X-Ray Telescope Onboard Astro-E. II. Ground-Based X-Ray Characterization

X-ray characterization measurements of the x-ray telescope (XRT) onboard the Astro-E satellite were carried out at the Institute of Space and Astronautical Science (Japan) x-ray beam facility by means of a raster scan with a narrow x-ray pencil beam. The on-axis half-power diameter (HPD) was evaluated to be 1.8?-2.2?, irrespective of the x-ray energy. The on-axis effective areas of the XRTs for x-ray imaging spectrometers (XISs) were approximately 440, 320, 240, and 170 cm(2) at energies of 1.49, 4.51, 8.04, and 9.44 keV, respectively. Those of the x-ray spectrometer (XRS) were larger by 5-10%. The replication method introduced for reflector production significantly improved the imaging capability of the Advanced Satellite for Cosmology and Astrophyics (ASCA) XRT, whose HPD is ~3.6?. The increase in the effective area by a factor of 1.5-2.5, depending upon the x-ray energy, compared with that of the ASCA, was brought about by mechanical scale up and longer focal lengths. The off-axis HPDs were almost the same as those obtained on the optical axis. The field of view is defined as the off-axis angle at which the effective area becomes half of the on-axis value. The diameter of the field of view was ~19? at 1.49 keV, decreasing with increasing x-ray energy, and became ~13? at 9.44 keV. The intensity of stray light and the distribution of this kind of light on the focal plane were measured at the large off-axis angles 30? and 60?. In the entire XIS field of view (25.4 mm x 25.4 mm), the intensity of the stray light caused by a pointlike x-ray source became at most 1% of the same pointlike source that was on the optical axis.     

X-ray telescope onboard Astro-E. III. Guidelines to performance improvements and optimization of the ray-tracing simulator

We present a detailed study of the performance of the Astro-E x-ray telescope (XRT) onboard the Astro-E satellite. As described in preceding papers the ground-based calibrations of the Astro-E XRT revealed that its image quality and effective area are somewhat worse than that expected from the original design. Conceivable causes for such performance degradation are examined by x-ray and optical microscopic measurements at various levels, such as individual reflectors, sectors, and quadrants of the XRT and their alignments. We can attribute, based on detailed measurements, the degradation of the image quality to a slope error in the individual reflectors and the positioning error of reflectors. As for the deficit of the effective area, the shadowing of x rays within the XRT body is the dominant factor. Error budgets for the performance degradation of the Astro-E XRT are summarized. The ray-tracing simulator, which is needed to construct the response function for arbitrary off-axis angles and spatial distributions of any celestial x-ray sources, has been developed and tuned based on the results of detailed measurements. The ray-tracing simulation provides results that are consistent within 3% with the real measurement except for large off-axis angles and higher energies. We propose, based on knowledge obtained from all the measurements and simulations, several plans for future developments to improve the performance of the nested thin-foil mirrors.     

Modeling technique for the Hubble Space Telescope wave-front deformation

Images from the Hubble Space Telescope suffer from an overcorrected spherical aberration that is due to a conic-constant error in the primary mirror. Within the program known as the corrective optics space telescope axial replacement (COSTAR) simulators have been built to provide the point-spread function (PSF) of the telescope alone and of the telescope with the faint-object camera F/96. It was found that the experimental PSF's were identical to those in orbit, which was not the case when the PSF's were calculated with commonly used optical software. We explain this discrepancy and propose a modeling method that is based on the determination of the wave-front error at the exit-pupil level that gives results that are consistent with observations.     

Characterization of the supermirror hard-x-ray telescope for the InFOCmuS balloon experiment

A hard-x-ray telescope is successfully produced for balloon observations by making use of depth-graded multilayers, or so-called supermirrors, with platinum-carbon (Pt/C) layer pairs. It consists of four quadrant units assembled in an optical configuration with a diameter of 40 cm and a focal length of 8 m. Each quadrant is made of 510 pieces of coaxially and confocally aligned supermirrors that significantly enhance the sensitivity in an energy range of 20-40 keV. The configuration of the telescope is similar to the x-ray telescope onboard Astro-E, but with a longer focal length. The reflectivity of supermirrors is of the order of 40% in the energy range concerned at a grazing angle of 0.2 deg. The effective area of a fully assembled telescope is 50 cm2 at 30 keV. The angular resolution is 2.37 arc min at half-power diameter 8.0 keV. The field of view is 12.6 arc min in the hard-x-ray region, depending somewhat on x-ray energies. We discuss these characteristics, taking into account the figure errors of reflectors and their optical alignment in the telescope assembly. This hard-x-ray telescope is unanimously afforded in the International Focusing Optics Collaboration for muCrab Sensitivity balloon experiment.     

