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 ττ between a flat surface and the azimuthal rotation axis of the sample holder is described. When the incidence angle θθ and readout of the azimuthal angle ϕϕ 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 a small residual oscillation of the scattering plane remains.