Patent Number: 050254642
Section: summary

FIELD OF THE INVENTION This invention relates generally to transmission gratings used in the extreme ultraviolet and soft X-ray region of the spectrum and, in particular, is concerned with a support structure consisting of horizontal and vertical bars. The positions of the vertical bars are chosen so as to reduce or eliminate a diffraction pattern coincident with that of the grating itself in the diffraction plane. DESCRIPTION OF THE PRIOR ART Diffraction gratings of the transmission type for producing spectra have been known for decades. For use in the optical wave band, diffraction gratings generally consist of a number of fine, equidistant and parallel lines ruled on a support such as plate glass or optically worked glass. For use in the EUV and soft X-ray wave band transmission gratings thus comprise, typically, regularly spaced opaque wires (as many as several thousand per millimeter) which have a thickness of only a few microns in width and are supported on each end. Of course, the wire spacing and the width are crucial to the performance of the grating in producing desirable spectra. In such construction, and because of their very small cross-section, the wires are structurally weak, so it becomes important to confine their length so as not to exceed a few tens of microns. Furthermore, to be useful, gratings must be of a size larger than one square centimeter in area. Because of these extremely rigid requirements, a support structure is essential in order to support the wires at intervals not to exceed a few tens of microns. The support structure, itself, must be reasonably rigid. Furthermore, it is not only highly desirable but extremely important that the support structure produce no additional diffraction pattern in the diffraction plane of the grating from which spectral analysis is expected. Many of the cosmic X-ray sources are apt to be strong emitters of soft X-ray and extreme ultraviolet (EUV) radiation. For the most part, hot stellar coronae are expected to emit optically thin plasma emission, while other types of sources (e.g., hot white dwarf stars, compact X-ray binaries, and active galactic nuclei) are likely to display predominantly continuum emission spectra. In addition, the intervening material has its own spectral character which should appear as lines and edges superposed on the spectra of the cosmic sources. Much of the future growth of X-ray and EUV astronomy will depend on the development of dispersive instruments with resolving powers sufficient to determine Doppler shifts and line profiles as well as to resolve spectral features. Such measurements would greatly improve the current concepts of temperature, density, composition, structure, and dynamics of astrophysical sources and of the intervening material. Prominent among the many types of dispersive instruments that have been described for astrophysical applications are objective grating spectrometers which incorporate a transmission grating placed between the elements of a grazing-incidence Wolter telescope and the telescope focus. In these designs, the telescope is used not only to concentrate the relatively weak cosmic source radiation, but also works in conjunction with the transmission grating to form sharp spectral lines. Such objective grating designs, while simple in concept, involve some complications. Because of the weak radiation fluxes from cosmic sources, the area of typical grazing-incidence telescopes is large and usually consists of thin annuli of large diameter, and the telescopes usually have long focal lengths. For example, the AXAF grazing-incidence telescope design has a focal length of about ten meters and consists of six nested primary and secondary mirrors, the outermost of which has a diameter of about one meter. In order to take advantage of the long focal length (which directly translates into dispersion in the focal plane), the most desirable location for the transmission grating is just behind the telescope mirrors. This means that the dimensions of the grating must be almost as large as those of the telescope. In order to achieve high effective areas, the gratings must have as much open area as possible consistent with its diffracting role. Usually, the gratings are formed by etching ectangular holes or slits in thin metal sheets. Because no known material transmits strongly in the soft X-ray/EUV wave band, the gratings cannot be mounted on a substrate and must the self-supporting. In a typical grating, long slits -1 .mu.m wide are etched in a sheet of gold -1 .mu.m thick, so that it is unable to endure the rigors of launch and handling if made in large unsupported areas. Some means is therefore required to strengthen the gratings in a manner which produces minimum interference with the placement of the slits. Typically, the structural integrity of the gratings is provided by a system of progressively coarser and stronger support structures overlaid and bonded onto the grating, incurring a loss of 25% to 50% of the grating throughput. Coarse structures have little impact on the diffraction pattern of the grating, but often a fine support structure is required which contains open areas only an order of magnitude or so larger than the dimension of the slits. Such fine support structures produce diffraction patterns of their own which are superposed on the desired pattern of the slits. There are a number of computer algorithms by means of which it is possible in principle to deconvolve the true spectrum from the data. However, in practice, statistical fluctuations in the spectrum due to the low photon fluxes typical of observations in soft X-ray and EUV astronomy tend to produce increased uncertainties in the neighborhood of diffraction maxima which may mask the presence of weak spectral lines when strong lines are also present. An optimum support structure design is thus one whose artifacts are absent from the diffraction plane. Numerous methods have been suggested in the prior art for supporting a transmission grating. They have generally been of the following three types. The one common type of grating support has taken the form of thin plastic films bonded to the grating wires. This form has yielded less than satisfactory results because the thin plastic film is found to be partially opaque to the soft X-ray and extreme ultraviolet radiation which tends to seriously degrade the overall performance of the grating. Another method suggested in the prior art to support the transmission grating is to adopt regularly spaced horizontal and vertical supports (perpendicular to and parallel to the grating wires, respectively) and bond them to the grating wires. Such a construction has been known to yield only marginal benefits because the regular spacing of the support structure produces a diffraction pattern in the diffraction plane of the grating which tends to confuse the spectra of interest. In a third approach taken by the prior art to support transmission gratings, researchers have adopted a random support structure which consists of randomly oriented and randomly spaced supports bonded to the grating wires. It has been found in this type of construction that the randomly oriented and spaced support structure is difficult if not impossible to be made completely random while still providing the necessary element of support and desired maximum throughput. Modifications of the randomly oriented design which were able to provide the needed support have unfortunately produced added diffraction patterns which likewise interfere with the spectra of interest. SUMMARY OF THE INVENTION The transmission grating support structure of the present invention consists of regularly spaced horizontal bars which are perpendicular to the grating wires and randomly spaced vertical support bars which are parallel to the grating wires. The vertical supports are regularly spaced in each row. However, in each row the vertical supports are employed advantageously by having their position relative to the position of the vertical supports in all of the other rows at a distance which is a pseudo-random integer of grating wire spacings. The arrangement embodying the invention, therefore, assigns to this pseudo-random integer (phase number), k, a number from 0 to N-1, where N is the distance from one vertical support to the other next in a single row, measured in terms of grating wire spacings. In addition, the preferred embodiment requires that the number of rows be 2MN where M is an integer. Further, the diffraction patterns above and below the diffraction plane are mirror images if the phase number, k, of row m is the same as for row 2MN-m, which is a desirable feature. In any case, the number of rows having the same phase number is restricted to 2M, that is, there are 2M rows for each value of the phase number (k=0, 1, 2, . . . , N-1). This restriction guarantees that in the diffraction plane of the grating, the diffraction pattern produced by the support structure coincides with that produced by the grating wires in the diffraction plane. Accordingly, an object of the invention is a new grating in the transmission grating field. Another object of the invention is a novel grating support. A further object of the invention is a transmission grating whose diffraction pattern coincides with that produced by the grating wires in the diffraction plane. Another feature of the invention is to completely eliminate from the diffraction plane any artifacts contributed by the support structure in a transmission grating. Still another feature characterizing the present invention is a support structure in transmission gratings altered to reduce significantly its diffraction pattern in the diffraction plane. Other objects of the invention will become apparent from the following description of the embodiments of the present invention taken in conjunction with the accompanying drawings.