Several lamellar gratings of 18-in. aperture, with 300 diffraction elements per inch, have been made for determining stellar spectra with wide angle telescopes. Central orders are missing at λ4800, and weak at adjacent wavelengths. The two, equal first orders are about 1m weaker than spectra by a prism or echelette grating. Dispersion is linear. The procedure of manufacture is adaptable to larger size. The wavelength at which the central order is missing can be controlled by varying the lamellae thickness. The lamellae are evaporated strips of quartz of 0.5-μ thickness, covering half the face of the support—a spectacle crown, plane parallel plate of thickness. The lamellae were deposited by thermal evaporation.
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