KAIT Telescope Drive

Motor Controller

We use the the Oregon Microsystems model PC58 8 axis motor controller board, to control most of the motors at the observatory. It has both stepper and dc servo axes and 12 general purpose digital I/O lines.

Hour Angle

The telescope is driven using a direct friction drive with a 48:1 reduction. H.A. drive

The hour angle drive uses a d.c. servo fed back from Renishaw tape encoder system. H.A. encoder This drive system provides smooth tracking and fairly accurate small angle offsets but tends to slip when moved over larger angles. Hall effect sensors (Sony corp) are used to generate a very accurate index pulse at the ha=dec=0 degree position.


Dec drive

A d.c. servo similar to the HA axis is used to control the declination axis thru a 36:1 direct drive.

Computer control

All of the data acquisition at the observatory is done with commercial cards placed in IBM-PC clone computers. Each computer runs Linux and communicates with the master computer running UNIX via an ethernet connection.


The Renishaw tape encoders provide the hour angle and declination readout. The Sony Magnescale provide the index positions. Time is provided from Network Time Protocol. The corrections to apparent position include: Pointing corrections include The mean uncorrected errors are on the order of 10 arc-seconds; limited mainly by engineering time.


The dome azimuth is controlled by an 1/2 horse power 3 phase motor controlled by a ABB electronic drive. The position is encoded why a series of 168 stripes on the dome that are read by a retro reflector; this gives 672 quadrature pulses per revolution. The absolute position is denoted by a home marker at azimuth 132 degrees.


The telescope secondary is mounted on three stepper driven micrometer screws denoted T, U, and V. The U actuator is to the West, the T to the North East and the V to the South East. secondary


The slit is opened by a 12 volt d.c. motor, powered by a battery that rides atop the dome. A solar cell charges the battery since we do not have slip rings to send a.c. to the dome slit. Two large coils of wire are placed around the dome, one that rotates with the dome and the other on the base. To open the dome a 100kHz tone is fed through the lower coil. A phase lock loop in the upper loop detects the tone and causes the circuit to open the slit.

In the event of a power failure, the tone will go away and the slit will close. A interval timer is also used as a safety feature. The computer must tell the dome to stay open every 3 minutes (or so) otherwise the timer will fail and the slit will close.

Last Updated August 31, 2000 by
Email: rtreffers@astro.berkeley.edu