Interferometric Laboratory Optical Layout

The pupil transfer, from the individual telescopes to the interferometric laboratory, is an unique feature of the VLTInterferometer allowing to have a 2arcsec interferometric field available at the instruments entrance. This capability is theresult of a careful analysis pursued from the very beginning of the VLTI until today in the interferometric laboratory layout.For this goal it has been necessary to develop a new optical device, the Variable Curvature Mirror (VCM), and also todesign all the optical systems located after the delay-lines, as the beam compressors for instance, according to theseinterferometric field-of-view and pupil transfer requirements.This pupil transfer and the role/design of the various optical systems are presented for the major configurations of theVLTI. A special section is dedicated to the VCM system as this component is the most critical one and required specialstudies, using large deformation theory of elasticity, and advanced techniques in optical fabrication. The final performancesof the VCM are reviewed. As these performances had an important influence on the design of the other systems in theinterferometric laboratory, the trade-off between the instruments requirements and the VCM capabilities is presented.Keywords: VLTI, active optic, deformable mirror, variable curvature, pupil transfer, beam compressor