Physics – Optics
Scientific paper
Jan 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005esasp.576...35d&link_type=abstract
"Proceedings of the Gaia Symposium "The Three-Dimensional Universe with Gaia" (ESA SP-576). Held at the Observatoire de Paris-Me
Physics
Optics
7
Gaia, Astrometry, Accuracy
Scientific paper
Gaia's scientific objective can be summarised as: to observe the complete sample of all stars brighter than V ˜ 20 mag with end-of-life astrometric accuracies of 10 µas at V = 15 mag. To first order, Gaia's astrometric capabilities are a function of the satellite's operational strategy (mainly scanning law and mission lifetime) and the properties of its optical and detector system (e.g., CCD time-delayed integration). This paper elaborates on the following points: 1. As a result of the scanning law, astrometric accuracies vary with direction on the sky. Moreover, for any given direction, there is a difference between attainable position, proper motion, and parallax accuracy: end-of-life position and proper motion errors will be, respectively, 21% and 44% smaller than end-of-life parallax errors. 2. The properties of Gaia's optical and detector system are such that for stars brighter than V ˜ 12 mag, photon noise settles to a low level. The end-ofmission astrometric accuracies for these stars will amount to a few µas (the `bright-star noise floor'). For magnitudes between V ˜ 12 and 20 mag, photon noise determines the line-spread-function centroiding accuracies, and the expected end-of-life astrometric accuracies are 10 - 20 µas at V = 15 and a few hundred µas at V ˜ 20 mag (see the Appendix). At fainter magnitudes, star detection and confirmation statistics and sky-background effects enter, and astrometric accuracies reach milli-arcsecond (mas) levels. 3. At a given V magnitude, astrometric accuracy also depends on apparent star colour (i.e., intrinsic star colour combined with interstellar reddening) through, e.g., the wavelength-dependent properties of the CCD charge diffusion modulation transfer function (partly responsible for the `image quality') and quantum efficiency of the CCDs and the transmission of the telescope optics. Generally, redder stars, at a given V magnitude, have smaller astrometric errors. ∗On behalf of the Astrometry and Astrometric Error Budget Working Group
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