Hipparcos calibration

Abstract

The methods and results of various Hipparcos in-flight calibrations are described. The geometrical calibrations allow the determination of the instrument distortions in the main field of view to better than a milli-arcsecond. A systematic decrease of the basic angle at a rate of two milli-arcseconds per month has been measured during the first 7 months of operation. Results of the photometric calibration of the main instrument show a decrease in sensitivity of about 1.5% per month. No significant decrease has been found for the star-mapper sensitivity.