Using single-mode fibers to monitor fast Strehl ratio fluctuations

Abstract

The theory of starlight coupling into singlemode 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 point is illustrated with experimental data obtained at the 3.6 m La Silla telescope corrected with the ADONIS adaptive optics system. A 37 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.