![]() The situation can be improved somewhat by using toroidal grating substrates however, their use is restricted because of high costs. Their chief deficiency lies in their wavelength-specific imaging properties, which leads to astigmatism, which in turn limits the exit slit size (and, consequently, the energy throughput). Hence, concave grating systems are preferred in the entire ultraviolet region. Two mirrors, each reflecting 20% of the light incident on them, will reduce throughput by a factor of twenty-five. This is par-ticularly important in the far vacuum ultraviolet region of the spectrum, for which there are no good normal-incidence reflectors. The great advantage in using concave ruled or holographic gratings lies in the fact that separate collimating and focusing optics are unnecessary. Collimating lenses are rarely used, since mirrors are inherently achromatic.įor special purposes, plane reflection gratings can be made on unusual materials, such as ceramics or metals, given special shapes, or supplied with holes for Cassegrain and Coudé-type telescopic systems. Both achieve spectral scanning through rotation of the grating. A single mirror arrangement (the Ebert-Fastie mount) can also be used. The most popular arrangement for plane reflection gratings is the Czerny-Turner mount, which uses two spherical concave mirrors between the grating and the entrance and exit slits. ![]() Plane gratings have been used for ultraviolet, visible and infrared spectra for some time they are also used increasingly for wavelengths as short as 110 nm, an extension made possible by special coatings that give satisfactory reflectivity even at such short wavelengths Master gratings as large as 320 x 420 mm have been ruled. ![]() The choice of existing plane ruled or holographic reflection gratings is extensive and continually increasing. ![]()
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