A Vacuum-Compatible Cylindrical Inertial Rotation Sensor with Picoradian Sensitivity

We describe an inertial rotation sensor with a 30-cm cylindrical proof-mass suspended from a pair of 14-$\mu$m thick BeCu flexures. The angle between the proof-mass and support structure is measured with a pair of homodyne interferometers which achieve a noise level of $\sim 5\text{ prad}\mathrm{/}\sqrt{\text{Hz}}$. The sensor is entirely made of vacuum compatible materials and the center of mass can be adjusted remotely.