IRMA: a prototype infrared Michelson stellar interferometer

We describe a prototype near-infrared Michelson stellar interferometer used to estimate stellar diamater by measuring visibility amplitudes. In particular, details are given for the performance of the device, including the stability, limiting sensitivity and sources of errors in the visibility amplitudes. We compare the results for stars measured in common with the French interferometer at CERGA. An improved diameter for the red supergiant alpha Her is also presented.