  The accuracy requirements on station-keeping for constellations of
free-flying collectors coupled as (future) imaging arrays in space for
astrophysics applications are examined. The basic imaging element of these
arrays is the two-element interferometer. Accurate knowledge of two quantities
is required: the \textit{projected baseline length}, which is the distance
between the two interferometer elements projected on the plane tranverse to the
line of sight to the target; and the \textit{optical path difference}, which is
the difference in the distances from that transverse plane to the beam
combiner. ``Rules-of-thumb'' are determined for the typical accuracy required
on these parameters. The requirement on the projected baseline length is a
\textit{knowledge} requirement and depends on the angular size of the targets
of interest; it is generally at a level of half a meter for typical stellar
targets, decreasing to perhaps a few centimeters only for the widest attainable
fields of view. The requirement on the optical path difference is a
\textit{control} requirement and is much tighter, depending on the bandwidth of
the signal; it is at a level of half a wavelength for narrow (few %) signal
bands, decreasing to $\approx 0.2 \lambda$ for the broadest bandwidths expected
to be useful. Translation of these requirements into engineering requirements
on station-keeping accuracy depends on the specific details of the collector
constellation geometry. Several examples are provided to guide future
application of the criteria presented here. Some implications for the design of
such collector constellations and for the methods used to transform the
information acquired into images are discussed.
