Patent Number: 050162659
Section: summary

BACKGROUND OF THE INVENTION This invention relates to x-ray telescopes and more particularly to variable magnification ultra-high spectral resolution stigmatic glancing incidence x-ray telescopes capable of simultaneously producing multiple high spatial and ultra-high spectral resolution images of solar and stellar sources at numerous well defined spectral wavebands. For applications of obtaining ultra-high spatial resolution observations with high sensitivity detectors, such as CCD's or Multi-Anode MicroChannel Arrays (MAMA'S), variable magnifications are highly desirable. For maximum information of plasma diagnostics, ultra-high spectral resolution two dimensional x-ray/extreme ultraviolet images are very important. However, this capability does not at present exist. Very high resolution telescopes, such as the optical system currently under development for the Advanced X-Ray Astrophysics Facility (AXAF) have a fixed focal length and fixed field of view as dictated by the fundamental parameters of the primary mirror. These telescopes can perform spectroscopy of point sources but are extremely limited when performing simultaneous high resolution spectrography and imaging of extended sources. They have been designed with the greatest emphasis placed upon the harder rather than the softer components of the x-ray spectrum. The ability to produce images of sources at x-ray energies up to 10 keV is of profound significance to the solution of many of the most important problems of astrophysics and solar physics. An instrument for simultaneously performing high spatial resolution images of the sun and of astrophysical sources at numerous well defined spectral wavebands is disclosed in applicant's copending application (Ser. No. 756,979) filed on Aug. 15, 1985, entitled Multispectral Glancing Incidence X-Ray Telescope. In that application a telescope system was disclosed which made high resolution and magnification imaging of solar and stellar x-ray and extreme ultraviolet radiation possible. The telescope system there disclosed images over a broad band of hard x-ray and extreme ultraviolet radiation, in the range of 30 angstroms and below using Wolter type optics without increasing the physical size of the telescope. This was accomplished by combining ellipsoidal layered synthetic microstructure (LSM) mirrors operating at inclined orientations in combination with a glancing incidence Wolter I system with off-axis x-ray detector means with the LSM optics positioned behind the primary focus of the Wolter I primary mirrors system, the LSM mirrors being concave and positioned behind the primary focus of the Wolter I primary mirror system. The apparatus therein disclosed thus made it possible to obtain high spatial and spectral resolution images of point sources or of extended sources of x-ray emission at wavelengths shorter, i.e., higher energies, than could be imaged with the spectral slicing x-ray telescope disclosed in applicant's earlier U.S. Pat. No. 4,562,583 dated Dec. 31, 1985, which operated at normal incidence with all optical elements positioned on the optical axis. Layered synthetic microstructure (LSM) coatings have during the past few years come to be more commonly called "multilayer coatings" or simply "multilayers", and hence the more modern terminology will be used in the present application. In the prior art, Wolter x-ray telescopes have been used with single or nested mirrors to focus x-rays from astronomically distant point or extended sources. These telescopes use x-ray mirrors which operate at a glancing or grazing angle of incidence. The mirrors may be used uncoated or may be coated with a high-Z material such as gold, platinum or iridium. The solar x-ray telescopes which were flown on SKYLAB operated at grazing angles of 54 arc minutes and could effectively reflect only x-rays of energies lower than the 0.5 keV (wavelengths&gt;6 angstroms). These Wolter Type I mirrors use internally reflecting, coaxial and confocal paraboloidal and hyperboloidal mirrors. Astrophysical telescopes, such as HEAO, XMM and AXAF, have been designed to operate at glancing angles in the range of 20 to 50 arc minutes, making it possible for them to focus and image x-rays with energies up to 8 to 10 keV (wavelengths &gt;1.2 angstroms). Images with these systems are typically recorded on high resolution photographic film or other solid-state or gas filled detectors such as CCD's Position Sensitive Proportional Counters, Multi-Anode Micro-Channel Arrays (MAMAS). Techniques for coupling Wolter telescopes to solid state detectors by means of convex hyperboloid mirrors were described in the aforesaid U.S. Pat. No. 4,562,583. However, this device is not capable of operating over the entire wavelength range which can be covered by glancing incidence x-ray telescopes due to the difficulty of fabricating Layered Synthetic Microstructure (LSM) coatings capable of operating at wavelengths significantly less than 30 angstroms when cofigured at normal incidence. Some spectral information has been achieved by means of bandpass filters placed in front of the prime focus of glancing incidence telescopes, as on ATM Experiments S-054 and S056 which were flown by NASA on its first orbiting space station, SKYLAB. However, this technique provides very crude, low spectral resolution filtergrams which do not have adequate spectral resolution for proper diagnostics of the solar or of stellar plasmas. Grating spectroscopy instruments were also flown on SKYLAB for