Seven plane gratings of high quality having ruled widths of more than 10 in. have now been produced on the M.I.T. servo controlled ruling engine, monitored with interference fringes. Resolving powers closely approaching the theoretical 900 000 in the 12th order green, and above 1 200 000 in the 26th order of 2537 A, have been obtained with several gratings in a 40-ft spectrograph having 10-in. concave mirrors. The wave fronts produced by the gratings, ruled with 7442 grooves per in. in some cases and 300 grooves per mm in others, have been studied with a large Michelson-Twyman interferometer at angles of incidence and reflection having sums up to 150°, in accordance with the method described by Stroke. Ghost and satellite intensities in all orders, as well as the blaze properties of the gratings, have been measured on a 5-m Littrow spectrograph with 8-in. lens. Rowland ghosts in several cases are found to be well below 0.1% in the 12th order green, and below one part in 100 000 at the angles at which ghost intensities are usually measured for purposes of comparison. In the better gratings satellite intensities total less than 1%, with no single satellite stronger than 0.3%. The stronger satellites reported in our previous 8-in. gratings were found to originate from tilting of the interferometer mirrors and the grating blank arising from variations along the ways. Since the gratings have 50 sq in. of ruled area and are well blazed they are extremely fast, 20 sec being adequate to give a satisfactory exposure to ordinary sources at dispersion of 12 mm per A. The blaze angle was varied from one grating to the next. Some of the gratings concentrate more than 50% of the incident green light in a single high order.
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