The VLTI Calibrators Program: Getting Ready for High Accuracy Measurements

Abstract

The need for high angular resolution, high sensitivity measurements has increased since a few decades. More large, long baseline interferometers (VLTI, Keck Interferometer KI, CHARA) are coming on line to fulfill these needs. To exploit fully the scientific potential of an interferometer, astronomical calibrators have to be observed in close sequence with the scientific targets to provide a measurement of the Instrumental Transfer Function which will be used to remove the instrumental and atmospheric effects in the scientific data. This requires establishing a network of astronomical calibrators, either unresolved or with a diameter known with an accuracy of only a few tenths of percent to be comparable to the internal accuracy of the instruments. The aim of the ESO/NEVEC VLTI Calibrators Program is to build such a database for the VLT Interferometer. This database will include sets of high accuracy calibrator objects suitable for the different configurations of the VLTI with the different instruments (VINCI, MIDI, AMBER) and for a wide range of baselines (up to 200 m). After an introduction on the VLTI instrument and a short description of some of the problems of calibration in Interferometry, statistics on the preliminary list of VLTI calibrators will be shown. Since the first fringes in the K band with the commissioning instrument VINCI, data on calibrators have been accumulated with several baselines. We will explain the strategy used to observe and process the data. Some of the preliminary measurements will be shown and some of the limitations will be discussed.