While the location of the `satellite target' (the part of the sky on which the detector is centred for the particular observation) is well defined for single stars, being the best estimate of the star's position at the epoch of the satellite observation, the situation is not so straightforward for double or multiple systems. For such systems, the choice of the target position may be:
[]The nature of the target for the satellite observation depends
upon the separation and difference of magnitudes between the
components, and has been dictated by considerations of the observation
and data reduction procedures (Turon et al. 1989b). The brighter
component is chosen when the signal perturbation due to the other
component(s) can be neglected (
mag).
For 
mag, there are two possibilities according to the
geometry of the system and the availaibility of data for
the components. The geometric centre has
been selected when the positions and magnitudes of each component are
known. A `geometric centre' may also have been adopted for some
multiple systems when the two brightest components dominate the other
components. The photocentre has been selected if the separation is such
that only one position or magnitude has been obtained for the system as
a whole, or in the case of multiple systems. When the component
separation is smaller than 3 arcsec, the entry is considered as a
photocentre irrespective of the difference of magnitude between the
components.