Figure a: The Gaia Sky Mapper has been used in a special mode to image the sky close to the Galactic plane for a short while. In nominal operations the image is not transmitted to the ground. Instead, Gaia is analysing it on the fly to detect point sources.

The first strip of the astrometric CCDs is used to confirm the detections from the Sky Mapper. Figure b illustrates the outcome of this process. Figure b is the same Sky Mapper image as in Figure a with an overlay of confirmed sources in the first strip of the astrometric CCD. Red markers are for the brightest stars (G<13 mag) which will be downlinked as small images. Yellow markers are for stars 13<G<16 mag; they will be binned in across-scan direction. The cyan markers are the faintest stars with 16<G<20 mag for which across-scan binning is done as well. In operations a fraction of the detected stars are used to control the Gaia spin rate. Figure b indicates, with square shape markers, those stars which the software has selected for attitude control. It is worth noting that in operations the data sent down around each object is much smaller than the marker in Figure b.

Read more about Gaia source detections in an article on the ESA Gaia blog.