Optical path difference model for the Very Large Telescope Interferometer

The optical path difference model (OPD model) determines where to position the delay lines in order to compensate for on-axis delay as seen from an astronomical object of known coordinates. This model is equivalent to a pointing model but applied to the interferometric delay. The objective is to reduce the time to search for fringes and to improve the delay lines blind tracking accuracy. This aspect is of prime importance not only when considering the overall operational efficiency of the interferometer but also its ability to quickly observe a set of program objects even after relocation of the auxiliary telescopes. The optical path difference model is based on a precise knowledge of the interferometer configuration by including a set of calibration measurements. This paper describes the main characteristics of the model and includes the results of a simulation developed to fit telescope axis misalignments which contribute to optical path difference errors.