An actuator for a separator for separating contaminants from a fluid stream which includes entrained contaminants. The actuator is arranged to move along an actuator axis to adjust an open cross-sectional area of at least one aperture of the separator, and comprises a flexible diaphragm, and a support assembly for the flexible diaphragm. The support assembly is movable along the actuator axis carrying the flexible diaphragm and has an upper support member mounted on a lower support member. A portion of the flexible diaphragm is located between the upper and lower support members. The lower support member comprises an axially extending support portion coupled to a radially extending support portion. The upper support member comprises a diaphragm anti-inversion feature coupled to a radially extending support portion. The diaphragm anti-inversion feature extends such that it axially overlaps at least part of the axially extending support portion of the lower support member.