Michel Faucherre

Prototype for the European Southern Observatory VLTI fringe sensor

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, preceding 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 being put on monomode optical fiber related items. Finally, current plans for the testing and the future use of the prototype are outlined.

Michelson- versus Fizeau-type beam combination: is there a difference?

Pros and cons of Michelson and Fizeau type interferometer configurations are compared for systems in space having baselines in the 5 to 100 meter range, when a large field of view is requested for off-axis tracking. The size of the coherent field-of-view is larger in a Fizeau type interferometer but the tolerances required by wide field operation are easier to achieve in the Michelson type. Furthermore, aberration compensation, which calls for more than three optical elements per beam to assure that the beams overlap in the Fizeau field of view, makes it necessary to split the on and off-axis fields before the f/250 final combination plane in both cases. Two categories of configurations are defined: for baselines B less than 10 meters, a Fizeau type is preferable; conversely a Michelson type should be adopted when B is greater than 10 meters. After summarizing the difficulties to overcome in each type, a solution for each type in space environment is proposed.

Observational Speckle Interferometry

Comparison between theoretical and observed speckle data at large telescopes allow a better accuracy in star diameters measurements by speckle interferometry techniques. Astrophisical results will be presented, based on observations from 3.6 meter ESO and CFH ( Canada-France-Hawaii) telescopes.

VLT interferometer: proposed implementation

The Very Large Telescope Interferometer (VLTI) is one of the operating modes of the VLT. In addition to consisting of the four stationary 8-meter-diameter telescopes, it includes a number of movable Auxiliary Telescopes which both complement the (u,v) plane coverage of the large telescopes and provide a powerful interferometric facility by itself (available 100 percent of the time). The current plans for the implementation of the VLTI are described. These plans will be finalized after the choice of the VLT site in 1990.