Orren C. Mohler

Experimental Determination of Absolute f-Values for Methane

The absolute f-values of rotational lines in the positive and negative branches of the 2ν3 overtone band of CH4 have been derived from absorption measurements in the laboratory by two independent methods. The first method involves the use of the curve of growth and requires the observation of weak lines for which the total absorption is independent of the damping constant. In the second method the lines are broadened artificially, by the introduction of about 3 atmos of air, to half-widths that are about five times the slit width. The f-values are then determined by integration of the logarithm of the percentage absorption over the line profile. On the average, the f-values obtained by the two methods agree within 10 percent.

Solar Spectroscopy with a Cashman Cell

A description is given of a long-focus all-reflecting spectrometer of the Pfund type which has been designed and constructed at the McMath-Hulbert Observatory for initial application to the infra-red solar spectrum. The detectors employed thus far are two Cashman cells that permit the recording of the solar spectrum between 1.0μ and 3.6μ with high resolution. The amplifier was designed and constructed by W. Wilson. Radiation from overlapping orders is effectively eliminated by a pre-dispersing unit. Two plane reflection gratings, ruled 600 lines/mm and 200 lines/mm, are used for the regions 1.0μ–2.5μ and 2.5μ–3.6μ respectively. Observed line separations indicate that approximately 80 percent of the theoretical resolving power has been attained.

A Double-Pass Pfund Spectrograph for Solar Research

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.