1936/02/01 by N. U. Mayall · 1 citation
Engineering · Physics and Astronomy · Earth and Planetary Sciences · #Photocathodes and Microchannel Plates #History and Developments in Astronomy #Atmospheric Ozone and Climate
paper · pdf · doi:10.1086/124645
openalex publication_date 1936/02/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/05/22
The rapid development during the past few years of interest in spectra of faint objects has led to the construction at the. Lick Observatory of a new spectrograph especially adapted to their observation. This instrument has as its primary purpose the determination of the radial velocities of the brighter extra-galactic nebulae, but it is also available for a wide variety of researches requiring the observation of faint light sources beyond the reach of the usual stellar spectrograph, such, for example, as globular clusters, the Zodiacal Light, comets, and, of course, faint stars. Plate II shows the. general appearance of the spectrograph, the two photographs being taken with it (a) in its normal position, and (b) rotated 90° with respect to its supporting web. This form of construction, used in the primary focus of the Crossley 36-inch mirror, was adopted for two reasons : ( 1 ) to provide a field large enough to find a faint object, and (2) to make it possible to set the slit on an object too faint to be seen. The first is accomplished by turning the entire spectrograph about an axis parallel to the optic axis of the mirror. In this operation, the slit is replaced in position by a finder consisting of cross-wires, reflecting prism, and lens. Thus when the object has been located and centered on the wires, the slit is easily set on it by simply swinging the instrument back to the normal position. The means for performing the same operation for an invisible object are provided by having the finder cross-wires movable with micrometer screws mounted at right angles to each other. From setting the cross-wires on the nearest star seen in the field to placing the slit on the invisible object is a process so self-evident that details