An instrument for photographing the Sun at the wavelength of a strong Fraunhofer line. An image of the Sun produced by a telescope is focused on to a primary slit, and light from this slit passes to a grating or prism. The dispersed light so formed is then intercepted by a secondary slit positioned at a desired wavelength (e.g. a part of the Ηα‎ line) and a photographic plate is placed behind this slit. The primary slit is then moved across the image of the Sun and the secondary slit moved to maintain its spectral location, producing a spectroheliogram on the photographic plate or CCD.