An experimental set-up to measure the long-term stability of large-mirror supports

20-2000 Precision of the Cherenkov ring pattern recognition and reconstruction relies, apart from other factors, on highly precise and stable positioning of the spherical mirrors, which focus the Cherenkov light cones produced by the particle to form ring-images on the focal plane. Prototypes of adjustable mirror mountings that hold the mirrors on the support structure have to be tested for their long-term angular stability, in order to ensure that the mirrors will not change their precisely adjusted positions inside the detector vessel. A method, together with a set-up, for such a measurement is presented. As an example, measurement results from a mirror support prototype developed for the RICH2 detector in LHCb are described and discussed.