Hipparcos - Principles

Principles

Some key features of the observations were as follows:

• through observations from space, the effects of astronomical seeing due to the atmosphere, instrumental gravitational flexure and thermal distortions could be obviated or minimised;
• all-sky visibility permitted a direct linking of the stars observed all over the celestial sphere;
• the two viewing directions of the satellite, separated by a large and suitable angle (58°), resulted in a rigid connection between quasi-instantaneous one-dimensional observations in different parts of the sky. In turn, this led to parallax determinations which are absolute (rather than relative, with respect to some unknown zero-point);
• the continuous ecliptic-based scanning of the satellite resulted in an optimum use of the available observing time, with a resulting catalogue providing reasonably homogeneous sky density and uniform astrometric accuracy over the entire celestial sphere;

• the various geometrical scan configurations for each star, at multiple epochs throughout the 3-year observation programme, resulted in a dense network of one-dimensional positions from which the barycentric coordinate direction, the parallax, and the object's proper motion, could be solved for in what was effectively a global least squares reduction of the totality of observations. The astrometric parameters as well as their standard errors and correlation coefficients were derived in the process;
• since the number of independent geometrical observations per object was large (typically of order 30) compared with the number of unknowns for the standard model (five astrometric unknowns per star) astrometric solutions not complying with this simple five-parameter model, could be expanded to take into account the effects of double or multiple stars, or non-linear photocentric motions ascribed to unresolved astrometric binaries;
• a somewhat larger number of actual observations per object, of order 110, provided accurate and homogeneous photometric information for each star, from which mean magnitudes, variability amplitudes, and in many cases period and variability type classification could be undertaken.