The reference pixels of MIDI are the two pixels of the detector onto
which the centroids of the target images must fall in order to ensure a
proper beam overlap. They are measured using the center of a pinhole,
in the 3 set-ups OPEN, HIGH_SENS and SCI_PHOT, for telescopes beams A
and B feeding MIDI, for the interferometric channnels 1 and 2 of MIDI,
and for the photometric channels PA and PB of MIDI (in SCI_PHOT only).