Abstract

The double-passing of the McMath–Hulbert spectrometer on Mount Wilson has been achieved by the addition of four mirrors and an intermediate slit. It is shown theoretically that in a double-pass instrument (1) for a given luminosity the resolving power should be at least 1.6 times that in the single-pass, and (2) for a given resolving power the luminosity should be at least twice that in the single-pass. For the highest resolving powers, where diffraction losses penalize the single-pass, the gain factors should be as high as 3.7 and 7.9, respectively. The effect of the presence of Rowland ghosts in the single-pass may be as large as 5%. The residual scattered light in the double-pass is estimated to vary from 0.2 to 0.4% of the continuum intensity. The directly observed central intensities of the Na D1 and D2 lines in the solar spectrum are 5.4 and 4.6%, respectively. The influence of “ghosts of ghosts” in double-pass spectra is evaluated. The instrument is well suited for direct observation of solar line profiles. If Doppler shapes are assumed for the instrumental and the solar line profiles, the observed half-widths and central intensities are, respectively, not more than 4% and 0.4% larger than their true values.

© 1964 Optical Society of America

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