Motions in the Sun at the Photospheric Level: I. Methods

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Abstract
Methods for determining sight-line velocities at specified points on the solar photosphere are described. The portion of the solar image, the spectrum of which is to be photographed, is observed with an angular magnification of 390 times. The image may be guided, independently of the telescopic controls, and the photograph taken at the instant of best seeing. The spectra are measured against a centre spectrum by Evershed's positive-on-negative method on a self-recording micrometer; possible line displacements due to the asymmetry of the horizontal apparatus function of the spectroscope are thus eliminated. An equation of condition between the three rectangular velocity components of motion in the photosphere and the measured sight-line velocity is derived.

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