program objects and it was quite rare that we had to choose between multiple optical counterpart candidates within the geometrical constraints of the instrument. As a consequence, our target selection is nearly unbiased. The only selection effect that should be considered was related to objects with multiple counterpart candidates. In these cases we usually selected the object in the appropriate magnitude range for the particular mask, but in general we tried to revisit these objects – unless the first one turned out to be clearly the counterpart.

We also took advantage of the extremely high accuracy of the robotic masks: in some cases, we reconfigured [*some*]{} of the slits between read-outs, without changing the telescope pointing to observe many (brighter) optical counterparts. This way, the integration time on bright objects could be shortened and we could use the remaining time on a different object, while maintaining longer integration times for the faint ones.

During our last two runs (in November and December 2001) we were also using the prefabricated masks (MXU mode – only available for FORS2), as opposed to movable robotic slitlets (MOS mode). For our survey, the only important difference between the two modes is more freedom in 