Evaluation of precision tilt sensors for measuring telescope position

This report describes a method for using precision tilt-sensors to measure the position of an equatorial telescope relative to the local horizontal plane. Unlike conventional systems which measure the telescope position using position encoders coupled to the telescope axes, this method avoids many sources of non-repeatable error, such as hysteresis in the telescope structure due to inelastic flexure of the fork or yoke, or random slippage in the couplings between the position encoders and the telescope axes. In this respect, it shares many of the advantages of optical gyros, but achieves these at much lower cost. We present a design for a compact and relatively inexpensive dual-axis tilt-table whose frame is rigidly attached to the telescope's primary mirror cell. The table contains two precision tilt-sensors, aligned orthogonally with the tilt axes of the table. The sensors are used as nulling devices to close a servo loop which keeps the table level at all times. This provides a precise and stable reference against which the telescope position is measured. A high resolution incremental encoder is directly coupled to each tilt-table axis and measures the angle by which that axis rotates to keep the table level. A mathematical transform converts these two encoder readings into local hour angle and declination. Preliminary tests of the tilt sensors and of a single-axis prototype tilt-table are reported, and future plans described. The use of tilt-tables for measuring the positions of non-equatorial telescopes is also briefly examined.