is the average of both exposures. To easily reject moving solar system objects from our detection catalogues, we also produced a ”minimum“ image of both exposures. This combined image contains the lower of the pixel values of the two exposures, and is effectively devoid of all those objects that moved by more than one FWHM between the exposures. We carried out the analysis using the double-image mode of *Sextractor* [@Bertin.1996], with source detection on the ”minimum” image and photometric measurements on the average image, using PSF fitting. The different analysis steps are described here:

-   Spatially variable PSF models of the images are built with PSFex [@Bertin.2010], using single stars within the image as prototypes..

-   The sources on the minimal image are identified using *Sextractor*. Their astrometry and photometry are derived by PSF fitting. (The latter is discarded from thereon, but is included in the fit). Since moving objects are at different positions in the two exposures that were stacked to produce the minimal image, they do not appear in the resulting source list.

-   A flux-only PSF fitting at the position of the sources identified on the minimal sum stacked image is then carried out on the average 