Some Results with the Coudé Spectrograph of the Mount Wilson Observatory.
Abstract
The development of the coudé spectrograph in use with the 100-inch telescope is traced through its successive stages to its present form. At present three Schmidt cameras with focal lengths of 32, 73, and r 14 inches, respectively, may be used in conjunction with a concave off-axis collimating mirror and a Wood aluminium on pyrex-glass plane grating. The grating has great concentration of light in the red of the first order and the ultraviolet of the second. The linear scale of the 2o-rnch spectrograms in the second order of the 114-inch Schmidt camera is 2.9 A/mm. Illustrations of the application of the spectrograph consist of five investigations. The first of these is a determination of the solar parallax from 37 spectrograms of a Boötis. The range among the ob- servations is 0.398 km/sec, and the resulting parallax is 8'~8o5 ± 0'~007. The radial velocity of the star is -5.621 ± 0.005 km/sec. Seven spectrograms of Mars in the region about X 7000 have been used in a study of the water-vapor lines. Four of the spectrograms were taken when Mars was approaching the earth, and three when it was receding. Measurements of the wave lengths of the water-vapor lines show no certain displace- ments from their normal terrestrial positions and indicate an upper limit for the amount of water vapor in the equatorial regions of Mars certainly not exceeding 5 per cent of that in the earth's atmosphere. A high-dispersion spectrogram of o Ceti taken about one month after maximum of light provides evi- dence that the multiple structure of at least some of the emission lines of hydrogen is due to superimposed absorption lines of other elements. Comparison of the displacements of a large number of absorption lines of different excitation levels indicates a small systematic difference with level, lines of the lowest excitation level showing the smallest positive displacement. The doubling of a considerable number of lines in the spectra of supergiant M-type stars was dis- covered with the coudé spectrograph. The separation and the relative intensities of the two components vary greatly in different stars, the separation being largest and the difference of intensity greatest in a Orionis, and less in a Scorpii, ar Herculis, and o Ceti. All the double lines arise from near the ground state of the neutral or ionized atom, and the amount of widening of other lines appears to depend upon their excitation levels. The position of the mean of the two components agrees well with that of the normal line, but the effect can hardly be ascribed to reversal and is probably due to Doppler shifts in the extensive atmospheres of these stars, as has been suggested by Spitzer. The original discovery with the coudé spectrograph of several previously unobserved sharp inter- stellar lines in the ultraviolet and violet spectra of some stars of early type has been supplemented by re- cent observations of the bright star ~ Ophiuchi with higher dispersion. A number of additional lines has been found; and the presence of several lines of CH, predicted by McKellar, has been established. The existence of molecules of CH in interstellar space, first suggested by Swings and Rosenfeld, and given much greater probability by the work of McKellar, and of CN as suggested by McKellar, now seems to be beyond reasonable doubt
- Publication:
-
The Astrophysical Journal
- Pub Date:
- January 1941
- DOI:
- 10.1086/144237
- Bibcode:
- 1941ApJ....93...11A