visual tests). At brighter magnitudes, where deblending would be near impossible (e.g. detecting a R $\sim25$ X-ray object in the halo of a R $\sim19$ galaxy), the probability of field galaxy contamination is negligible. Stellar contamination is negligible at our high galactic latitude.

It is also important to point out that these estimates of field galaxy contamination are for the probability of finding an unrelated object in our X-ray error circle. We can also ask a technical question: what is the probability of finding a field object on a slit? Taking a $20\arcsec\times2\arcsec$ area (the typical slit length in FORS-1 is around 20$\arcsec$), we expect to find a R $<23$ galaxy in 5% of the slits and we expect statistically a field galaxy with magnitude R $<25$ in every second slit. This means that one has to be extremely careful in the data reduction and do a very careful book keeping in the process.

OBSERVATIONS AND DATA REDUCTION
===============================

Data were obtained during 11 nights in 2000 and 2001. A summary of the observations is presented in Table \[tblobs\]. All observations were using the ‘150I’ grism (150I+17 in FORS-1 and 150I+27 in FORS-2). These grisms provide a pixel scale (dispersed) 