Predicted astrometric transits during the Gaia mission

During its operational lifetime, Gaia will continuously scan the sky, roughly along great circles, according to a carefully selected pre-defined scanning law. The characteristics of this law, combined with the across-scan dimension of the astrometric fields of view, result in the above pattern for the distribution of the predicted number of transits on the sky in ecliptic and galactic coordinates. The fixed solar aspect angle (ξ = 50 degrees) , i.e., the angle between the Sun and Gaia's spin axis, favours transits/observations of stars around ecliptic latitudes plus and minus 90 - ξ = 40 degrees. (In galactic coordinates this region of favoured transits appears as a ring centered on the ecliptic poles.) The predicted number of astrometric observations, both astrometric fields combined and averaged over the sky, equals 83.

Technical details for these images:
Coordinate system = ecliptic / galactic (the above image alternates between these two coordinate systems);
Sky projection = Hammer;
Transverse size astrometric field of view = 0.65 deg;
ω = 60 arcsec/s;
ξ = 50 deg;
S = 4.095; K = 5.200 revolutions/yr;
Spin axis precession period = 365.25/K = 70.24 days;
Full mission duration = 5.5 yr (start-to-end of operations, including dead time);
Total dead time = 0.5 yr (randomly selected throughout the mission).

Image courtesy of Jos de Bruijne.

[High resolution versions available: ecliptic coordinates; galactic coordinates]

[Published: 15/09/2003]