Using single-mode fibers to monitor fast Strehl ratio fluctuations - Application to a 3.6 m telescope corrected by adaptive optics

The theory of starlight coupling into single-mode fibers is reviewed to sho w how i) the central obstruction in a telescope pupil deteriorates coupling efficiency much more than one would expect from the loss of collecting are a, and ii) a single-mode fiber and a photometer can be used to monitor fast Strehl ratio fluctuations. This last paint is illustrated with experimenta l data obtained at the 3.6m La Sills telescope corrected with the ADONIS ad aptive optics system. A 37 x gain in coupling efficiency was demonstrated b y turning on the adaptive? optics system, but periodic fluctuations in the coupled signal revealed a vibration of the telescope tube that could not, h ave been detected otherwise.