Ronchi test for flat mirrors

One of the RICHes in the velocity spectrometers of the proposed CKM experiment requires a flat mirror, situated in the high intensity kaon beam. To reduce the interaction background for the experiment, this mirror has to be as thin as possible. First glass prototypes were produced in Mexico. To test the surface quality of these prototypes, we extended the Ronchi method so flat mirrors can also be tested. We present the methods and report on results of our measurements.