Patent Number: 050162659
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT This invention relates to a high resolution, variable dispersion glancing incidence imaging x-ray spectroscopic telescope of variable magnification. The telescope is capable of producing overlapping high spatial resolution images each at a single line or line multiplet in selected narrow wavebands of the x-ray/extreme ultraviolet portion of the spectrum. The field of view of the telescope and the magnification (and hence resolution) of the resultant image may be varied by selection of the multilayer ellipsoidal diffraction grating mirrors, such selection also allowing the precise wavelength band of interest, over the entire spectral range for which the primary glancing incidence mirror is sensitive to be selected, typically 2 to 100 angstroms. The telescope has particular applications to missions in space. FIG. 1 illustrates the telescope, designated generally at 10, as pointed from the payload bay 12 of an orbiting Space Shuttle Vehicle V, the telescope 10 being mounted on the pointing platform 14, which is used to precisely point the telescope at the sun or at the selected astrophysical source and to maintain it stable and free from vibration for the duration of the exposure. The telescope may be used in an orbiting observatory as utilized in the High Energy Orbiting Observatory launched by the United States National Aeronautics and Space Administration (NASA) or on a major Astrophysical Facility such as AXAF, or aboard the U.S. Space Station FREEDOM, which is currently under development by NASA. As hereinafter described, the variable magnification glancing incidence x-ray telescope 10 uses concave ellipsoidal multilayer mirrors to achieve ultra-high spectral resolution at selected narrow wavebands in the aforesaid portion of the spectrum, and to permit the image magnification and field of view to be varied, the mirrors being ruled with diffraction gratings prior to being coated. As known in the art, a diffraction grating comprises a series of very narrow, parallel diffracting surfaces which, when rays are incident upon it at an angle, produces a succession of spectra. When the rays are composed of various wavelengths, the corresponding images of any order will appear at different points and the result is a spectrum. Thus, the grating acts as a dispersion piece since it disperses the composite wavelength rays and transmits the rays of different wavelengths in different directions. Referring now to FIG. 2, the optical system is configured such that the first focus F1 of a multilayer diffraction grating concave ellipsoidal mirror 16, hereinafter merely designated as diffraction grating or just grating, forming a segment of an ellipsoid 18 lies at the prime focus of a conventional single Wolter I or Wolter/Schwarzschild glancing incidence x-ray telescope system typically comprising a glancing incidence parabolidal mirror 20 followed by a glancing incidence coaxial and confocal hyperboloidal mirror 22. Alternatively, nested Wolter I mirrors may be used or the mirrors 20 and 22 may have surface configurations based upon the Wolter II design (internal hyperboloid followed by an externally reflecting hyperboloid), the Narai design (hyperboloid-hyperboloid), or other aspheric-aspheric design configuration of the optical system, without departing from the present invention. The first focus F1 and the center of the ellipsoidal diffraction grating 16 lie on the optical axis 24 of the glancing incidence Wolter telescope optics. The ellipsoid 18 has a second focus F2 and a high resolution contoured x-ray detector 26 is located at the second focus F2 off the optical axis, the detector being a contoured Charge Coupled Device (CCD), a contoured Multi-Annode Microchannel Array, (MAMA) or a camera carrying x-ray sensitive photographic film curved to conform to the Rowland Circle. X-rays strike the mirrors 20, 22 at less than their critical angle and are effectively reflected to produce an image in the focal plane F1 of the mirror system, the incident beam of x-ray radiation 28 being reflected by the Wolter telescope mirrors 20 and 22 to become a convergent beam 30. After passing through principal focus F1, the x-ray beam diverges as illustrated at 32 until it strikes the concave ellipsoidal diffraction grating 16, located behind the primary focus F1. The diffraction grating 16, which has a ruled grating and is coated on its concave surface with an x-ray reflecting multilayer coating 33, is inclined relative to the optical axis 24, preferably 60 degrees or less, so that x-rays of shorter wavelengths can be reflected than are possible with normal incident multilayer optics, the