds. The computer power of the telescope was an IBM PC/XT running a 4.77MHz. An ISA bus 12 bit Analog to Digital / Digital to Analog card was designed along with two 16 bit counters which interfaced the telescope drive to the computer. Many of the CCD images shown in the CCD Photo Gallery were taken using this drive system.

In July 1996, the DC servo motor drive system was replaced with a stepper motor system purchased from Soft-Tec Systems at a cost of $1250.00. This new drive system allowed a greater pointing resolution of 0.50139 arc seconds and slew rates up to 3.5 degrees per second. The drive system uses two small Vexta 0.9 degree/ half step (400 step /evolution) 2-phase stepper motors. The picture at the right shows the stepper motor mounted on the DEC arm of the mount. Each stepper motor uses an 18:1 gearhead. The output of the gearhead drives a 1.6:1 pulley that connects the stepper motor to the worm gear. The worm and main gears have a 359:1 ratio. Both the R.A. and DEC. motors use the same the gear reduction ratios. On the opposite end of the stepper motor gearhead is a yellow damper wheel. This damper wheel is used to smooth out the shaft speed of the motor during the pause between steps. The computer used for this drive system is an AST 166MHz Pentium notebook computer. The computer drives the stepper motors using its line printer port. At the telescopes maximum slew rate of 3.5 degrees per second, the computer is sending 25,000 pulses per second to the drive electronics. A hand controller connected to the computers serial port allows the operator to control both the telescope's movement and computer menu options.

If you would like more information regarding the stepper motor system above please contact Soft-Tec Systems at (817) 292-1940 or visit their web site at  www.robotics.com/skyprobe/