  We developed an apparatus to couple a 50-micrometer diameter
whispering-gallery silica microtoroidal resonator in a helium-4 cryostat using
a straight optical tapered-fiber at 1550nm wavelength. On a top-loading probe
specifically adapted for increased mechanical stability, we use a
specifically-developed "cryotaper" to optically probe the cavity, allowing thus
to record the calibrated mechanical spectrum of the optomechanical system at
low temperatures. We then demonstrate excellent thermalization of a 63-MHz
mechanical mode of a toroidal resonator down to the cryostat's base temperature
of 1.65K, thereby proving the viability of the cryogenic refrigeration via heat
conduction through static low-pressure exchange gas. In the context of
optomechanics, we therefore provide a versatile and powerful tool with
state-of-the-art performances in optical coupling efficiency, mechanical
stability and cryogenic cooling.