x-rays being reflected by diffraction as an array of converging beams 34a, 34b, 34c, etc. (only three of which are illustrated) toward their respective second focus F2a, F2b, F2c, etc. (only three of which are illustrated) of the ellipsoid 18, the respective second focus being on the Rowland circle. Thus, the x-rays are reflected to the location of the curved surface coincident with the contour of the face of the detector 26 producing an array of overlapping images of high spatial and high spectral resolution at a magnification and field of view on the detector 26 as established by the contour and location of the ellipsoidal surface of the diffraction grating. As hereinafter described the grating 16 may be withdrawn from the x-ray beam by selection means such as a solenoid activated lever arm 36, which is not illustrated in FIG. 2 for purposes of clarity of presentation but is illustrated in FIGS. 6, 7 and 8, to permit the diverging beam 32 to continue aft until it is intercepted by another concave ellipsoidal diffraction grating mirror 38 forming a segment of an ellipsoid of revolution 40 larger than the ellipsoid 18, but sharing the common foci F1 and F2a, F2b, F2c, etc., the grating 38 like the grating 16 also being behind the primary focus F1. This diffraction grating also has ruled gratings and is coated on its concave surface with an x-ray reflecting multilayer coating 41, and is also inclined relative to the optical axis 24. This will produce a lower magnification and relatively larger field of view image of the source on the detector 26, since the magnification is given by the equation M=d2/d1, where d1 is the distance from the first focus F1 to the concave ellipsoidal mirror and d2 is the distance from the concave ellipsoidal mirror to the second focus F2. Referring to FIG. 3, the ellipsoid of revolution 18 which determines the surface contour of ellipsoidal grating substrate or mirror 16 employed in the instant invention is illustrated. Referring to FIG. 4 it can be seen that the ellipsoidal mirror substrate 16 includes long sides 16b and corresponding ends 16d. The grating substrate is ruled by mechanical or holographic ruling or anisotropic etching techniques with a high precision diffraction grating 100 set at an appropriate blaze angle .alpha. on the concave surface 16a. Prior to the coating of the surface with the precision rulings 100, the concave surface 16a must be polished to a high degree of smoothness, in the order of 3-10 angstroms RMS, for imaging in soft x-ray/XUV range and to a precision of 0.5-3 angstroms RMS for producing high quality images in the x-ray to hard x-ray regime. The best final grating can be realized with the best possible mirror substrate. Consequently, the superior results of ultra-smooth surfaces which can be achieved by the recently developed Ion Polishing and Advanced Flow Polishing methods are to be preferred. These techniques can produce ultra-smooth mirror surfaces (0.5.ANG.-3RMS). The mirror substrates should be of a stable material capable of receiving such an ultra-smooth surface finish and which can be contoured to the proper figure. Ideal substrates include Zerodur, Cervit, Fused Silica, ULE Fused Silica and some more exotic materials, such as sapphire and glassy carbon. Low expansion coefficient is highly desirable for optics which will receive a significant thermal loading. For solar telescopes, the use of a heat rejecting pre-filter is desirable, and will permit materials such as Hemlite grade sapphire or glassy carbon to be used. These materials can yield the ultimate (0.2-0.7.ANG. RMS) in ultra-smooth surfaces, but they have a somewhat higher thermal coefficient of expansion than materials such as Cervit or Zerodur. The grating spacing is greatly exaggerated in FIG. 4, and typical gratings are simple amplitude or laminar gratings with rulings of 500-1500 lines/mm. Such gratings can provide spectral resolutions as high as .lambda./.DELTA..lambda.&gt;2000 at normal incidence. All constructive interference should occur at constant angle with respect to the zero order Bragg angles. The concave ellipsoidal multilayer diffraction grating is then capable of producing an array of overlapping images, one for each of the diverse spectral lines or multiplets emitted by the source and lying within the bandpass of the multilayer coating 33 at the diffraction order of interest. The multilayer coating is thereafter deposited upon the concave surface 18a of the grating and consists of multiple precise alternating layers of high-z diffractor material separated by low-z spacer material layers. D is the thickness of the diffractor plus spacer layer. The 2D spacing and the materials selected for the