Fringe envelope tracking at COAST

We report on a new fringe envelope tracking system installed at the Cambridge Optical Aperture Synthesis telescope (COAST). This currently uses the existing photon-counting avalanche photo diode (APD) detector system to allow real-time fringe tracking on up to 3 baselines simultaneously. This system has been recently tested on the sky and has proved to successfully track the fringe envelope on a 38m baseline. The algorithm based on an envelope method has also been implemented and tested at the Infrared-Optical Telescope Array (IOTA) interferometer.