Hipparcos

Abstract

The advantages of space astrometry in comparison with Earth-based instru­ments are obvious:

• Absence of atmospheric refraction and turbulence so that the image is a fixed diffraction pattern which is much more accurately defined — and hence analysed — than on the Earth.

• Quasi-absence of mechanical torques that modify the position of the image of the centre of the field when the instrument changes orientation.

• Possibility to observe the entire sky with a single instrument.