x-ray multilayer coating 33 are chosen so as to reflect the desired band of x-ray emission. Since these mirrors reflect radiation by Bragg diffraction, the precise wavelength at which the peak reflectivity occurs is determined by the 2D spacing of the multilayer coating and the angle of incidence at which the radiation strikes the mirror. The optical properties of the diffractor and spacer components at the wavelength of interest must be taken into consideration in order to select the optimal composition. Tungsten/Carbon, Rhodium/Carbon, Molydenum/Silicon and other material combinations have been proven to have superb properties of long term stability. Excellent reflectivities (approaching theoretical limits) have been achieved in practice with these materials. Reflectivities at normal incidence in the soft x-ray/XUV regime as high as 65% have been documented. At smaller angles of incidence, reflectivities of hard x-rays with reflection efficiencies in excess of 70% have also been measured. Referring now to FIG. 5, which illustrates a side elevational view of a multilayer diffraction grating the grating substrate 16 is polished to a high degree of smoothness and then ruled or anistropically etched with a grating 100 of spacing S.sub.g. Incident polychromatic radiation x-ray/XUV beam B strikes the grating at the Bragg angle .alpha. with respect to the grating surface. The grating surface is coated with a uniform array of multilayer diffractor layers 100d separated by a uniform array of multilayer spacer layers 100s. The Bragg diffracted beam is reflected as the zeroth order beam 0. The grating dispersed Bragg light is diffracted of in first order as beam 1, in second order as beam 2, etc. The negative orders are diffracted as beams -1, -2, etc. When the source has several spectral lines within the bandpass of the multilayer grating, an array of overlapping images will be produced, one image for each spectral line in the bandpass. The intensity of the light in the image is related to the brightness of the source at that particular spectral line. This provides an incredibly powerful tool for plasma diagnostics for complex astrophysical sources such as the sun, active galaxies, binary systems, supernova remnants, etc. The ellipsoid of revolution shown in FIG. 3 has the important optical property that radiation which emanates from one focus F1 of the ellipsoid is re-focused to the second focus F2 of the ellipsoid. For some embodiments, it may also be desirable to use a mirror surface which comprises a segment of a toroid of revolution or a spheroid, and this remains within the spirit and scope of the present invention. Mirror substrate element 16 however, is preferably a concave, inclined ellipsoidal element. As aforesaid, the ellipsoidal element is configured such that one of its foci coincides with the principal focus F1 of the Wolter mirror system and the high resolution x-ray detector 26. Referring now to FIG. 6, a telescope 10 according to the present invention is illustrated having a mount tube 42 affixed to a mounting plate structure 44 for mounting the telescope to the pointing platform of the vehicle V as illustrated in FIG. 1. The mirrors 20 and 22 are housed within a mirror mount cell 46 which maintains them in alignment and has a mounting flange 48 for mounting the mirrors to the telescope mount tube 42. In the preferred embodiment, the mirror mount cell 46 and the mount tube 42 may comprise filament wound fiber epoxy material, although other material such as Beryllium, Aluminum, or Invar may be suitable if requirements related to outgassing properties, thermoexpansion coefficient or weight should dictate their selection and if economy permits. An optical reference cube 50 may be used for aligning the optical axis of the telescope 10 to other instruments (not illustrated) which may be flown on the same spacecraft to collect simultaneous data at other wavelengths. Heat shield or heat rejection plates 52 mounted at the forward end of the telescope may be used for solar studies to eject unwanted solar heat so as to protect the telescope from excessive heating which could cause de-focus effects. A front aperture stop 54 is utilized to prevent radiation from traveling directly through the center of the Wolter optics and reaching the concave ellipsoidal mirrors without first being reflected by the Wolter optics. The incident radiation beam 28 enters the telescope through an entrance annulus 56 which is covered with a visible light rejection pre-filter 58, the pre-filter typically being 2000.ANG. of aluminum on a nickel mesh support structure 60. After the incident radiation beam 28 is reflected by the primary mirror system 20 and 22, the