Grazing Incidence Telescope Research Articles

3-D x-ray mirror metrology with a vertical scanning long trace profiler (abstract)

The long trace profiler (LTP) was originally developed at Brookhaven National Laboratory for the specific purpose of measuring the surface figure of large cylindrical mirrors used at grazing incidence in synchrotron radiation (SR) beamlines. In its original configuration, it could measure only along one line down the center of the cylinder. A single linear profile is often sufficient to gauge the quality of the optical surface on these kinds of mirrors. For some applications it is necessary to measure the topography of the entire surface, not just along one line but over a grid that covers the entire surface area. We have modified a standard LTP to enable measurement of the complete surface of Wolter telescope optics in a vertical configuration. The vertical scanning LTP (VSLTP) is capable of producing a complete 3-D map of the surface topography errors relative to the ideal desired surface on complete segments of paraboloids and hyperboloids. The instrument uses a penta prism assembly to scan the probe beam in the longitudinal direction parallel to the mirror symmetry axis and uses a precision rotary stage to provide scans in the azimuthal direction. A Risley prism pair and a dove prism are used to orient the probe beam in the proper direction for the azimuthal scans. The repeatability of the prototype instrument is better than 20 nm over trace lengths of 35 mm with a slope measurement accuracy of about 1 microradian.

Spectroscopy, imaging and Compton-scatter polarimetry with a germanium strip detector

Germanium strip detectors combine the excellent energy resolution possible with germanium detectors with fine two-dimensional spatial resolution determined by orthogonal strip electrodes. We are testing a detector with an active volume of 5/spl times/5/spl times/1 cm/sup 3/ and a total of 50 electrodes, 25 on each side with a 2 mm pitch. Potential astrophysics applications include use as a focal plane for a coded-aperture telescope in the energy range /spl sim/5-500 keV, as the focal plane of a grazing-incidence telescope in the /spl sim/5-100 keV band, or in a Compton scatter telescope. The utilization of germanium strip detectors for these applications requires proof of their performance capabilities in terms of energy and spatial resolution, and efficiencies for single and multi-pixel interactions. In this paper, we demonstrate the excellent spectroscopy and imaging performance of a 2 mm pitch strip detector, and report on the results of Monte-Carlo simulations of photopeak efficiencies for single and multi-pixel interactions. These simulations show that 2-pixel interactions are a significant fraction of the photopeak efficiency in the 80-500 keV range. We demonstrate that these interactions can be used to measure linear polarization of a normally incident beam.

Application of multilayers to VUV and X-ray astronomy

The progress of multilayers makes it possible to fabricate a normal incidence telescope with high angular resolution and wide field of view in the wavelength region 30–300A, though the sensitive wavelength region is limited by the Bragg condition. The sensitive energy region of a grazing incidence telescope is drastically extended up to 100keV by means of depth-graded multilayers which satisfy the Bragg condition in the broad energy band. A multilayer coated grating is also useful dispersive element with high efficiency and spectral resolution in the 1–300A region. This development is expected to open up a new field in astronomical imaging and spectroscopic observations, which are not accessible by present telescopes.

The pn-CCD as focal plane detector for the XMM satellite mission

The X-ray Multi Mirror (XMM) satellite mission is one of the major European Space Agency (ESA) space missions scheduled for launch by the end of this decade. Its goal is to observe the x-ray-emitting sky in the 100 eV to 15 keV photon energy range with high spectral and spatial resolution. XMM's scientific instruments consist mainly of three independent Wolter telescopes each having an imaging charge-coupled-device (CCD) detector in the focal plane. Two of the three telescopes are equipped with CCDs based on the conventional MOS-CCD technology. The third is equipped with pn-CCDs, a new developement of the semiconductor laboratory of the Max-Planck-Institut fur extraterrestrische Physik. The pn-CCD has several features such as full detector depletion, parallel readout and good radiation hardness due to the replacement of the MOS structures with pn-diode electrodes. The design of the pn-CCD detector for XMM is shown and measurements with pn-CCD prototype units are presented.

Polarizing properties of grazing-incidence x-ray mirrors: comment

We show that grazing-incidence telescopes, like those used for x-ray imaging, present negligible instrumental polarization. This property does not depend on the number of reflections the telescope employs to lead light from the entrance pupil to the focal plane. The result applies to the various mirrors of the advanced x-ray astrophysics facility satellite. In this particular case we have quantified the residual instrumental polarization to be between 10(-3) and 5 × 10(-5), depending on the size of the resolution elements.

