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.

The limitations of the coherent field of view in Optical Space Interferometry are presented. The size of the field required is inferred from the number of photoevents and from stellar density. Then we examine the limitations of the FOV in both "Michelson" -or planar arrayand "Fizeau" -the equivalent of a masked giant telescope-cases in order to assess their maximum size; it is shown that in all cases, the field is too small to include a reference star in the "Michelson" final image field (< a few arcsec) when the input pupil is too diluted; conversely this is not the case for the "Fizeau ", but alignment tolerances are extremely severe. The two main limitations in the Michelson type, field curvature effect and pupil geometry conservation, are then derived in terms of tolerances on the optics. The problem of spatial frequency plane coverage is addressed and we finally propose a mission concept that accomodates most of the problems raised in this paper and nevertheless makes use of two off -axis stars for tracking purposes by splitting the fields before recombination: a "Michelson" with flat collectors, making use of fiber optics in the reference channels.

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The operation of stellar interferometers suffers from turbulence-induced random fluctuations of optical pathlength difference between collected fields. Active compensation needs an error signal which is provided by a "fringe sensor". A phase A study for a fringe sensor for the ESO Very Large Telescope Interferometer (VLTI) has been conducted at OCA, leading to a proposition for the completion of a prototype. In this article, the goals and the principle of the sensor are recalled (see Gay and Rabbia, preceeding paper, in this symposium). Its optimal working wavelength is discussed. Results of a numerical simulation of the sensor operation are reported, comprising sensitivity estimates. The proposed setup is then described in the details, emphasis beeing put on monomode optical fiber related items. Finally, current plans for the testing and the future use of the prototype are outlined. 08194.149561'941'$600 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/15/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx measurement of the position of the envelope of the interference pattern which can be used to remove the n?L uncertainty on the OPD. These measurements are performed by a digital lock-in amplifier type algorithm. If we call 2/fi the period of the OPD SPIE Vol. 2200 / 205 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/15/2015 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx