DescriptionBritannica Spectroheliograph diagram.png	English: Mechanical diagram of a spectroheliograph, a scientific instrument for making photographs of the sun at specific wavelengths
Date	published 1911
Source	Encyclopædia Britannica, 11th ed., 1911, v. 25, between p. 619
Author	George Ellery Hale
Permission (Reusing this file)	Public domainPublic domainfalsefalse This image comes from the 13th edition of the Encyclopædia Britannica or earlier. The copyrights for that book have expired in the United States because the book was first published in the US with the publication occurring before January 1, 1929. As such, this image is in the public domain in the United States.

Description:

It consists of a heavy cast-iron platform (a) mounted on four steel balls (b) which run in V guides of hardened steel. Most of the weight of the instrument is floated on mercury contained in three troughs (c, c, c) which form part of the cast-iron base. The platform carries the two slits, the collimator and camera objectives and the prism-train. An image of the sun, about 6.7 inches (170 mm) in diameter, is formed by the Snow telescope on the collimator slit (d). This slit is long enough [8.5 inches (220 mm)] to extend entirely across the solar image and across such prominences of ordinary height as may happen to lie at the extremities of a vertical diameter. After passing through the slit the diverging rays fall upon the 8 in. collimator objective (e), which is constructed in the manner of a portrait lens in order to give a sharp field of sufficient diameter to include the entire solar image. In the Snow telescope the ratio of aperture to focal length is 1 : 30. Hence light from any point on the slit will fill a circle about 2 inches (51 mm) in diameter on the collimator objective, as its focal length is 60 inches (1,500 mm). Since the diameter of the solar image is 6.7 inches (170 mm) there is a slight, but inappreciable loss of light from points in the image at the extremities of a vertical diameter.

The rays, rendered parallel by the collimator objective, meet a plane mirror (f) of silvered glass, which reflects them to the prisms (g, g'). These are of dense flint-glass (Schott 0.102), and each has a refracting angle of 63° 29'. Their width and height are sufficient to transmit (at the position of minimum deviation) the entire beam received from the collimator. After being deviated 180° from the original direction, the dispersed rays fall on the camera objective (h), which is exactly similar to the collimator objective. This forms an image of the solar spectrum in its focal plane on the camera slit (i). Beyond the camera slit, and almost in contact with it, the photographic plate-carrier (j) is mounted on a fixed support. In order to bring a spectral line upon the camera slit, the slit is widely opened and the plane mirror (f) rotated until the line is seen. A cross-hair, in the focal plane of an eyepiece, is then moved horizontally until it coincides with the line in question. The slit is narrowed down to the desired width, and moved as a whole by a micrometer screw, until it coincides with the cross-hair. The eyepiece is removed and the photographic plate (k) placed in position. An electric motor, belted to a screw (l or l') connected with the spectroheliograph, is then started. The screw moves the spectroheliograph at a perfectly uniform rate across the fixed solar image. (Two screws, of different pitch, are provided, to give different speeds.) Thus a monochromatic image of the sun is built up on the fixed photographic plate.