Chromatic Aberration Of An All-Reflective Telescope

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Abstract
The satellite HIPPARCOS of the European Space Agency aims to build a catalog of the astro-metric parameters of 100,000 stars. The satellite carries a special telescope which scans the sky ; it images the stars on a modulating grid and the angular distance between two stars is deduced from the dephasage between the two modulated signals observed after the grid. The study showed an effect induced by the wavelength dependency of the diffraction. This effect, which occurs even when the system is all-reflective (and so when there is not classical chromatism) has been called "CHROMATICITY". The theoretical reasons of this aberration are explained and numerical values of the HIPPARCOS telescope CHROMATICITY are computed ; the accuracy of the results is then discussed.

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