extreme ultraviolet spectroscopy, but these instruments were not capable of functioning at x-ray wavelengths below 171.ANG. and had very low sensitivity below 304.ANG.. However, the information produced was of crucial importance for solar x-ray plasma diagnostics. The primary disadvantages of using an x-ray telescope with filters to produce spectral data is that the bandpasses are so wide as to encompass tens, hundreds or even thousands of spectral lines resulting from plasma in the atmosphere of the sun or any stellar source. The emission lines originate in plasmas at vastly varying temperature and emanating from widely differing heights in the solar or stellar atmosphere. In the applicant's copending application Ser. No. 756,979 entitled Multispectral Glancing Incidence X-Ray Telescope, a system was disclosed having the capability of obtaining high resolution images in different spectral bands over the entire wavelength range that the glancing incidence primary optic was capable of reflecting (1.ANG.-100.ANG.). Disclosed in that application was a high resolution x-ray telescope having a rotatable cylindrical carrier on which a plurality of concave mirrors were mounted, the mirrors being coated with different coatings, and the carrier being rotated to place a selected mirror in the path of the reflected incoming beam to obtain high resolution images of different wavelengths dependent upon which mirror was selected. Even that instrument only provides high spectral resolution images, with the bandpasses determined by the spectral bandpass of the multilayer coating of the ellipsoidal optic. In some regions of the solar atmosphere, a bandpass of only a few angstroms may include many spectral lines from low temperature plasma located in the upper chromosphere or transition region combined with emission from spectral lines from high temperature plasma from the solar corona. During the Oct. 23, 1987 flight of the Stanford/MSFC Rocket X-Ray Telescope, in which we produced the first high resolution, full disk x-ray images of the sun with multilayer x-ray optics (Science, Vol. 241, 1725-1868), the 171-175.ANG. images are dominantly produced by Fe IX (171.075.ANG.) and Fe X (175.534.ANG.) emission at 1 million degrees, but those images are contaminated by some undefined low intensity component of emission at 500,000 degrees due to the presence of lower temperature emission from O V (172.174.ANG.) and the O VI doublet (172.936.ANG. and 173.081.ANG.) from the plasma in the cooler transition region. As an example of the complexity of the solar atmosphere, it should be noted that within the narrow (171-176.ANG.) bandpass of that Cassegrain multilayer x-ray telescope, there exists 21 different spectral emission lines from several different ionization states of Iron, Nickel and Oxygen. At the shorter wavelengths, the number of closely adjacent spectral lines from diverse ionization states becomes even more acute. These pictures of the sun are the first images to show the presence of the solar network (super-granulation) structure at coronal temperatures. However, that important discovery is somewhat confused by the presence of the lower temperature Oxygen lines in the instrument bandpass. Even though those lines are believed to be sufficiently weak to have produced a non-observable contribution to the images their exact contribution must await further studies. SUMMARY OF THE INVENTION Accordingly, it is a primary object of the present invention to provide an ultra-high spectral resolution stigmatic x-ray spectroscopic telescope capable of producing high spectral resolution solar and stellar images with variable magnification and field of view at wavelengths selected over the x-ray and extreme ultraviolet range of coverage. It is another object of the present invention to provide a high sensitivity glancing incidence x-ray telescope capable of producing high spatial resolution images, with ultra-high spectral resolution and with variable magnification and variable field of view, of solar and stellar x-ray and extreme ultraviolet radiation sources, the spectral bandpass being readily selectable from a plurality of narrow wavebands in the entire wavelength range of coverage of the glancing incidence primary optic (2.ANG.-100.ANG.). It is a further object of the present invention to provide a high sensitivity variable magnification and field of view glancing incidence x-ray telescope capable of producing ultra-high spectral resolution and high spatial resolution images of solar and stellar x-ray and extreme ultraviolet radiation sources, the spectral bandpass being readily selectable from a plurality of multilayer diffraction grating mirrors aft of the primary focus of the primary glancing incidence mirrors, the image being resolved onto one or more x-ray detectors. It is a still further object of the present invention to provide a high sensitivity variable magnification and field of view glancing incidence x-ray telescope capable of producing ultra-high spectral resolution and high spatial resolution images of solar and stellar x-ray and extreme ultraviolet radiation sources, the spectral bandpass being readily selectable from a plurality of multilayer diffraction grating mirrors aft of the primary focus of the primary glancing mirrors on a rotatable carrier, and the magnification and field of view being selectable from a plurality of such carriers, the image being resolved onto one or more x-ray detectors. Accordingly, the present invention provides an optical system utilizing a plurality