Efficiency of an elliptically shaped x-ray mirror

Curved reflecting mirrors are widely used as x-ray optical elements for both laboratory and synchrotron radiation sources. In general, the mirror parameters are optimized by numerical simulation. We discuss an analytical approach that is useful for deriving the mirror parameters, including eccentricity, length, angular acceptance, and magnification. We have examined in particular an elliptical surface from which we learned that, given the distance between the foci of the ellipse, the magnification, and the critical angle of total external reflection, it is possible to find analytically the optimal eccentricity that maximizes the angular acceptance and the optimal mirror length. We found that the last-named parameter, in a first approximation, depends only on the distance between the foci of the ellipse and on the magnification factor. We present as well a comparison of optimal parameters obtained with analytical calculation and with ray-tracing simulation that yielded good agreement.     

X-ray telescope onboard Astro-E: optical design and fabrication of thin foil mirrors

X-ray telescopes (XRT's) of nested thin foil mirrors are developed for Astro-E, the fifth Japanese x-ray astronomy satellite. Although the launch was not successful, the design concept, fabrication, and alignment procedure are summarized. The main purpose of the Astro-E XRT is to collect hard x rays up to 10 keV with high efficiency and to provide medium spatial resolution in limited weight and volume. Compared with the previous mission, Advanced Satellite for Cosmology and Astrophysics (ASCA), a slightly longer focal length of 4.5-4.75 m and a larger diameter of 40 cm yields an effective area of 1750 cm(2) at 8 keV with five telescopes. The image quality is also improved to 2-arc min half-power diameter by introduction of a replication process. Platinum is used instead of gold for the reflectors of one of the five telescopes to enhance the high-energy response. The fabrication and alignment procedure is also summarized. Several methods for improvement are suggested for the reflight Astro-E II mission and for other future missions. Preflight calibration results will be described in a forthcoming second paper, and a detailed study of images will be presented in a third paper.     

Advanced laser-backlit Grazing-Incidence X-Ray Imaging Systems for Inertial Confinement Fusion Research. II. Tolerance Analysis

Two example ultrahigh-spatial-resolution laser-backlit grazing-incidence x-ray microscope designs for inertial confinement fusion (ICF) research have been described [Appl. Opt. 40, 4570 (2001)]. Here details of fabrication, assembly, and optical surface errors that are characteristic of present state-of-the-art superpolished multilayer-coated spherical mirrors are given. They indicate that good image qualities can be expected; in particular, <0.5-mum spatial resolution at very high x-ray energies (up to 25 keV) appears to be feasible: Existing ICF imaging diagnostics approach ~2 mum spatial at low (<2 keV) energy. The improvement in resolution compared with that of other grazing-incidence devices is attributed to a fortuitous residual on-axis aberration dependence on short wavelengths; recent advances in mirror fabrication, including a new thin-film deposition technique to correct figure errors precisely in one dimension; and novel design. For even higher resolutions, a means of creating precise aspherical mirrors of spheric-quality microroughness may be possible by use of the same deposition technique.     

Supermirror hard-x-ray telescope

The practical use of a grazing x-ray telescope is demonstrated for hard-x-ray imaging as hard as 40 keV by means of a depth-graded d-spacing multilayer, a so-called supermirror. Platinum-carbon multilayers of 26 layer pairs in three blocks with a different periodic length d of 3-5 nm were designed to enhance the reflectivity in the energy range from 24 to 36 keV at a grazing angle of 0.3 deg. The multilayers were deposited on thin-replica-foil mirrors by a magnetron dc sputtering system. The reflectivity was measured to be 25%-30% in this energy range; 20 mirror shells thus deposited were assembled into the tightly nested grazing-incidence telescope. The focused hard-x-ray image was observed with a newly developed position-sensitive CdZnTe solid-state detector. The angular resolution of this telescope was found to be 2.4 arc min in the half-power diameter.     

