The secondary mirror of an infrared telescope is attached to a unit which causes the mirror to oscillate, thus modulating the infrared signal. Since the plane of oscillation of the modulating unit is fixed, the change in direction of modulation necessary during observations must be obtained by releasing the unit from the supporting shaft, rotating it to the required angle, and then returning it to its seating with high precision. This series of operations is made possible in the device described here by the activation of an electromagnet, the field of which frees from its seating the shaft which carries the unit. After rotation, the shaft is recalled to its seating by two disk springs. Constructional details are given of the whole support system which includes the electro-magnetic device, supporting shaft, focusing system, and spider attachment to the upper ring of the telescope. The completed structure has been tested and mounted on the Italian National Infrared Telescope at the Gornergrat.