variations over the sky of the astrometric performance

Numerical factor to be applied to the sky-averaged astrometric errors for the different astrometric parameters as function of ecliptic latitude β, excluding the effect of the variation of the number of transits over the sky. The quantity Nobs in column 4 denotes the end-of-mission number of focal-plane passages for AF, BP, and RP (both fields of view combined). For RVS, the number of focal-plane passages is reduced by a factor of 4/7. The transit numbers in column 4 are based on an assumed 6.2% dead time / data loss. A more realistic data loss to consider is ~10% for stars brighter than 13th magnitude, increasing to ~20% for stars around magnitude 20 and fainter.

Note: the variation with ecliptic latitude β of the number of focal-plane passages is not included in the factors in the last five columns and needs to be accounted for as well! This table provides the numerical factors including this effect.

 

  |sin(β)|     βmin [°]     βmax [°]     Nobs   α* δ ϖ μα* μδ
0.025 0.0 2.9   61     0.920     0.666     1.045     0.651     0.478  
0.075 2.9 5.7 61 0.916 0.667 1.044 0.648 0.478
0.125 5.7 8.6 62 0.907 0.667 1.043 0.642 0.478
0.175 8.6 11.5 62 0.898 0.668 1.041 0.636 0.479
0.225 11.5 14.5 63 0.887 0.672 1.042 0.628 0.482
0.275 14.5 17.5 65 0.881 0.675 1.044 0.624 0.485
0.325 17.5 20.5 66 0.871 0.678 1.046 0.618 0.488
0.375 20.5 23.6 68 0.853 0.679 1.046 0.607 0.489
0.425 23.6 26.7 71 0.833 0.682 1.048 0.594 0.490
0.475 26.7 30.0 75 0.814 0.689 1.053 0.581 0.495
0.525 30.0 33.4 80 0.786 0.697 1.058 0.563 0.498
0.575 33.4 36.9 87 0.751 0.708 1.067 0.536 0.502
0.625 36.9 40.5 98 0.717 0.725 1.088 0.513 0.515
0.675 40.5 44.4 122 0.679 0.774 1.154 0.484 0.545
0.725 44.4 48.6 144 0.646 0.822 1.156 0.451 0.573
0.775 48.6 53.1 106 0.656 0.738 0.943 0.457 0.515
0.825 53.1 58.2 93 0.668 0.711 0.852 0.467 0.496
0.875 58.2 64.2 85 0.679 0.695 0.789 0.473 0.486
0.925 64.2 71.8 80 0.686 0.686 0.741 0.479 0.478
0.975 71.8 90.0 75 0.684 0.690 0.700 0.479 0.479
Mean     81 0.787 0.699 1.000 0.556 0.496

 

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