  We describe an apparatus for the implementation of hybrid optomechanical
systems at 4 K. The platform is based on a high-finesse, micrometer-scale fiber
Fabry-Perot cavity, which can be widely tuned using piezoelectric positioners.
A mechanical resonator can be positioned within the cavity in the
object-in-the-middle configuration by a second set of positioners. A high level
of stability is achieved without sacrificing either performance or tunability,
through the combination of a stiff mechanical design, passive vibration
isolation, and an active Pound-Drever-Hall feedback lock incorporating a
reconfigurable digital filter. The stability of the cavity length is
demonstrated to be better than a few picometers over many hours both at room
temperature and at 4 K.
