A separator for separating contaminants from a fluid stream which includes entrained particulate contaminants, comprising: a first chamber having a cylindrical wall and a first inlet (22, 27) through which a fluid stream enters the first chamber, to flow axially along the first chamber, the cylindrical wall having a plurality of apertures (29, 29', 29", 29a - 29i) formed in it through which the fluid stream can pass out of the first chamber, an impaction surface located downstream of the first chamber, separated from the interior of the first chamber by the cylindrical wall, the impaction surface being arranged relative to the apertures so that the fluid stream leaving the first chamber through the apertures is incident upon the impaction surface, so that the fluid stream is deflected and contaminants separated from the fluid stream, and an actuator which is arranged to move along an actuator axis which is parallel to the axis of the first chamber to adjust the open cross-sectional area of the apertures in the cylindrical wall according to a pressure differential between fluid pressure in the first chamber and a pressure reference by moving along the cylindrical wall so as progressively to occlude the apertures, the actuator comprising a diaphragm which separates the first chamber from a pressure reference, and comprises a tubular membrane which is fastened towards one end of the cylindrical wall and is arranged to fold and to unfold progressively in response to a change in the differential pressure across the diaphragm so as respectively to uncover and to cover the apertures, the separator being characterised in that: the apertures have straight edges which extend perpendicular to the actuator axis, and in which a first aperture and a second aperture of said plurality of apertures (a) are spaced apart so that there is no overlap between them along the actuator axis, and (b) are the closest spaced to one another along the actuator axis of any pair of the apertures which are spaced apart along the axis, and in which the ratio of the spacing (a) between the closest edges of the first and second apertures along the actuator axis to the height (y) of whichever of the first and second apertures has the smaller height is not more than 1.5, the closest edges of the first and second apertures along the actuator axis being straight along substantially all of their lengths and extending perpendicular to the actuator axis. a first chamber having a cylindrical wall and a first inlet (22, 27) through which a fluid stream enters the first chamber, to flow axially along the first chamber, the cylindrical wall having a plurality of apertures (29, 29', 29", 29a - 29i) formed in it through which the fluid stream can pass out of the first chamber, an impaction surface located downstream of the first chamber, separated from the interior of the first chamber by the cylindrical wall, the impaction surface being arranged relative to the apertures so that the fluid stream leaving the first chamber through the apertures is incident upon the impaction surface, so that the fluid stream is deflected and contaminants separated from the fluid stream, and an actuator which is arranged to move along an actuator axis which is parallel to the axis of the first chamber to adjust the open cross-sectional area of the apertures in the cylindrical wall according to a pressure differential between fluid pressure in the first chamber and a pressure reference by moving along the cylindrical wall so as progressively to occlude the apertures, the actuator comprising a diaphragm which separates the first chamber from a pressure reference, and comprises a tubular membrane which is fastened towards one end of the cylindrical wall and is arranged to fold and to unfold progressively in response to a change in the differential pressure across the diaphragm so as respectively to uncover and to cover the apertures, the separator being characterised in that: the apertures have straight edges which extend perpendicular to the actuator axis, and in which a first aperture and a second aperture of said plurality of apertures (a) are spaced apart so that there is no overlap between them along the actuator axis, and (b) are the closest spaced to one another along the actuator axis of any pair of the apertures which are spaced apart along the axis, and in which the ratio of the spacing (a) between the closest edges of the first and second apertures along the actuator axis to the height (y) of whichever of the first and second apertures has the smaller height is not more than 1.5, the closest edges of the first and second apertures along the actuator axis being straight along substantially all of their lengths and extending perpendicular to the actuator axis.