Imaging Sunlight Using a Digital Spectroheliograph by Ken M. Harrison

Author:Ken M. Harrison
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

The term ∆n/∆λ is the index of refraction—a measure of the prism material dispersion and can vary from 0.013 to 0.0036 (crown glass at 4000 and 6000 Å) to 0.051–0.01 (flint glass at similar wavelengths). A 60° flint prism with a base length of 30 mm would therefore give a resolution R = 1530 at 4000 Å and only R = 300 at 6000 Å. To increase the dispersion and hence the resolution it was not uncommon to see trains of up to seven prisms used in early astronomical spectrographs.

Hale used a 4″, 568 l/mm Rowland grating in his initial Kenwood SHG (1892), but changed to two 60° prisms in his larger Rumford design (1902). He felt that the prisms gave less scattered light and a brighter spectrum than the early Rowland grating.

One downside to the use of prism dispersers is that the use of long slits causes curvature in the recorded absorption lines. The field angle effect at the extremes of the slit length causes the light path through the prism to be longer and the deviation greater. This results in the absorption line appearing as an arc, with the ends curving towards the blue side of the spectrum. The radius of this curvature is approximately equal to the focal length of the imaging lens.

The quality and reasonable cost of the replica reflective gratings as well as the ease of mounting and aligning the grating optical elements has made the use of prisms almost obsolete in the construction of the SHG. For the dedicated amateur who is up for the challenge and wants to replicate the earlier work of Hale etc. good 60° flint prisms can still be obtained from suppliers like ThorLabs and Edmund Optics.

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