reflected convergent beam 30 converges toward the principal focus F1 and then diverges as a diverging beam 32 behind the principal focus F1 to strike the multilayer coated grating surface of a selected one of either a first or a second set of inclined ellipsoidal gratings 116, 138 as hereinafter described, the first focus of each mirror coinciding with principal focus F1 of the primary Wolter I x-ray mirror system. The beam after striking a grating is reflected as a narrow selected wavelength band, dependent upon the grating selected, and is brought to focus on the single contoured detector 26 in the embodiment of FIG. 6, the detector 26 being disposed at the focal plane of the focus F2 of the ellipsoidal gratings. In the preferred embodiments, the detector 26 is a photographic film contoured into the curve of the Rowland circle carried on a spool 62 and pressed in the focal plane F2 by a curved platen 64. The film is advanced by a motor drive 66 in accordance with electronic signals received by drive electronics (not illustrated). The film and drive assembly may be mounted within a camera housing 68 equipped with a handle 70 to permit an astronaut to remove and replace the film during an EVA. The camera housing 68 is mounted to the telescope housing 42 by means of a flange 72 and an adapter plate 74. Although a film camera is illustrated in the preferred embodiment, other detectors such as CCD's. MAMA's, etc. may be readily utilized in accordance with the present invention, the front surface of the detector being curved to match the Rowland circle geometry of the gratings. The first set of gratings 116 comprises a plurality of inclined concave ellipsoidal multilayer coated gratings 116a, 116b, 116c, 116d, mounted on a cylindrical carrier 76 substantially parallel to the axis of the carrier intermediate the ends thereof, the carrier being oriented at a desired angle and being positioned with respect to the optical axis 24 to present each grating 116a, 116b, 116c, 116d, at a desired inclination to the axis and the radiation bcam 32. Each of the gratings 116a through 116d is of the same ellipsoidal section of the ellipsoid 18, illustrated in FIG. 2, so that the primary image focused at F1 is always re-imaged onto the image plane of the detector 26 at focus F2. The exact multilayer coating for each grating element 116a through 116d is different, so that each grating mirror will reflect a different x-ray wavelength. Furthermore, the blaze angle and dispersion characteristics of the gratings, may differ so as to permit sources to be imaged with wider separation between images from adjacent spectral lines. A drive motor in the form of a stepper motor 78 is provided for selectively rotating the carrier 76, the motor driving the carrier by means of a belt 80 trained about pulleys at the ends of the respective motor and carrier. Although a stepper motor is the preferred form of drive mechanism, other drives such as a Geneva mechanism, or other drive and coupler means, such as sprocketed wheel and chain, etc. for accurately positioning the cylinder to dispose a selected grating onto the optical axis may be utilized to select one of a plurality of x-ray wavelengths. While only four gratings are illustrated, it is to be understood that any number of such gratings may be employed, each with a different multilayer coating, and possibly different ruling characteristics or blaze angles, the greater the number of gratings utilized, the greater the number of different wavelengths that may be recorded on the detector 26. The cylindrical drive carrier 76 is mounted on the retractable solenoid activated lever arm 36 so that the carrier may be withdrawn from the beam 32 to allow the beam to continue aft to allow it to expand until it is intercepted by a selected one of the second set of gratings 138. The second set of gratings 138 comprises a plurality of inclined concave ellipsoidal multilayer coated gratings 138a, 138b, 138c, 138d, mounted on a second cylindrical carrier 82 in the same manner in which the gratings 116a through 16d are mounted on the first carrier 76. The carrier 82 is oriented at a desired angle and positioned with respect to the optical axis 24 to present each grating 138a, 138b, 138c, 138d, at the desired inclination relative to the axis 24 and the incoming radiation beam 32. Preferably, in the embodiment illustrated in FIG. 6, both carriers are inclined at substantially the same angle to reflect the radiation from their respective grating to the single detector 26. Drive motor means 84 similar to the drive motor 78 is provided for selectively rotating the cylindrical carrier in a similar manner and for the same purpose that the motor 78 