Production and performance of the inFOCmicroS 20-40-keV graded multilayer mirror

The International Focusing Optics Collaboration for microCrab Sensitivity (InFOCmicroS) balloonborne hard x-ray telescope incorporates graded Pt/C multilayers replicated onto segmented Al foils to obtain the significant effective area at energies previously inaccessible to x-ray optics. Reflectivity measurements of individual foils demonstrate our capability to produce a mass quantity of multilayered foils with a rms roughness of 0.5 nm. The effective area of the completed mirror is 78 and 22 cm2 at 20 and 40 keV, respectively. The measured half-power diameter is 2.0 +/- 0.6 are min (90% confidence). The successful completion of this mirror demonstrates its applicability to future x-ray telescopes such as Constellation-X.     

Reflection and refraction of narrow Gaussian beams with general astigmatism at tilted optical surfaces: a derivation oriented toward lens design

The formulas for the reflection and refraction of a narrow Gaussian beam with general astigmatism at a tilted optical surface are derived by ray-tracing techniques. The propagation direction of the reflected and refracted beams is computed by tracing the central ray of the incident beam, and the characteristic parameters of the respective wavefronts are worked out by applying the formulas developed for the generalized ray tracing. Moreover, the Gaussian form of the reflected and refracted amplitude distributions along the transverse coordinates is determined by requiring the matching of the incident, reflected, and refracted light spots on the optical surface. No limiting assumptions are made regarding the form of the optical interface or the orientation of the incident astigmatic wavefront. In the end, to illustrate a simple application of these formulas, the reflection of a Gaussian beam at a conicoid is considered, and a simple property of the conicoidal mirrors is reported.     

W/SiC x-ray multilayers optimized for use above 100 keV

We have developed a new depth-graded multilayer system comprising W and SiC layers, suitable for use as hard x-ray reflective coatings operating in the energy range 100-200 keV. Grazing-incidence x-ray reflectance at E = 8 keV was used to characterize the interface widths, as well as the temporal and thermal stability in both periodic and depth-graded W/SiC structures, whereas synchrotron radiation was used to measure the hard x-ray reflectance of a depth-graded multilayer designed specifically for use in the range E approximately 150-170 keV. We have modeled the hard x-ray reflectance using newly derived optical constants, which we determined from reflectance versus incidence angle measurements also made using synchrotron radiation, in the range E = 120-180 keV. We describe our experimental investigation in detail compare the new W/SiC multilayers with both W/Si and W/B4C films that have been studied previously, and discuss the significance of these results with regard to the eventual development of a hard x-ray nuclear line telescope.     

Design and performance considerations of cat's-eye retroreflectors for use in open-path Fourier-transform-infrared spectrometry

A ray-tracing analysis of cat's-eye retroreflectors for use in active open-path Fourier-transform-infrared (OP/FT-IR) spectrometry and the results of testing f/0.5 and f/1.75 cat's-eye retroreflectors built in our laboratory with a commercial active OP/FT-IR spectrometer are presented. The ray-tracing model is based on the optical characteristics of a commercial single-telescope monostatic OP/FT-IR spectrometer and explores trends in cat's-eye behavior in practical but rigorous field conditions encountered during transportable outdoor use. All mirrors modeled are paraboloids for which the focal ratios of the primary mirror are f/0.5, f/1.75, and f/3. The effect of the focal ratio of the primary mirror, the focal length of the secondary mirror, and the off-axis alignment of the primary and the secondary mirror have been evaluated as a function of path length, including variable input-beam divergence, between the spectrometer and the cat's-eye. The paraboloidal mirrors comprising the primary and secondary of the cat's-eye retroreflectors tested were made in our laboratory by spin casting liquid epoxy-graphite composite mixtures followed by in situ polymerization with no postpolishing.     

Measurements of the hard-x-ray reflectivity of iridium

In connection with the design of a hard-x-ray telescope for the Constellation X-Ray Observatory we measured the reflectivity of an iridium-coated zerodur substrate as a function of angle at 55, 60, 70, and 80 keV at the National Synchrotron Light Source of Brookhaven National Laboratory. The optical constants were derived from the reflectivity data. The real component of the index of refraction is in excellent agreement with theoretical values at all four energies. However, the imaginary component, which is related to the mass attenuation coefficient, is 50% to 70% larger at 55, 60, and 70 keV than theoretical values.     

