The VLTI Delay Line

Schematic representation of the VLTI Delay Line, showing the retro-reflector on its moving base. The Delay Line is a mechanical-optical system that will compensate the optical path differences of the light beams from the individual telescopes. Such a system is necessary to ensure that the light from all telescopes arrive in the same phase at the focal point of the interferometer. Otherwise, the very sharp interferometric images cannot be obtained.