An optimized fringe tracker for the VLTI/PRIMA instrument

This paper presents new concepts for a Fringe Sensor Unit (FSU) optimized for high accuracy and low flux operation. This concept has been studied for the VLTI/PRIMA instrument in the H (and K) bands. To optimize both photon use and accuracy, an efficient spatial achromatic discrete modulation is chosen. For optical path difference measurements, most of the photons are used in a single polychromatic quadrature while the adjustable remaining part is dispersed for simultaneous group delay tracking. Integration time can be very short since no moving device is used. This FSU can also be turned to a classical two quadratures FSU if needed, for differential delay or visibility measurements. Optical designs for these FSUs are proposed. These simple designs are also very well suited to future space instruments. Theoretical performance and simulation results are finally given and compared to other existing devices.