Optical radiation generated by an auroral event is gathered by a fish-eye optical objective (12, 21, 22) and focussed by a telecentric lens (23) to a focal region where a two-passband filter (24) is located. The filter (24) passes two narrow bands of optical radiation, each band of which is centered on a specified wavelength. Rays in the two specified wavelengths are passed by a condenser lens (25) and a collimator lens (26) through an aperture stop in which a blocking filter (27) is located. The blocking filter (27) comprises two side-by-side half-size filters, each of which transmits a corresponding one and suppresses the other of the two specified wavelengths. A prism (28) diffracts rays each of the specified wavelengths by a different amount so that separated images in the two specified wavelengths are formed. A reimaging lens (29) relays the separate images to correspondingly different portions of a detector (30).