  We report on the implemented design of a two-colour instrument based on
electron multiplying CCD (EMCCD) detectors. This instrument is currently
installed at the Danish 1.54m telescope at ESO's La Silla Observatory in Chile,
and will be available at the SONG (Stellar Observations Network Group) 1m
telescope node at Tenerife and at other SONG nodes as well. We present the
software system for controlling the two-colour instrument and calibrating the
high frame-rate imaging data delivered by the EMCCD cameras. An analysis of the
performance of the Two-Colour Instrument at the Danish telescope shows an
improvement in spatial resolution of up to a factor of two when doing
shift-and-add compared with conventional imaging, and that it is possible to do
high-precision photometry of EMCCD data in crowded fields. The Danish
telescope, which was commissioned in 1979, is limited by a triangular coma at
spatial resolutions below 0.5" and better results will thus be achieved at the
near diffraction limited optical system on the SONG telescopes, where spatial
resolutions close to 0.2" have been achieved. Regular EMCCD operations have
been running at the Danish telescope for several years and have produced a
number of scientific discoveries, including microlensing detected exoplanets,
the detection of previously unknown variable stars in dense globular clusters
and the discovery of two rings around the small asteroid-like object (10199)
Chariklo.
