Abstract: A spectrometer for temporally separating electromagnetic radiation (10) includes a cavity (105) having first and second reflecting mirrors (1, 2, 4, 5). The first mirror (1, 2) has an aperture (8) arranged to allow electromagnetic radiation (10) to be input into the cavity (105). The spectrometer also includes an imaging device (3) between the first and second mirrors (1, 2, 4, 5) that defines an optical axis of the cavity (105) and performs spatial Fourier transforms of the electromagnetic radiation (10). The first and/or second mirrors (1, 2, 4, 5) has a normal that is arranged at a non-parallel angle to the optical axis, such that the position and/or angle of incidence of electromagnetic radiation (10) on the second mirror is shifted after each round trip.