of the X-ray-to-optical flux ratios are shown in Table \[fxfo\_classes\]. As can be seen from the table, the new X-ray-to-optical flux ratio is not significantly different from the canonical one. The typical ranges are a bit wider, but this just a consequence of [*converting*]{} the ranges instead of directly determining it from large surveys. With our new normalization, we can use the original ranges [@stocke1991] to make an educated guess on the galactic/extragalactic nature of objects.

TARGET SELECTION
================

Target selection was primarily based on our deep VLT/FORS imaging data [@giacconi2002], reaching a depth of R $\sim26.5$. In regions not covered by this VLT/FORS deep imaging, we used somewhat shallower VLT/FORS imaging in the R-band obtained as part of the survey.

Possible optical counterparts of X-ray sources were selected based on the estimated astrometry error of the X-ray object (for a relatively bright point source at zero off-axis angle the astrometry rms error is $\sim 0\farcs5$). We used the automatically generated optical catalog, however, [*every*]{} object was visually inspected for deblending problems and artefacts.

The surface density of our X-ray objects is very well suited to MOS spectroscopy with FORS/VLT. We could fill a large part of the masks with 