  Preliminary results are described for emission-line imaging of the field of a
quasar at z=0.9 with a unique new instrument --- the TAURUS Tunable Filter
(TTF). At least fourteen [O II]-emitting galaxy candidates are found within +/-
750 km/s of the quasar redshift with a significance of >3 sigma. Another eight
candidates (>3 sigma) are also found with relative velocities +/- 750-1500 km/s
from the quasar, indicative of a large velocity dispersion in the field.
Together with the existence of a group of red galaxies in the field identified
through broad-band imaging, we suggest that the TTF data are probing cluster
merging which is ongoing at z=0.9.
