An optically referenced inclinometer with sub-microradian repeatability

A pendulum inclinometer has been built and tested, suitable for measuring the deflection of the gravitational vertical to an accuracy of 1 μrad. The design is based on measuring the differential capacitance between a pendulum bob and two closely spaced parallel electrodes. Two independent pendulums are oriented orthogonally to measure both coordinates. A collimated optical beam is stably attached to the inclinometer pair and provides a means for making stellar measurements relative to the gravitational vertical. For optimum performance, the inclinometer is operated at its null position by supporting the entire assembly on a rigid tilting plate. Good repeatability is a key feature of this inclinometer.