Using single-mode fibers to monitor fast Strehl ratio fluctuations

Application to a 3.6 m telescope corrected by adaptive optics

¹ Observatoire de Paris/DESPA, 5 place Jules Janssen, 92195 Meudon Cedex, France
² 2 rue de la chicane, 34130 Saint-Aunès, France
³ European Southern Observatory, Karl-Schwarzschildstra$\beta$e # 2, D-85748 Garching bei München, Germany

Send offprint request to: V. Coudé du Foresto; e-mail: vincent.foresto@obspm.fr

Received: 30 September 1999
Accepted: 31 March 2000

Abstract

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 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.