![]() Energy distribution on either side of the blaze peak is very similar to that of reflection gratings in the scalar domain. Experimentally, transmission gratings of 1200 g/mm have been used. In most cases, relatively coarse groove frequencies are preferred for transmission gratings, although gratings up to 600 g/mm are furnished routinely. The material used to form the substrate must also be chosen for its transmission properties and for the absence of bubbles, inclusions, striae and other imperfections, none of which is a concern for reflection gratings. However, since the substrate is now part of the imaging optics, special substrates are used, made to tighter specifications for parallelism, and those used in the visible region are given a magnesium fluoride (MgF 2) antireflection coating on the back to reduce light loss and internal reflections. Transmission gratings can be made by stripping the aluminum film from the surface of a reflection grating. Unlike ruled gratings, concave holographic gratings can be generated on substrates whose radii are fairly small (< 100 mm) and whose curvatures are fairly high (~ f/1 or beyond). Since the fringe pattern formed during the recording process is three-dimensional, a curved substrate placed in this volume will record fringes. The advent of the holographic method of generating gratings has made the manufacture of concave gratings commonplace. One must not expect such gratings to have a resolving power more than that of any single section, for such an achievement would require phase matching between the grating segments to a degree that is beyond the present state of the art. The resulting bipartite or tripartite gratings are very useful, as available energy is otherwise low in the short wavelength regions. The automatic energy limitation that is thereby imposed can be overcome by ruling multipartite gratings, during which the ruling process is interrupted once or twice so that the diamond can be reset at a different angle. The ruled width is unfortunately limited by the radius of the substrate, since the diamond cannot rule useful grooves when the slope angle of the substrate exceeds the blaze angle. Concave gratings are not only more difficult to rule than plane gratings, since the tool must swing through an arc as it crosses the substrate, but they require the spherical master substrate to have extremely high surface accuracy and tight tolerances on surface irregularity.Īnother limitation of ruled concave gratings appears when they are ruled at shallow groove angles. Though most ruled gratings are flat, curved substrates can be ruled as well if their curvatures are not extreme. ![]() 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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