[en] The authors describe their design of a liquid helium temperature prototype secondary mirror assembly (PSMA) under development for the NASA Space Infrared Telescope Facility (SIRTF) program. The SIRTF secondary mirror assembly must operate below 4 K and provide the functions of highly precise 2-axis dynamic tilting (chopping) in addition to the conventional functions needed by the SIRTF observatory. Their PSMA design employs a fused quartz mirror kinematically attached at its center to an aluminium cruciform. The mirror/cruciform assembly is driven in tilt about its combined center of mass using a unique flexure pivot and a four-actuator control system with feedback provided by pairs of differential position sensors. The voice coil actuators are mounted on a second flexure-pivoted mass enhancing servo system stability and isolating the telescope from vibration-induced disturbances. The mirror/cruciform and the reaction mass are attached to opposite sides of an aluminum mounting plate whose dimensional characteristics are nominally identical to that of the aluminum flexure pivot material. The mounting plate is connected to the outer housing by a six degree of freedom focus and centering mechanism using pivoted actuation levers driven by lead screw/harmonic drive/stepper motor assemblies