Active correction of the tilt angle of the surface plane with respect to the rotation axis during azimuthal scan

A procedure to measure the residual tilt angle $\tau$ between a flat surface and the azimuthal rotation axis of the sample holder is described. When the incidence angle $\theta$ and readout of the azimuthal angle $\phi$ are controlled by motors, an active compensation mechanism can be implemented to reduce the effect of the tilt angle during azimuthal motion. After this correction, the effective angle of incidence is kept fixed, and only the small residual oscillation of the scattering plane remains.