Calibration of MIDI , the Mid - infrared Interferometer for the VLTI

Calibration of MIDI, the Mid-infrared Interferometer for the VLTI MIDI is the MID-infrared Interferometric Instrument at the Very Large Telescope Interferometer (VLTI). It will perform long-baseline stellar interferometry in the N band (8. . . 13 μm). In the run-up of setting up MIDI, some preparatory tasks had to be solved concerning the calibration of the instrument in the laboratory. Being a crucial sub-system of MIDI, the Internal Delay Line was thoroughly tested in order to characterise its performance. Light sources for different purposes were provided and equipped with appropriate optical layout. Spectral references were also included. The interrelation of a calibrated Internal Delay Line and spectral calibration is presented. For calibration of the instrument during observations, calibrator stars are used. As part of the selection process of suitable objects for a database of such stars, a sub-sample of candidates was checked by interferometric observations in the K′ band (2.0. . . 2.3 μm). Special attention was payed to differential effects in narrow sub-bands. The potential of stellar interferometry is shown in the case of some late type stars. High spatial resolution data were used to constrain model parameters of the circumstellar envelopes of these objects.