Optimizing a Differential Interferometric Instrument

AMBER, the VLTI near infrared instrument has been designed to measure the variation of interferometric phase with wavelength. This has applications in most of AMBER astrophysical programs including the detection and spectroscopy of hot giant extra solar planets. However it has been necessary to make several trade offs in its design, which might limit the ultimate performances of this mode mainly because all the causes for rapidly variable nanometric differential dispersion could not be eliminated. This paper summarizes the limitations and analyzes ideas to build a VLTI instrument dedicated to the measurement of differential phases with accuracy down to the photon noise limit, which can be as low as a 10–6 fringes in the K band. Such a performance should allow, for example, high SNR medium resolution spectroscopy of more distant or smaller giant extra solar planets.