H-{alpha} Photometry of Northern Late-Type Stars I.

Errors of the order of +- 2A in the wavelength setting, however, were found experimentally to have no effect on the results, and corrections for the Doppler shift due to the stellar radial velocity were therefore unnecessary. The "H-{alpha} ratio" R, was defined by the expression R = [B/B(L)]/[A/A(L)] where B, A represent the photomultiplier output counts for a star, in the side bands and H-{alpha} band respectively, and B(L), A(L) represent the corresponding counts for the lamp.

One might expect this ratio to be partially determined by features, other than H-{alpha}, situated within the central and comparison regions of the stellar spectrum, but examination of a high resolution spectrogram of Arcturus (a K2 giant star typical of the stars on the observing program) taken by Griffin (1962) showed that these other features affected the H-{alpha} ratio by less than 0.3%. This is only of the same order as the estimated observational error in the H-{alpha} ratios, and we shall assume therefore that the H-{alpha} ratio is determined solely by the strength of the H-{alpha} absorption line.

Griffin (1961MNRAS.122..181G) describes the three-channel spectrophotometer with which observations were made at the coude focus of the Cambridge Observatory 36 in reflecting telescope. Griffin and Redman (1960MNRAS.120..287G), Deeming (1960MNRAS.121...52D), and Griffin (1961MNRAS.122..181G) describe the method of Observation and reduction.