An ultra-stable, 3-axis goniometer for precise angular rotations for optical metrology of X-ray mirrors

Deterministic polishing of X-ray mirror for synchrotron light and XFEL sources requires metrology instruments capable of accurately measuring optics with slope errors < 50 nrad RMS and height errors < 1 nm peak-to-valley. To improve the performance of the Diamond-VeNOM slope profiler \*, we have developed an ultra-stable, 3-axis rotation stage \** to orient the mirror under test. The goniometer employs a spherical air-bearing, actuated by three piezo-walkers via flexure struts. This combination provides high stiffness, zero friction, and minimal parasitic errors. Linear interferometers provide positional feedback to the piezo actuators for fast, closed-loop control of 3D angles. Temperature controllers and forced air stabilisation minimise thermal drifts. FEA and dynamic model-ling optimised all components via mechatronic principles. The goniometer can accommodate X-ray mirrors up to 500 mm long and 10 kg in mass. It has an angular range of ± 10 mrad in 3 orthogonal directions, a minimal incremental step of < 100 nrad, and thermal drift of ~ 100 nrad over 30 minutes. Shielding of heat sources reduces air turbulence for probing autocollimators or laser beams. The system is controllable via EPICS to enable dynamical synchronisation with other motion stages and detectors