A shutter assembly comprising a first planar member and a second planar member opposed from one another and forming a sleeve having a cavity therebetween, the sleeve having a pair of side rails adjacent the cavity along sides of the sleeve. A first shutter member having a first end is disposed in the cavity and slidingly disposed along one of the side rails, and a second shutter member having a second end is disposed in the cavity and slidingly disposed along the other side rail. The first end is opposed to the second end and is configured to be selectively advanced towards, and retracted from, the second end so as to define an aperture therebetween having a first shape when disposed in a first position, and wherein the aperture has a second larger shape when the first end is disposed in a second position. The first shutter member and the second shutter member maintain a thermal contact with the side rails and the planar members in all positions. The shutter assembly is well suited to be used at a cryogenic temperature and in a high vacuum environment.