Electron beam collimator

An electron beam collimator comprising a tube structure of rectangular cross-section and which is defined by first substantially rigid wall members each of which comprises two planar wall portions which meet to form a corner of the tube structure and by further intermediate rigid planar wall members, each side of the tube structure comprising one of the wall portions of one of said first wall members and one of the wall portions of another of said first wall members and at least one intermediate planar wall portion formed by one of the intermediate planar wall members, and each wall portion in each side being arranged in overlapping and sliding relationship with an adjacent wall portion in that side so that the cross-section of the tube structure, and hence of the beam emerging therefrom, can be varied, and means for moving the wall portions relative to one another to vary the cross-section of the tube structure, said means comprising first and second elements which are movable towards and away from one another, and third and fourth elements which are movable towards and away from one another in a direction at right angles to the direction of movement of the first and second elements, one of the first wall members connected to the first and third elements for movement with each of these two elements relative to the other, another of the first wall members being connected to the first and fourth elements for movement with each of these two elements relative to the other, another of the first wall members being connected to the second and third elements for movement with each of these two elements relative to the other, the other of the first wall members being connected to the second and fourth elements for movement with each of these two elements relative to the other, and the intermediate wall member or members in each side of the tube structure being connected to a respective one of the first, second, third and fourth elements for movement therewith.