A low resolution scanning multiple Fabry‐Perot spectrometer has been built for the observation of very weak extended light sources. The instrument consists of three sets of 7.6 cm diam Fabry‐Perot plates operated in central order; their spacings are in a 3–4–5 ratio and are servo controlled using reference light provided by a single master grating monochromator. The instrument is scanned by varying the wavelength setting on the grating instrument. The resolution of the instrument is variable through the range 0.5–15 A, and its luminosity is equal to that of a 5 cm interference filter with a 65% peak transmittance. The free spectral range is about 125 times the peak half‐width. Wavelengths in that range are transmitted at a level of 10−4or less of the central transmission peak.