  We apply an efficient selection method to identify potential weak Very Long
Baseline Interferometry (VLBI) target quasars simply using optical (SDSS) and
low-resolution radio (FIRST) catalogue data. Our search is restricted to within
12" from known compact radio sources that are detectable as phase-reference
calibrators for ASTRO-G at 8.4 GHz frequency. These calibrators have estimated
correlated flux density >20 mJy on the longest ground-space VLBI baselines. The
search radius corresponds to the primary beam size of the ASTRO-G antenna. We
show that ~20 quasars with at least mJy-level expected flux density can be
pre-selected as potential in-beam phase-reference targets for ASTRO-G at 8.4
GHz frequency. Most of them have never been imaged with VLBI. The sample of
these dominantly weak sources offers a good opportunity to study their radio
structures with unprecedented angular resolution provided by Space VLBI. The
method of in-beam phase-referencing is independent from the ability of the
orbiting radio telescope to do rapid position-switching manoeuvres between the
calibrators and the nearby reference sources, and less sensitive to the
satellite orbit determination uncertainties.