Polarization aberration analysis of the advanced x-ray astrophysics facility telescope assembly: errata

Reference 1 contains a serious error that renders most of the results inapplicable to grazing-incidence telescopes and creates an incorrect impression as to the polarization properties of grazing-incidence telescopes. The errors arise from an incorrect application of the sign convention for the Fresnel equations. The Fresnel equations are correct as stated in Eqs. (2) and (3) but are incorrectly assembled into the Jones matrix in Eq. (8), where the minus sign of ap should be removed. This equation is at the heart of the technical development, causing errors in many of the remaining equations. The retardance is incorrectly calculated by Eq. (5) and is plotted incorrectly in Fig. 4. Near grazing incidence, the linear retardance associated with reflection from metals, approaches zero ( 0) as the angle approaches grazing incidence (i 900). The paper incorrectly states that -> r rad. Thus a grazing-incidence mirror acts as a nonpolarizing element, not as a half-wave linear retarder. The polarization aberration function for the primary mirror in Jones matrix form [Eq. (16)] should be 1 0 J(P, 0) 0 1), (1)

Characterization of x-ray transmission gratings

Self-supporting transmission gratings with periods of 1 microm or below are used in combination with grazing-incidence telescopes in celestial x-ray astronomy. They can be produced with sizes up to only a few cm(2); therefore, several hundreds or even thousands of individual elements are needed in order to cover the aperture of a telescope. This large number leads to the problem of characterization of the gratings regarding their x-ray performance. We demonstrate that spectrometry in the resonance domain using H polarization is a suitable method for the determination of the grating wire profile and deviations of the grating surface from a plane. Although developed originally for microwave applications it can be shown that the methods of strict solution of the Helmholtz equation are able to explain even small effects related to imperfections of periodic submicrometer structures.

Transmission grating spectroscopy and the Advanced X-Ray Astrophysics Facility

The use of transmission gratings with grazing-incidence telescopes in celestial x-ray astronomy is reviewed. The basic properties of transmission grating spectrometers and the use of "phased" gratings to enhance the diffraction efficiency are outlined. Special attention is given to the AXAF high-energy transmission grating (HETG) being fabricated at MIT. The HETG operates over the range 0.4 to 8 keV, gives resolving powers of 100 to 1000, effective areas of 10 to 300 cm², and a minimum detectable line flux of 1 to 10 x 10^-6 photons cm^-2 s^-1. The instrument consists of a single array with two types of membrane-supported grating facets: medium-energy gratings (0.6-μm period, 0.5-μm-thick silver) mounted behind the outer three AXAF mirrors, and high-energy gratings (0.2-μm period, 1.0-μm-thick gold) mounted behind the inner three mirrors. The materials and thicknesses are selected to maximize efficiency throughout the energy band. The facets are fabricated at MIT using a process involving x-ray lithography. AXAF will also carry a low-energy transmission grating (LETG) supplied by the Laboratory for Space Research at Utrecht. It uses mesh-supported grating facets of 1.0-μm period and is optimized for operation down to 80 eV. Gratings are most effective for the study of point sources, but they also give moderate resolution spectra of slightly extended sources and can be used to map the spatial distribution of line-emitting regions.

The interpretation of the spectral line intensities from the CHASE spectrometer on Spacelab 2

In this paper we analyse the solar spectral intensities observed with the CHASE grazing incidence telescope and spectrometer flown on NASA's Spacelab 2 Mission in 1985. Our main purpose has been to investigate the sources of error that arise in the application of the differential emission measure technique used to analyse such data. We suggest methods by which these sources of error may be investigated.

Formula for the rms blur circle radius of Wolter telescope based on aberration theory

A formula for the rms blur circle for Wolter telescopes has been derived using the transverse ray aberration expressions of Saha, Appl. Opt.24, 1856– 1863 ( 1985), Saha, Proc. Soc. Photo-Opt. Instrum. Eng.444, 112– 117 ( 1984), and Saha, Proc. Soc. Photo-Opt. Instrum. Eng.640, 10– 19 ( 1986). The resulting formula for the rms blur circle radius over an image plane and a formula for the surface of best focus are based on third-, fifth-, and seventh-order aberration theory predict results in good agreement with exact ray tracing. It has also been shown that one of the two terms in the empirical formula of VanSpeybroeck and Chase, Appl. Opt.11, 440– 445 ( 1972), for the rms blur circle radius of a Wolter I telescope can be justified by the aberration theory results. Numerical results are given comparing the rms blur radius and the surface of best focus vs the half field angle computed by skew ray tracing and from analytical formulas for grazing incidence Wolter I-II telescopes and a normal incidence Cassegrain telescope.