of off-axis ellipsoid mirrors operating at angles of incidence inclined relative to the optical axis, preferably less than 60 degrees, polished to a high degree of smoothness, ruled with a precision diffraction grating configured at a selected blaze angle preferably ranging up to 30.degree., and coated with selected multilayer coatings. A plurality of coated diffraction grating mirrors preferably are carried by each of at least a pair of rotatable carriers which are placed behind the prime focus of a glancing incidence mirror and utilize concave optics. Primary Wolter-type mirrors focus the incoming x-rays to the primary focus of the glancing incidence optics which is coincident with the first focus of the ellipsoidal multilayer diffraction grating mirrors, and at least one high sensitivity, high resolution detector curved to receive the multiple overlapping images produced along the Rowland circle set is placed at the other focus of the ellipsoidal diffraction grating optics. Selection of a carrier places a first set of diffraction grating mirrors in the path to receive the incoming beam to provide a first magnification and field of view, and selection of a diffraction grating mirror of the first set provides a selected wavelength. Rotating the carrier changes the selected diffraction grating mirror and thus the selected wavelength. Changing the selected carrier changes the magnification and dispersion. In the preferred embodiment x-rays of the selected wavelength are reflected and diffracted to produce an overlapping array of images to a detector at the second focus of the elliptical diffraction mirrors, each image corresponding to the emission from the plasma in a single spectral line. Preferably, the different diffraction grating mirrors on each rotating carrier have the same surface contour but are coated with multilayer coatings of different multilayer composition or 2D parameter. Selection of the carrier is provided by retracting at least the first carrier from the beam to allow the x-ray beam to continue to diverge until it strikes the selected diffraction grating mirror on a second rotatable carrier which also focuses the radiation to the same detector, but an image at a different magnification and dispersion is produced from that produced by the first carrier. Fine control over the magnification dispersion and field of view may be achieved by the use of a large number of carriers, each with its own array of wavelength selecting multilayer diffraction grating coated concave ellipsoidal mirrors which may have different blaze angle and dispersion characteristics to permit wider separation between images from adjacent spectral lines. In an alternate embodiment, a plurality of such gratings operating at different wavelengths and capable of providing different magnifications and fields of view are selectable to produce images onto a plurality of x-ray detectors. This permits different x-ray detectors with different performance characteristics to be matched to the optical properties of the imaging system as the magnification, dispersion and field of view are varied. The significance of the magnification feature will be appreciated by considering that when the spectroscopic telescope is used at low magnification to image extended astrophysical sources, e.g., Supernova Remnants, clusters of galaxies, etc. or to produce full disk images of the Solar Corona, a low magnification and wide field of view (1 degree or more) are required. When detectors with fixed pixel sizes such as CCD's or MAMA's, are used, the spatial resolution will suffer at these low magnifications. However, even with high resolution photographic films, where resolution is not a problem, the ability to alter magnification is still of value, as the lower magnification images will record higher flux densities on the film for the same region, and permit fainter features to be observed, even though at lower spatial resolution. Thus after an interesting region of the supernova remnant or the sun has been observed in the low resolution wide field mode, introduction of a different ellipsoidal mirror into the beam will allow the same region to be investigated at much higher magnification and spatial resolution. The very high sensitivity, low magnification mode is very useful for pointing the telescope precisely at faint galaxies or stars, wherein they could then be studied in detail by the lower sensitivity and yet higher magnification and enhanced spatial resolution component of the instrument. The coating constitutes a synthetic Bragg crystal, and is comprised of a large number (50-1000) of alternating layers of high-z diffractor material separated by low-z spacer material and determines the narrow bandpass over which the gratings will be utilized. X-rays which strike the coating are reflected by Bragg diffraction in accordance with the Bragg relation: n(.lambda.)=2DSin(.phi.), where n is the diffraction order, .lambda. is the wavelength of radiation for which the peak reflectivity occurs, D is the multilayer parameter which is the sum of the thickness of one diffractor layer plus one spacer layer in the multilayer stack, and .phi. is the angle at which the incident x-ray strikes the mirror surface. It may be pointed out that glancing angles such as are usually required for Wolter systems are not required for multilayer mirrors designed to cover the wavelengths of x-radiation which can be reflected by conventional x-ray telescopes, however, such small angles might be chosen for some particular applications.