Development of a variable curvature mirror for the delay lines of the VLT interferometer

This paper presents the variable curvature mirror (VCM) serving for beam management purposes in the interferometric mode of the Very Large Telescope (VLT). This special device located in the delay line system provides for a precise positioning of the pupil image of an individual telescope in the recombination laboratory. The particularity of this active mirror is its continuous variable curvature from (2800 mm)(-1) to (84 mm)(-1) which has to be achieved with a reasonable optical quality. This large curvature variation needed in the system, lead us to extend the classical theory of elasticity to the range of large deformations (i.e. the achieved flexions are larger than the mirror's thickness). This new approach, in the domain of active optics, gave us a theoretical support to determine the physical parameters of the mirror (thickness distribution, loading configuration). Today the VCM system has been thoroughly tested and its performances evaluated. The experimental results have shown good agreements with the theory.