  Context. High accuracy astrometric instruments like Gaia aiming at an
accuracy of 1 microarcsecond cannot be considered as point-like observers in
the framework of relativistic modelling of observable quantities.
  Aims. Special-relativistic effects on the imaging by a non-point-like
arbitrarily moving optical instrument are discussed.
  Methods. A special-relativistic reflection law for a mirror of arbitrary
shape and motion is derived in the limit of geometrical optics. The aberration
patterns are computed with ray tracing using a full pecial-relativistic model
for two simple rotating optical instrument.
  Results. It was found that the effect of special-relativistic reflection law
on the photocenters of aberration patterns of an optical system rotating with a
moderate angular velocity of $60 \arcsec/{\rm s}$ may be at the level of 1
microarcsecond if the system involves mirrors significantly inclined relative
to the optical axis.
  Conclusions. Special-relativistic optical modelling of the future astrometric
instrument is generally speaking indispensable if a level of a few
microarcseconds is envisaged.
