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 show how i) the central obstruction in a telescope pupil deteriorates coupling efficiency much more than one would expect from the loss of collecting area, and ii) a single-mode fiber and a photometer can be used to monitor fast Strehl ratio fluctuations. This last paint is illustrated with experimental data obtained at the 3.6m La Sills telescope corrected with the ADONIS adaptive optics system. A 37 x gain in coupling efficiency was demonstrated by turning on the adaptive? optics system, but periodic fluctuations in the coupled signal revealed a vibration of the telescope tube that could not, have been detected otherwise.