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The British–German interferometric gravitational-wave detector GEO600 uses two high-finesse triangular ring cavities of 8 m optical pathlength each, as an optical mode-cleaning system. The modecleaner system is housed in an ultrahigh-vacuum environment to avoid contamination of the optics and to minimize both the influence of refractive index variations of the air and acoustic coupling to the optics. To isolate the cavities from seismic noise, all optical components are suspended as double pendulums. These pendulums are damped at their resonance frequencies at the upper pendulum stage with magnet-coil actuators. A suspended reaction mass supports three coils matching magnets bonded onto the surface of one mirror of each cavity, allowing length control of the modecleaner cavities to maintain resonance with the laser light. A fully automated control system stabilizes the frequency of the slave laser to that of the master laser, the frequency of the master laser to the length of the first modecleaner and the length of the first to the length of the second modecleaner. The control system uses the Pound–Drever–Hall sideband technique and operates autonomously over long time periods with only infrequent human interaction. The duty cycle of the system was measured to be 99.7% during an 18 day period. The throughput of the whole modecleaner system is about 50%. In this article, we give an overview of the mechanical and optical setup and the achieved performance of the double modecleaner system. © 2003 American Institute of Physics.