Tests of a precision tiltmeter system for measuring telescope position

We have previously described a system that derives the pointing coordinates of an equatorial telescope by measuring the angular position of a dual-axis tilt-table whose frame is rigidly attached to the telescope's primary mirror cell.(1) In that system, two precision tilt-sensors aligned orthogonally and mounted in the plane of the table are used as nulling devices to close an active servo loop which holds the table level as the telescope moves. Rotary encoders measure the angle by which each tilt-table axis rotates, and a mathematical transform converts those encoder readings into telescope hour angle and declination. Recent work has indicated the feasibility of several simplifications to that system. First, by use of suitable low friction bearings on the tilt-table axes, along with non-contacting encoders, the active servo loop is no longer needed to level the tilt-table. Rather, a simple suspended weight keeps the platform almost level, with the residual small tilt error measured by the precision tilt sensors. Second, by suitable orientation of the weight and the tilt sensors relative to the telescope polar axis, the system can measure telescope hour angle and declination directly, eliminating the need for the complex mathematical transform. Experimental results using these ideas are presented.