An example of the scan-coincidence fraction as a function of separation on the sky. The coincidence level should be approaching 1 at separations much smaller than the size of the field of view (as it is for the Hipparcos data), but is found to be between 0.55 and 0.7 in the TGAS data. The red dots represent correlations (in ecliptic coordinates) in the north and south quadrants (between ± 45° from the north or south directions), the blue dots in the east and west quadrants (between ± 45° from the east or west directions). The scan coverage is significantly different between north-south and east-west directions. The data are for all stars in HEALPix (level 2), pixel 1, an area of about 833 square degrees (equatorial coordinates).
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