LONGITUDINAL DISPERSION COMPENSATION FOR THE VERY LARGE TELESCOPE INTERFEROMETER

The Very Large Telescope Interferometer [1,13] will be operated in air which will introduce small optical path fluctuations due to internal turbulence [4,6] and dispersion effects. Both may contribute to fringe contrast decrease. Longitudinal dispersion effects can be corrected b y inserting an appropriate glass of variable thickness in one arm of t he interferometer [3,5,11,12]. This paper presents a new method applie d to VLTI in order to select the optimum glass, according to both the observing wavelength and the spectral bandpath, and to calculate its t hickness. Results are presented in terms of improvement on the fringe contrast.