Abstract

Hipparcos is the most recent scientific program of the European Space Agency. Its primary mission is to provide measurement of the five astrometric parameters of 100 000 selected stars with an accuracy of a few milliarc sec. The basic principle of HIPPARCOS consists in measuring the angular distance of pairs of stars by scanning great circles on the celestial sphere. This is accomplished by superimposing two fields of view distant by 58° at the focal plane of a single common telescope.

The paper deals with some essential aspects of the HIPPARCOS optical system on which extremely stringent requirements are placed. The selected design, based on an all-reflective folded Schmidt Telescope is described, together with a general discussion of the important design drivers relating to the particular procedure used to measure the star position.

Especially important are the short term stability of the angle between the two fields of view and the chromaticity of the optics whereby an error in position of stars having different spectral types might be experienced ; here, the optical phase transfer function is essential to describe this particular performance.

Also presented is the trade-off carried out to select the baseline.

© 1982 AIAA