Schmidt objective-prism spectroscopy at moderately high dispersion.

Abstract

view Abstract Citations (1) References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Schmidt objective-prism spectroscopy at moderately high dispersion. Miller, Freeman D. Abstract Two ~~-inch objective prisms were included in the list of auxiliary equipment planned for the Curtis Schmidt telescope at the Portage Lake Observatory. The experience of others has demonstrated the value of a single prism working at a dispersion of about 250 A/mm at H~: one of the two prisms of the Curtis telescope has a 40 refracting angle, and provides this dispersion. In writing the specifications for the second prism, consideration was given to the predominantly astrophysical interests of this observatory. The 40 prism is suitable for many investigations of a statistical nature; if it were to be combined with a prism of higher dispersion, it was felt that the Curtis Schmidt would find numerous applications in the spectroscopy of individual, moderately faint stars; and in investigations of a statistical character, involving stars somewhat too faint to be conveniently observed in large numbers with slit spectrographs. The second prism has a refracting angle of 60, and, used singly, a dispersion of 180 A/mm at H~. The two prisms in combination give a dispersion of 110 A/mm at H~, intermediate bet~een the two lowest dispersions available with the existing slit spectrographs of the University of Michigan ~7-inch reflector. The performance of the two-prism combination has been examined with respect to quality of spectra and limiting magnitude. In making this evaluation, the principle was adopted that the images should be of a density comparable to that considered desirable in slit spectroscopy. Still fainter spectra may occasionally be useful, but, for such stars it is generally more satisfactory to make use of a single prism. Inasmuch as this two-prism combination is substantially more powerful than any currently in use with Schmidt telescopes of comparable aperture, a summary of the performance evaluation may be of general interest. The theoretical resolving power is 2.6 X I0~. If it be assumed that the diameter of the tremor disc is 2", the purity is 101, and faint lines in the photographic region of the spectrum should be resolved if separated by 4 A. In practice, this resolution is attained with ease on short exposures on IIa-O emulsion. The combined optics (mirror, correcting plate and two prisms) are thus shown to be of excellent quality. Maximum useful exposure for io3a-O plates is limited by sky fog to about 35 minutes. On 30-minute, io3a-O plates, stars of photographic magnitude 10.3 form spectral images 0.4 mm wide of satisfactory density. The quality of the spectra on the longer exposures is very sensitive to the care used in widening. It is anticipated that rate control of the diurnal drive will ultimately prove satisfactory, but to date the best results have been obtained with manual widening in increments of about 3", by means of a standard position micrometer installed on the guiding telescope. On plates of good quality, exposed for 30 minutes on ~o~a-O emulsion, and widened as described, luminosity-sensitive lines `commonly employed for luminosity classification of late- type stars are well defined. Microdensitometer tracings of spectra on these plates exhibit satisfactory detail, very nearly comparable to that seen on tracings of bright stars photographed with the same prism combination. In view of the results described briefly above, it may be concluded that the two-prism combination will be an efficient instrument for the purposes for which it was designed. University of Michigan Observatory, Ann Arbor, Mich. Publication: The Astronomical Journal Pub Date: March 1953 DOI: 10.1086/106813 Bibcode: 1953AJ.....58...44M full text sources ADS |