The ruling of diffraction gratings at the Mount Wilson Observatory.

The newer ruling engine of the Mount Wilson Observatory is now producing sizeable diffraction gratings of high quality in all respects. Several recent gratings are 7~41 inches wide with grooves 543 inches long, and one is 8 by 5~' inches. Curved-edge ruling diamonds, developed here, have been used in blazing these gratings to specifications for astronomical use. High luminous efficiency is combined with practically complete absence of scattered light, either general or local in the spectrum. Resolving power of 500,000 is achieved. Rowland ghost intensity is held to about 0.00004 in the first order of 15,000 line per inch rulings and is negligible. Most ruling to date has been at 300, 400, 600, or 900 grooves per millimeter, but other spacings are available. The rather considerable modifications of the Rowland-type engine are described, with particular reference to the monorail diamond carriage, the coupling of the nut to the grating carriage, the end-thrust bearing of the screw, the use of Xitralloy steel ways, and the spacing mechanism. That the principles of this entirely mechanical, single-screw machine are thoroughly sound is attested by the quality of its products. Blazed plane gratings have almost entirely supplanted prisms in fast stellar spectrographs of both short and long focus. Our methods of testing gratings are outline~ and a formula is proposed for the evaluation of gratings. Mount Wilson and Palomar Observatories, Pasadena, Ca 1sf.