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The Tycho-Gaia Astrometric Solution:
|The 2.5 million stars contained in the Tycho-2 catalogue, illustrated in the all-sky map above, offer a unique opportunity for a first full-sky verification of the Gaia instrument and astrometric data processing at a very early point in the mission. The feasibility of a joint Tycho-Gaia astrometric solution (TGAS), determining the positions, parallaxes, and proper motions of the Tycho-2 stars with only half a year of Gaia observations, was studied in an upcoming A&A publication (preprint on arXiv).|
|The map of the field-of-view transits above illustrates one of the TGAS simulations. The colours show the varying number of observations per star in a six month period. The regular pattern of loops around the ecliptic (cyan line) is a feature of the scanning law. The irregular white stripes and wedges correspond to data gaps present in early datasets. If only Gaia data are used, these data only allow the mean positions of the stars to be determined, not their parallaxes or proper motions. This also makes the solution of limited value for checking the internal consistency of the data. The authors show that a sensible astrometric solution including parallaxes and proper motions is feasible when using the Tycho-2 positions as additional information.|
The formal uncertainties of the simulated TGAS solution, shown in this figure, depend strongly on the sky coverage. However, in many well-observed areas the accuracy of the resulting five astrometric parameters is potentially in the sub-milliarcsecond range. This assumes that several very preliminary calibrations can actually be achieved with the necessary precision on just half a year of data.
Attempting TGAS within DPAC thus provides the opportunity of identifying issues in Gaia's processing chain early on, and an interesting challenge to test our readiness on real data.
credits: Daniel Michalik, Lennart Lindegren, David Hobbs (Lund University)
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