central axis 10 of rotation relative to fixed support structure 13 points directly towards the centre of the field of view of the apparatus and a fixed mirror 9 mounted within the hollow rotating primary drum is inclined at 45 u00b0 with respect to said axis so as to reflect onto the reflecting facets of the primary drum 11 radiation entering the drum from the field of view from an open end of the drum as before such radiation is focused by an optical element which in fig3 is a fixed focusing lens 8 which is mounted within the primary drum and which is located between the open end of the drum and the mirror 9 the axis of the lens 8 is roughly parallel to the axis 10 about which the primary polygon or drum 11 rotates the fixed focusing lens 8 like the concave mirror 2 in the embodiment of fig1 and 2 converges the incoming radiation towards a focus this converging radiation is reflected from the fixed plane mirror 9 onto that one of the reflective inwardly directed faces of the primary drum or polygon 11 which is currently suitably positioned with respect to the mirror 10 radiation reflected from this face of the primary drum or polygon passes through a hole in the centre of plane mirror 9 and is directed towards a focus close to the axis of rotation 10 in the same way as in the embodiment of fig1 and 2 before reaching the focus the radiation is relayed by a meniscus lens not shown in fig3 onto a reflective secondary drum or polygon not shown in fig3 from whence it is reflected to the receivers not shown in fig3.