The automatic alignment system of GEO 600

Abstract This paper gives an overview of the automatic mirror alignment system of the modecleaner and main interferometer of the GEO 600 gravitational wave detector. In order to achieve the required sensitivity of the detector, the eigenmodes of all optical cavities have to be aligned with respect to the incoming beams (or vice versa) and kept aligned for long measuring periods. Moreover the beam spots have to be centred on the mirrors to minimize coupling of residual angular mirror motion into changes of the optical path length. An overview of the principles and setup for the automatic alignment is given, and first results of the modecleaner and 1200 m cavity alignment system are presented, including the error-point spectra of mirror angular motions, which are smaller than 10−8 rad Hz−1/2 below 10 Hz.