Formula for the rms blur circle radius of Wolter telescope based on aberration theory

A formula for the rms blur circle for Wolter telescopes has been derived using the transverse ray aberration expressions of Saha, Appl. Opt. 24, 1856-1863 (1985), Saha, Proc. Soc. Photo-Opt. Instrum. Eng. 444, 112-117 (1984), and Saha, Proc. Soc. Photo-Opt. Instrum. Eng. 640, 10-19 (1986). The resulting formula for the rms blur circle radius over an image plane and a formula for the surface of best focus are based on third-, fifth-, and seventh-order aberration theory predict results in good agreement with exact ray tracing. It has also been shown that one of the two terms in the empirical formula of VanSpeybroeck and Chase, Appl. Opt. 11, 440-445 (1972), for the rms blur circle radius of a Wolter I telescope can be justified by the aberration theory results. Numerical results are given comparing the rms blur radius and the surface of best focus vs the half field angle computed by skew ray tracing and from analytical formulas for grazing incidence Wolter I-II telescopes and a normal incidence Cassegrain telescope.

Image defects from surface and alignment errors in grazing incidence telescopes

The rigid body motions and low frequency surface errors of grazing incidence Wolter telescopes are studied. The analysis is based on surface error descriptors proposed by Paul Glenn [Opt. Eng. 23(4), 384-390 (1984)1. In his analysis, the alignment and surface errors are expressed in terms of Legendre-Fourier polynomials. Individual terms in the expression correspond to rigid body motions (decenter and tilt) and low spatial frequency surface errors of mirrors. With the help of the Legendre-Fourier polynomials and the geometry of grazing incidence telescopes, exact and approximated first order equations are derived in this paper for the cornponents of the ray intercepts at the image plane. These equations are then used to calculate the sensitivities of Wolter type I and II telescopes for the rigid body motions and surface deformations. This theory also provides a tool to predict how rigid body motions and surface errors of the mirrors compensate each other.

XUV wide field camera for ROSAT

The ROSAT project is an international collaboration between the Federal Republic of Germany, the United Kingdom, and the United States. The satellite, due to be launched in June 1990, carries a payload of two coaligned imaging telescopes: the German X-Ray Telescope (XRT), which operates in the soft x-ray band (0.1 to 2 keV or 6 to 100 A), and the UK Wide Field Camera (WFC), which operates in the XUV band (0.02 to 0.2 keV or 60 to 600 A). ROSAT will perform two main tasks in its anticipated two to four year lifetime: a six-month all-sky survey in the soft x ray and XUV bands followed by a program of pointed observations for detailed studies of thousands of individual targets. In this paper we review the design and performance of the WFC. The instrument is a grazing incidence telescope comprising a set of three nested, Wolter-Schwarzschild Type I, gold-coated aluminum mirrors with a microchannel plate detector at their common focus. Thin plastic and metal film filters define the wavelength passbands.

Alternative set of surface error descriptors for grazing incidenceoptics

Orthonormal polynomials have long been used as a convenient tool to describe optic surface errors. Previously, the products of Fourier and Legendre polynomials have been employed to describe the surface errors for grazing incidence optics. We have applied an alternate set of polynomials, Fourier-Fourier polynomials, to describe the surface errors. This new set differs functionally from the Fourier Legendre set in that each term has a finite bandwidth frequency spectrum. The advantages of this difference with respect to predictions of grazing incidence telescope performance based on measured surface figure errors are discussed.

The Potential of the Wide Field Camera on ROSAT for Investigations of the XUV Background

The ROSAT X-ray astronomy satellite, due to be launched in early 1990, will carry two separate and complementary grazing-incidence telescopes with co-aligned axes. The German X-ray telescope (XRT) will cover the soft X-ray region in the range 0.15–2 keV (6–80 Å), while the U.K. XUV Wide Field Camera (WFC) will extend coverage to beyond 200 Å. The WFC is a joint project of Leicester and Birmingham Universities, the Mullard Space Science Laboratory, and the authors' institutes. The primary objective of ROSAT is to perform an all-sky survey over a period of six months. This will be followed by a guest-observer, “pointed” phase. We briefly discuss the sensitivity of the WFC to the soft X-ray/XUV background (SXRB) and the problems and techniques associated with distinguishing the astronomical background from other sources of background.