drives the first cylindrical carrier 76 by means of a drive belt 86. The second cylindrical carrier 82 may also be carried by a solenoid activated lever arm 88 for permitting the carrier 82 to be withdrawn from the radiation beam or re-inserted into the beam selectively if desired. Each of the gratings 138a through 138d is of the same ellipsoidal section of the ellipsoid 40, illustrated in FIG. 2, so that the primary image focused at F1 is always re-imaged onto the image plane of the detector 26 at F2 when one of the gratings 138a through 138b is inserted into the beam. As in the case of the first set of gratings 116, the specific multilayer coating for each respective grating element 138a through 138d will reflect a different x-ray wavelength. Although the carrier 82 contains ellipsoidal gratings belonging to another family of ellipisoids of revolution than those of carrier 76, the ellipsoids have common or coincident foci F1 and F2. Preferably the ellipsoidal gratings 116a through 116d on the carrier 76 have a greater magnification than the gratings 138a through 138d on the carrier 82 since they are closer to F1 and further from F2. Thus, when the first carrier 76 is disposed in the path of the incoming beam 32, a greater magnification and smaller field of view is reflected to the detector 26, but when a larger field of view at lower magnification is desired, the first cylindrical carrier 76 may be withdrawn from the beam by the solenoid activated lever arm 36 to permit the incoming beam to impinge upon one of the selected gratings on the carrier 82 and diffract and disperse the radiation over the surface of the detector 26 as an array of overlapping images in a specific wavelength band dependent upon the coated grating selected. When the telescope is subsequently pointed such that an interesting region lies on the optical axis 24, the solenoid activated lever arm 36 can then be engaged to move the first cylindrical carrier 76 into the beam to record the image at a greater magnification and smaller field of view onto the detector 26. Although only two carriers 76 and 82 are illustrated, the present invention contemplates the use of a plurality of such carriers and consequently the second carrier 82 includes the solenoid activated lever arm 88 so that both carriers may be withdrawn from the beam by the respective solenoid activated lever arm and permit a grating on a subsequent carrier to receive the beam. The second solenoid activated lever arm may also be useful to ensure that when a grating on the first carrier is selected, the second carrier is withdrawn from any refracted radiation reflected by a grating on the first carrier, and this is particularly important where space is critical. The multilayer coatings 33 and 41 can be deposited so as to be perfectly uniform if a broader spectral response is desired. If it is desired that the spectral response be as narrow as possible, multilayer coatings 33 and 41 will be deposited upon the ellipsoidal gratings while the substrates are inclined at the appropriate angle with respect to the sputtering source, rather than lying flat as is the usual case for coating optics by the magnetron sputtering process. This will result in a multilayer coating which has a diffractor and spacer layer thickness which varies as a function of position on the grating substrate. This type of wedge multilayer coating is called a "laterally graded multilayer coating", and the layers are thin wedges rather than plain parallel layers. With precisely the correct lateral grading of the mirror 2D parameter (for the particular angle at which the ellipsoidal grating will be operating) the effect of x-ray chromatic aberration can be removed. This effect is produced because the beam 32 diverges after passing through the principal focus F1 of the Wolter optics. Hence rays reflected from the top of the Wolter mirrors strike the ellipsoidal grating coating 33 at slightly different angles than the angle at which the rays reflected from the bottom of the Wolter mirror strike the ellipsoidal grating. Rays from the right and left sides strike at exactly the same angles. Properly coated graded multilayer mirrors can correct the x-ray chromatic aberration effects and ensure that the reflected radiation is confined to a narrow x-ray bandpass. The magnification M of the ellipsoidal grating as aforesaid is given by the relation: M=d2/d1, (where d1 is the distance from F1 to the grating and d2 is the distance from the grating to the detector at focal plane F2) so that when the first ellipsoidal grating which is nearest to the principal focus of the grazing incidence primary optic is used to intercept the beam, the highest magnification