Fabrication and alignment issues for segmented mirror telescopes

There is a great demand for new telescopes that use larger primary mirrors to collect more light. Because of the difficulty in the fabrication of mirrors larger than 8 m as a single piece, they must be made with numerous smaller segments. The segments must fit together to create the effect of a single mirror, which presents unique challenges for fabrication and testing that are absent for monolithic optics. This is especially true for the case of a highly aspheric mirror required to make a short two-mirror telescope. We develop the relationship between optical performance of the telescope and errors in the manufacture and operation of the individual segments.     

Development of a pumping laser system for x-ray laser research

A two-beam chirped-pulse-amplification Nd:glass laser system dedicated to x-ray laser research is described. Each beam provides an output energy of 20 J with a typical pulse duration of 1.3 ps. A prepulse of variable duration is generated by use of a novel, to our knowledge, optical system. A reflection optical system, comprised of an off-axis parabolic mirror and a spherical mirror, produces a line focus with 6-mm length and 165-microm width without chromatic aberration. By use of this pumping laser system, the nickel-like silver x-ray laser at a wavelength of 13.9 nm has been demonstrated.     

Reflection-wavelength control method for layer-by-layer controlled x-ray multilayer mirrors

A reflection-wavelength control method for a layer-by-layer controlled x-ray multilayer mirror without interface roughness is proposed. The reflection wavelength of the multiperiodic mirror is found to be simply determined by a combination ratio of periodic layers. Multiperiodic x-ray mirrors with reflectance wavelengths at 3.374 nm (C VI 1s-2p) and 3.950 nm (Ca XVIII 3d-5f) are successfully designed.     

Reflectivity and optical surface height requirements in a broadband coronagraph. 1. Contrast floor due to controllable spatial frequencies

We derive the broadband contrast floor in a coronagraphic telescope having nonideal optical surfaces. We consider only fundamental spatial frequencies within the control bandwidth of the coronagraph's deformable mirror. Cross terms arising from the beating of spatial frequencies beyond the deformable mirror control bandwidth will be considered in a second paper. Two wavefront control systems are analyzed:a zero-path difference Michelson interferometer with two deformable mirrors at a pupil image, and a sequential pair of deformable mirrors with one placed at a pupil image. We derive requirements on optical surface figure and reflectivity uniformity for both cases.     

Micropore x-ray optics using anisotropic wet etching of (110) silicon wafers

To develop x-ray mirrors for micropore optics, smooth silicon (111) sidewalls obtained after anisotropic wet etching of a silicon (110) wafer were studied. A sample device with 19 microm wide (111) sidewalls was fabricated using a 220 microm thick silicon (110) wafer and potassium hydroxide solution. For what we believe to be the first time, x-ray reflection on the (111) sidewalls was detected in the angular response measurement. Compared to ray-tracing simulations, the surface roughness of the sidewalls was estimated to be 3-5 nm, which is consistent with the atomic force microscope and the surface profiler measurements.     

Electroform replication used for multiple x-ray mirror production

Work done to produce x-ray mirrors via electroform replication is reported. Several advances have been made over previous work. We have produced lower grazing incidence angle (30 min) mirrors, obtained quantitative measurements up to higher energies (6.40 keV), produced about four times as many replicas from one mandrel, and obtained angular resolutions better than other work done with replicated metal mirrors.     

Variable Magnification With Kirkpatrick-Baez Optics for Synchrotron X-Ray Microscopy

We describe the distinction between the operation of a short focal length x-ray microscope forming a real image with a laboratory source (convergent illumination) and with a highly collimated intense beam from a synchrotron light source (Köhler illumination). We demonstrate the distinction with a Kirkpatrick-Baez microscope consisting of short focal length multilayer mirrors operating at an energy of 8 keV. In addition to realizing improvements in the resolution of the optics, the synchrotron radiation microscope is not limited to the usual single magnification at a fixed image plane. Higher magnification images are produced by projection in the limit of geometrical optics with a collimated beam. However, in distinction to the common method of placing the sample behind the optical source of a diverging beam, we describe the situation in which the sample is located in the collimated beam before the optical element. The ultimate limits of this magnification result from diffraction by the specimen and are determined by the sample position relative to the focal point of the optic. We present criteria by which the diffraction is minimized.