Instrumental set-up. A tunable narrowband imaging filter is obtained by coupling two Fabry-Perot etalons (FPE) with a high resolution spectrometer based on a reflection grating (G). The set-up includes the following: calibration optics (CO), a modulator (MOD), an interferometric pre-filter (IF), flat mirrors (M), collimating mirrors (CM), imaging mirrors (IM), a linear polarizer (LP), and reduction optics (RO).