Abstract: A variable aperture mechanism (VAM) comprises a cam assembly, a single motor capable of rotating the cam assembly, and a pair of aperture members which are coupled to the cam assembly and arranged to affect the size of an aperture, with the size of the aperture varying with the position of the cam assembly. The VAM would typically be used with a sensor having an associated optical field-of-view (FOV), with the aperture members moving in and out of the FOV with the rotation of the cam assembly such that the aperture can be set to multiple f-numbers. A thermal link between the aperture members and a cryogenically-cooled surface ensures that the aperture members are also cryogenically-cooled.