The Rayton short-focus spectrographic objective.
Abstract
Spectra of very faint stars and nebulae can be observed only with a camera objective of short focus. The California Institute of Technology has a lens of this type, later to be used with the 200-inch telescope, which is an eight-fold enlargement of a microscope objective, with a focal length of 32 mm and an aperture of 50 mm or a focal ratio of F/o.6. Tests made~at the 100-inch telescope of the Mount Wilson Observatory show excel- lent definition and a gain of 50 per cent in speed over the short-focus camera previously used. The spectrograph has a collimator of 24 inches focal length and two light flint prisms. The dispersion at A 4350 is 418 A per millimeter. Ordinary stellar velocities cannot be measured, since the probable error is of the order of 50-100 km/sec.; but an uncertainty of this amount is not important when large displacements, such as those occurring in the spectra of faint extra-galactic nebulae, are measured. A single nebula in the Ursa Major cluster has been observed in which the apparent velocity-shift is + ii ,~oo km/sec. The exposure time required was forty-five hours
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 1930
- DOI:
- 10.1086/143255
- Bibcode:
- 1930ApJ....71..351H