ASTROMETRY FROM SPACE - NEW DESIGN OF THE GAIA MISSION

An interferometric astrometric mission, aiming at accuracies at around the 10 microarcsec level, was recommended as a high priority concept within the new ESA Horizon 2000+ scientific programme. The original ou tline concept for such a mission, GAIA, presented its general feasibil ity but did not address many questions of implementation or optimisati on. Another concept of an interferometer for a scanning astrometric sa tellite is presented. It contains a simpler optical telescope and a mo re efficient detector system. The design utilizes the full resolution of all light in the dispersed fringes of a Fizeau interferometer. A pr eliminary optimization of the satellite indicates that two telescope u nits with a baseline of 100 cm will achieve a precision of 3, 8, 22, 6 8, 302 microarcsec for parallaxes of stars with V = 12, 14, 16, 18, 20 mag, respectively, from a 5 year mission. Simultaneous spectrophotome try of the entire spectrum of each star will be obtained with a resolu tion corresponding to intermediate band photometry. The expected preci sion of this photometry is about 0.003 mag for V = 16. The performance is good in crowded fields, at least up to one star per 5 arcsec(2). A Hipparcostype beam combiner of 150 cm width is placed in front of a t elescope with 4 square apertures of 50 cm. The assumed focal length is f = 60 m and the field 0.5 degree diameter. The detector consists of CCDs used for time delayed integration (drift-scan.).