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Table 1

Numerical factor to be applied to the sky-averaged astrometric standard errors of Eqs. (4) and (7) for the five astrometric parameters as a function of ecliptic latitude β, including the effect of the variation of the end-of-mission number of transits over the sky.

| sinβ | βmin [°] βmax [°] N obs α ∗ δ ϖ μ α ∗ μ δ
0.025 0.0 2.9 61 1.026 0.756 1.180 0.725 0.542
0.075 2.9 5.7 61 1.021 0.757 1.180 0.722 0.542
0.125 5.7 8.6 62 1.002 0.751 1.169 0.710 0.537
0.175 8.6 11.5 62 0.993 0.752 1.167 0.703 0.539
0.225 11.5 14.5 63 0.973 0.751 1.158 0.689 0.538
0.275 14.5 17.5 65 0.952 0.742 1.143 0.673 0.533
0.325 17.5 20.5 66 0.934 0.740 1.136 0.662 0.533
0.375 20.5 23.6 68 0.901 0.730 1.119 0.640 0.525
0.425 23.6 26.7 71 0.861 0.718 1.098 0.614 0.515
0.475 26.7 30.0 75 0.819 0.705 1.072 0.584 0.506
0.525 30.0 33.4 80 0.765 0.691 1.043 0.548 0.493
0.575 33.4 36.9 87 0.701 0.673 1.009 0.500 0.477
0.625 36.9 40.5 98 0.631 0.650 0.970 0.541 0.461
0.675 40.5 44.4 122 0.535 0.621 0.922 0.381 0.437
0.725 44.4 48.6 144 0.469 0.607 0.850 0.327 0.423
0.775 48.6 53.1 106 0.554 0.636 0.808 0.386 0.443
0.825 53.1 58.2 93 0.603 0.654 0.779 0.422 0.456
0.875 58.2 64.2 85 0.641 0.669 0.755 0.447 0.467
0.925 64.2 71.8 80 0.668 0.680 0.731 0.466 0.473
0.975 71.8 90.0 75 0.688 0.706 0.713 0.481 0.490
Sky-average 0.0 90.0 81 0.787 0.699 1.000 0.556 0.496

Notes. The quantity Nobs in Col. 4 denotes the end-of-mission number of focal plane passages for AF, BP, and RP (both fields of view combined; recall that Gaia DR1 is based on 14 months of data, corresponding on average to 16 field-of-view transits). For RVS, the number of focal plane transits is a factor 4/7 = 0.57 smaller (Sect. 3.3.7). The transit numbers in Col. 4 are based on an assumed 6% dead time (data loss). For the faintest objects (G ≳ 20 mag or GRVS ≳ 14 mag), the actual losses are larger (Sect. 5.3.1).

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