an astronomical spectroscopic instrument for photographing the solar disk in monochromatic light. The optical scheme of the spectroheliograph is the same as that of the spectrohelioscope except that a photographic plate is placed behind the second, exit slit. The exit slit isolates the desired spectral line or the desired narrow portion of the continuous spectrum. The movement of the two slits is coordinated so that different parts of the photographic plate correspond to the different parts of the solar disk that are successively projected on the first slit.
A photograph made with a spectroheliograph is known as a spectroheliogram. Such a photograph permits the observation of various formations on the solar disk or beyond its edge that emit or absorb light in the given spectral line and that are not visible in photographs made with white light, owing to the superposition of radiation of other wavelengths.
Spectroheliographs have two advantages over birefringent filters, which also can be used to obtain monochromatic photographs of the sun. These advantages are (1) the broad limits within which spectral lines can be selected for observation and (2) the possibility of choosing the degree of monochromatization, which is determined by the width of the exit slit.
For the most part, the H and K lines of ionized calcium and the Ha line of hydrogen are used to obtain spectroheliograms. Spectroheliograms taken at any wavelength of the continuous spectrum show all the details–such as granulation and spots–visible in photographs of the sun made in white light.
The first spectroheliograph designs were developed and implemented in the 1890’s by H. Deslandres of France, G. E. Hale of the USA, and J. Evershed of India. The construction of such an instrument, however, had been suggested somewhat earlier. The first spectroheliograms of prominences were made in 1891, and the first spectroheliograms of the entire solar disk were produced in 1892. In 1908 photographic plates sensitive to red light were developed, and the first spectroheliograms in Hα light were obtained.
Like spectrographs, spectroheliographs use prisms, diffraction gratings, and, sometimes, combinations of prisms and gratings as dispersing systems. Double spectroheliographs have been mounted on some telescopes to permit the simultaneous taking of two photographs of the sun in different spectral lines. With the development of research in space, spectroheliographs carried by rockets and artificial earth satellites have been used to obtain spectroheliograms in ultraviolet light and X-radiation.