In the course of the scientific validation of simulated test data covering a time period of 1.3 days, the Gaia First Look team discovered a feature in the sky coverage that was first thought to be an error in the simulation: gaps in the sky coverage.
The Figure above shows the coordinates of 200,000 stars which according to the simulations are measured in the direction of Gaia's two fields of view (FoVs) within 31 hours. During such a short period of time Gaia varies its scanning direction by only about 6 degrees (see Figure above). The coordinate "υ" (measured from -π to +π or -180° to +180°, respectively) on the abscissa is an angle measured around the mean scanning direction of the day; the perpendicular direction "r" (from -0.05 to 0.05 or -3° to +3°, respectively) is strongly stretched for better visibility.
For a given star the colour shows how often a star has been measured by one of the FoVs during the 31 hour period: as expected those stars close to the nodes of the scanning law - where the scans intersect - are measured more than five times (colour coded in yellow) while some stars ±90° away in along-scan direction (marked in blue) are scanned only once or twice (green colour).
Surprisingly, not only to the First Look team, the region to the right of the center of the Figure even showed gaps where stars were not measured at all, while on the other half of the scan circle a stronger overlap between consecutive scans was seen. The actual size of the gaps is about 100° along-scan and 0.16° across-scan.
Only a few days ago an explanation for these gaps has been found: it is due to the fact that the projections of the sky from the two different fields of view on a given CCD are offset in the across-scan direction. This effectively leads to a smaller or larger across-scan rotation for the two FoVs.
The goal of the Gaia First Look team is to permanently assess the quality of daily data from the Gaia satellite during the mission; the observed asymmetry in the scanning law for the two FoVs has consequences for the correct interpretation of the measurements. The inhomogeneity in the sky coverage caused by the asymmetry is much smaller than that anticipated from various operational data loss sources.
Figures courtesy of Stefan Jordan (Astronomisches Rechen-Institut, Heidelberg, Germany). Claus Fabricius and Jordi Portell (Universidad de Barcelona, Spain) and Uli Bastian (Astronomisches Rechen-Institut, Heidelberg, Germany) also contributed to the interpretation of the asymmetry.