the placement of slits in MXU mode. This improved our observing efficiency in the later phase of the survey, where we concentrated on fainter objects (with a higher surface density).

*The Reliability of the Target Selection*
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The reliability of X-ray follow-up surveys using optical (or near infrared) spectroscopy hinges on matching the X-ray source to the [*right*]{} optical object. This is primarily done through astrometry. Just how reliable are these identifications? Using deep galaxy number counts [@metcalfe2001], we expect roughly 0.02 galaxies in every square arcsec area that are brighter than R $\sim26$. Considering our best $3\sigma$ astrometry error ($1\farcs5$), we expect $\sim0.15$ [*field*]{} galaxies to fall within our error circle – even in the best, zero off axis angle case. In other words we expect one [*false*]{} candidate for every seventh X-ray object at R $<26$. Considering the roughly 250 X-ray sources we observed, we expect that for at least 35 of them, there [*will be*]{} a completely unrelated faint galaxy, even in an error circle of $1\farcs5$. Fortunately, the X-ray counterpart candidates typically have much brighter magnitudes (see Figure \[magdist\]). At these brighter magnitudes the probability of field galaxy contamination is much lower, so we should only 