and smallest field of view is recorded at detector 26. When a second ellipsoidal grating, which is farther away from the principal focus F1 is used to intercept the beam, lower magnification and wider field of view images are obtained. If a plurality of ellipsoidal grating carriers are utilized, they could be introduced to permit widely varying magnification and field of view so as to produce a "zoom" x-ray telescope with much finer adjustments in magnification than can be achieved with only two ellipsoidal grating carriers as shown herein. The construction illustrated in FIG. 6 utilizes a single detector 26, but as illustrated in FIG. 7, which depicts the focal plane for an alternate embodiment in which there are two retractable concave ellipsoidal grating sets 116, 138, and two independent detectors 26a and 26b are proposed, the gratings being segments of ellipsoids of revolution 18 and 40 which are inclined at different angles with respect to the optical axis 24 to have common foci F1 but different foci F2. The ellipsoidal gratings in the respective mirror sets 116, 138 represent different magnifications because of the relative placements with respect to the two foci F1 and F2, and permits a plurality of different spectral bands to be imaged. The gratings in the first set operate at a different angle of incidence than the gratings in the second set, and if they are constructed of multilayers of the same 2D spacing, different bandpasses of radiation will be reflected to the respective detectors 26a and 26b. Changing from one grating set to another changes the magnification as well as the wavelength reflected to the respective detector. By properly coating the mirrors, the same wavelength can be reflected from a mirror in the first mirror set 116 and another mirror in the second mirror set 138 despite the different angles of incidence. Also selection of the blaze and dispersion characteristics allows imaging with wider separation between adjacent spectral lines. Utilizing mirror sets inclined at different angles, FIG. 8 represents a modification of the embodiment illustrated in FIG. 6. Accordingly, the first cylindrical carrier 176 is inclined at a different angle from the second cylindrical carrier 182 to reflect the diverging beam of x-ray radiation 32 impinging upon their respective gratings 216a, 216b, 216c, 216d, and 238a, 238b, 238c, 238d respectively, to different detectors 126a and 126b respectively, the detectors 126a and 126b being located at respective foci F2' and F2". This permits a plurality of spectral bands to be covered with a plurality of magnifications and imaged upon redundant respective x-ray detectors 126a and 126b. In all other respects the embodiment illustrated in FIG. 8 is the same as that in FIG. 6, but since each detector preferably is photographic film, a duplication of the camera mounting construction is required for each detector. The detector 126a records a high magnification, narrow field of view images reflected by the gratings 216a through 216d of the carrier 176, while the detector 126b records a low magnification, wide field of view images reflected by the gratings 188a through 138d carried by the carrier 182. An electrical wiring harness 190a, 190b is illustrated for connecting the respective second camera by means of wiring 192a, 192b to the camera electronics controller (not illustrated). Although the two detectors illustrated in FIG. 8 are identical, for some applications it may be preferred that different detectors be utilized. For example, the low magnification detector could be a low resolution CCD or MAMA for real time precision pointing to x-ray areas of interest, and the high resolution narrow field images could then be recorded on high resolution photographic film. Such modifications of the present invention are intended to be included within the scope thereof. Consequently, it may be seen that by utilizing a plurality of inclined ellipsoidal multilayer gratings operating at different magnifications and wavelengths, it is possible to produce a spectroscopic telescope having variable dispersion glancing incidence imaging with variable magnification. The use of concave ellipsoidal grating elements operating at an inclined angle make it possible to magnify and image selected narrow spectral segments of the beam over the entire wavelength range of which the glancing incidence primary optics is capable of operating. Numerous alterations of the structure herein disclosed will suggest themselves to those skilled in the art. However, it is to be understood that the present disclosure relates to the preferred embodiment of the invention which is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.