Geometrical calibration and assessment of the stability of the HIPPARCOS payload

Abstract

The various geometrical calibrations of the Hipparcos telescope and detection system are described. It is shown that these calibrations can be executed with the required precision of better than a milli-arcsecond. Moreover, it is shown that variations in the parameters describing the calibration results are very slow, yielding the picture of a very stable instrument. With the exception of two short time periods where the thermal control of the instrument was not performing correctly, the basic angle is varying at a rate smaller than 2 milli-arcsec/month.