  Background. Relative astrometry at or below the micro-arcsec level with a 1m
class space telescope has been repeatedly proposed as a tool for exo-planet
detection and characterization, as well as for several topics at the forefront
of Astrophysics and Fundamental Physics. Aim. This paper investigates the
potential benefits of an instrument concept based on an annular field of view,
as compared to a traditional focal plane imaging a contiguous area close to the
telescope optical axis. Method. Basic aspects of relative astrometry are
reviewed as a function of the distribution on the sky of reference stars
brighter than G = 12 mag (from Gaia EDR3). Statistics of field stars for
targets down to G = 8 mag is evaluated by analysis and simulation. Results.
Observation efficiency benefits from prior knowledge on individual targets,
since source model is improved with few measurements. Dedicated observations
(10-20 hours) can constrain the orbital inclination of exoplanets to a few
degrees. Observing strategy can be tailored to include a sample of stars,
materialising the reference frame, sufficiently large to average down the
residual catalogue errors to the desired microarcsec level. For most targets,
the annular field provides typically more reference stars, by a factor four to
seven in our case, than the conventional field. The brightest reference stars
for each target are up to 2 mag brighter. Conclusions. The proposed annular
field telescope concept improves on observation flexibility and/or astrometric
performance with respect to conventional designs. It appears therefore as an
appealing contribution to optimization of future relative astrometry missions.