Imaging characteristics of the Extreme Ultraviolet Explorer microchannel plate detectors

The Extreme Ultraviolet Explorer (EUVE) satellite will conduct an all-sky survey over the wavelength range from 70 AA 760 AA using four grazing-incidence telescopes and seven microchannel-plate (MCP) detectors. The imaging photon-counting MCP detectors have active areas of 19.6 cm/sup 2/. Photon arrival position is determined using a wedge-and-strip anode and associated pulse-encoding electronics. The imaging characteristics of the EUVE flight detectors are presented including image distortion, flat-field response and spatial differential nonlinearity. Also included is a detailed discussion of image distortions due to the detector mechanical assembly, the wedge-and-strip anode, and the electronics. Model predictions of these distortions are compared to preflight calibration images which show distortions less than 1.3% RMS of the detector diameter of 50 mm before correction. The plans for correcting these residual detector image distortions to less than 0.1% RMS are also presented. >

Effect of various outcoupling options on free-electron ring laser performance

The power extraction and lowest-order transverse-mode structure of a free-electron ring laser with hyperboloid-paraboloid grazing incidence telescopes are studied as a function of the method used to outcouple the beam. Four different resonator configurations, with various outcoupling and intracavity optics, are modeled using a free-electron laser, wave optics computer code. A stable ring with a diffraction grating outcoupler gives better power extraction and transverse-mode structure than unstable rings and scrapers which outcoupled either an annular ring or central spot. Efforts to reshape the intracavity beam after scraper outcoupling in order to improve interaction with the electron beam in the wiggler did not produce power extraction equivalent to that of the stable ring. >

Design and performance of high-gain free-electron lasers with grazing incidence, unstable ring resonators

Grazing incidence, ring resonators have been proposed for high-grain free-electron lasers to alleviate the problem of inordinately high irradiance on intracavity optics. Such resonators are also relatively compact, and consequently, their alignment tolerances are more manageable. A geometric design algorithm and detailed computer analysis of a high-gain free-electron ring laser with hyperboloid-paraboloid grazing incidence telescopes is presented. A 3-D wave optics computer code is used to determine the loaded-cavity transverse-mode characteristics of the high-gain unstable ring and to examine the issue of transverse-mode control as a function of the key resonator parameters. Perturbation sensitivity of the individual elements is also determined and compared to that of a corresponding long stable resonator. >

The advanced X-ray astrophysics facility (AXAF)

NASA's Advanced X-ray Astrophysics Facility (AXAF) will be an orbiting X-ray observatory, designed to address fundamental questions in astronomy and astrophysics. Its importance derives from the fact that many of the fundamental processes in space are associated with high energy particles that radiate mainly X-rays, and that the AXAF will be extremely sensitive to X-ray emission. The United States National Academy of Sciences' Astronomy Survey Committee assigned AXAF as its highest priority among major new astronomy projects in recognition of the AXAF's scientific potential. AXAF is the successor to NASA's second High Energy Astronomy Observatory, the Einstein X-ray Observatory, which operated from 1978 to 1981. Like the Einstein, the AXAF will be built around a grazing incidence telescope capable of forming X-ray images. AXAF, however, will go far beyond the Einstein in capability having ∼ 10 times the angular resolution, ∼ 100 times the sensitivity for imaging, and up to ∼ 1000 times the sensitivity for spectroscopy. The AXAF will operate in space for 15 years, and be maintained and refurbished by use of NASA's Space Station. An AXAF observer program will make the great majority of the data available to the entire scientific community.

Design, construction, and performance of the ROSAT high-resolution x-ray mirror assembly

A grazing incidence telescope has been developed for the x-ray astronomy satellite ROSAT including a verification model and a flight model. The telescope consists of a fourfold nested Wolter type I mirror assembly of 84-cm front aperture and 240-cm focal length. The verification model has been built and fully assembled to a telescope. Full aperture x-ray measurements performed in our 130-m long-beam facility are pesented and successfully compared with model predictions based on mirror metrology data. An energy independent angular resolution of 4-sec of arc half-energy width for the encircled point spread function and a mirror surface microroughness of <3 A have been achieved. Metrology of the actual flight mirrors (although not yet assembled to a telescope) indicates an even better performance.