The prismatic spectroscope, used with the Oxford solar telescope, has a clear aperture of 15.2 cm and a focal length of 8.96 m. The linear dispersion varies from 2.8 mm A–1 at ? 4000 to 0.5 mm A–1 at ? 6500. The horizontal apparatus function, determined from the Kr line ? 4319.6, has a half-width of 0.048 A with a marked asymmetry to the red. The practical resolving power at this wave-length is 93 900, less than one-third of that predicted by diffraction theory. Because of the asymmetry in the apparatus function the centres of intensity of solar absorption lines are displaced to the red by amounts which vary as their equivalent widths; precautions must therefore be adopted in the measurement of Doppler displacements. The vertical apparatus function (up and down the slit) has a half-width of 0.167 mm. This tends to wash out the spectra of fine details on the solar surface, but theory and practical experience show that spectra of solar granules can be obtained. For unanalysed white light, incident on a slit with a height of 19 mm, the general scattered light in the focal plane is 1.6 per cent.
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