Search

NASA’s Marshall Space Flight Center (MSFC) maintains an active research program toward the development of high-resolution, lightweight, grazing-incidence x-ray optics to serve the needs of future x-ray astronomy missions such as Lynx. MSFC development efforts include both direct fabrication (diamond turning and deterministic computer-controlled polishing) of mirror shells and replication of mirror shells (from figured, polished mandrels). Both techniques produce full-circumference monolithic (primary + secondary) shells that share the advantages of inherent stability, ease of assembly, and low production cost. However, to achieve high-angular resolution, MSFC is exploring significant technology advances needed to control sources of figure error including fabrication- and coating-induced stresses and mounting-induced distortions.

In high energy solar astrophysics, imaging hard X-rays by direct focusing offers higher dynamic range and greater sensitivity compared to past techniques that used indirect imaging. The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload that uses seven sets of nested Wolter-I figured mirrors together with seven high-sensitivity semiconductor detectors to observe the Sun in hard X-rays through direct focusing. The FOXSI rocket has successfully flown twice and is funded to fly a third time in summer 2018. The Wolter-I geometry consists of two consecutive mirrors, one paraboloid and one hyperboloid, that reflect photons at grazing angles. Correctly focused X-rays reflect once per mirror segment. For extended sources, like the Sun, off-axis photons at certain incident angles can reflect on only one mirror and still reach the focal plane, generating a background pattern of singly reflected rays (i.e., ghost rays) that can limit the sensitivity of the observation to faint, focused sources. Understanding and mitigating the impact of the singly reflected rays on the FOXSI optical modules will maximize the instruments’ sensitivity to background-limited sources. We present an analysis of the FOXSI singly reflected rays based on ray-tracing simulations and laboratory measurements, as well as the effectiveness of different physical strategies to reduce them.

MSFC has a long history of developing full-shell grazing-incidence x-ray optics for both narrow (pointed) and wide field (surveying) applications. The concept presented in this paper shows the potential to use active optics to switch between narrow and wide-field geometries, while maintaining large effective area and high angular resolution. In addition, active optics has the potential to reduce errors due to mounting and manufacturing lightweight optics. The design presented corrects low spatial frequency error and has significantly fewer actuators than other concepts presented thus far in the field of active x-ray optics. Using a finite element model, influence functions are calculated using active components on a full-shell grazing-incidence optic. Next, the ability of the active optic to effect a change of optical prescription and to correct for errors due to manufacturing and mounting is modeled.

NASA's Marshall Space Flight Center (MSFC) engages in research, development, design, fabrication, coating, assembly, and testing of grazing-incidence optics (primarily) for x-ray telescope systems. Over the past two decades, MSFC has refined processes for electroformed-nickel replication of grazing-incidence optics, in order to produce high-strength, thin-walled, full-cylinder x-ray mirrors. In recent years, MSFC has used this technology to fabricate numerous x-ray mirror assemblies for several flight (balloon, rocket, and satellite) programs. Additionally, MSFC has demonstrated the suitability of this technology for ground-based laboratory applications-namely, x-ray microscopes and cold-neutron microscopes and concentrators. This mature technology enables the production, at moderately low cost, of reasonably lightweight x-ray telescopes with good (15-30 arcsecond) angular resolution. However, achieving arcsecond imaging for a lightweight x-ray telescope likely requires development of other technologies. Accordingly, MSFC is conducting a multi-faceted research program toward enabling cost-effective production of lightweight high-resolution x-ray mirror assemblies. Relevant research topics currently under investigation include differential deposition for post-fabrication figure correction, in-situ monitoring and control of coating stress, and direct fabrication of thin-walled full-cylinder grazing-incidence mirrors.

SuperHERO is a hard x-ray (20–75 keV) balloon-borne telescope, currently in its proposal phase, that will utilize high angular-resolution grazing-incidence optics, coupled to novel CdTe multi-pixel, fine-pitch (250 µm) detectors. The high-resolution electroformed-nickel, grazing-incidence optics were developed at MSFC, and the detectors were developed at the Rutherford Appleton Laboratory in the UK, and are being readied for flight at GSFC. SuperHERO will use two active pointing systems; one for carrying out astronomical observations and another for solar observations during the same flight. The telescope will reside on a light-weight, carbon-composite structure that will integrate the Wallops Arc Second Pointer into its frame, for arcsecond or better pointing. This configuration will allow for Long Duration Balloon flights that can last up to 4 weeks. This next generation design, which is based on the High Energy Replicated Optics (HERO) and HERO to Explore the Sun (HEROES) payloads, will be discussed, with emphasis on the core telescope components.

We are developing a balloon-borne hard-x-ray telescope that utilizes grazing incidence optics. Termed HERO, for High-Energy Replicated Optics, the instrument will provide unprecented sensitivity in the hard-x-ray region and will achieve milliCrab-level sensitivity in a typical 3-hour balloon-flight observation and 50 microCrab sensitivity on ultra-long-duration flights. A recent proof-of-concept flight, featuring a small number of mirror shells captured the first focused hard-x-ray images of galactic x-ray sources. Full details of the payload, its expected future performance and its recent measurements are provided.

As part of ongoing development efforts at MSFC, we have begun to investigate mounting strategies for highly nested xray optics in both full-shell and segmented configurations. The analytical infrastructure for this effort also lends itself to investigation of active strategies. We expect that a consequence of active figure control on relatively thin substrates is that errors are propagated to the edges, where they might affect the effective precision of the mounting points. Based upon modeling, we describe parametrically, the conditions under which active mounts are preferred over fixed ones, and the effect of active figure corrections on the required number, locations, and kinematic characteristics of mounting points.

The future of x-ray astronomy depends upon development of x-ray telescopes with larger aperture areas (approx. = 3 square meters) and fine angular resolution (approx. = 1 inch). Combined with the special requirements of nested grazing-incidence optics, the mass and envelope constraints of space-borne telescopes render such advances technologically and programmatically challenging. Achieving this goal will require precision fabrication, alignment, mounting, and assembly of large areas (approx. = 600 square meters) of lightweight (approx. = 1 kilogram/square meter areal density) high-quality mirrors at an acceptable cost (approx. = 1 million dollars/square meter of mirror surface area). This paper reviews relevant technological and programmatic issues, as well as possible approaches for addressing these issues-including active (in-space adjustable) alignment and figure correction.

Future x-ray telescopes will likely require lightweight mirrors to attain the large collecting areas needed to accomplish the science objectives. Understanding and demonstrating processes now is critical to achieving sub-arcsecond performance in the future. Consequently, designs not only of the mirrors but of fixtures for supporting them during fabrication, metrology, handling, assembly, and testing must be adequately modeled and verified. To this end, MSFC is using finite-element modeling to study the effects of mounting on full-shell grazing-incidence mirrors, during all processes leading to flight mirror assemblies. Here we report initial results of this study.

We are developing a Thomson-scattering-type polarimeter sensitive in the energy range from 10 keV to 20 keV for a small satellite. The polarimeter consists of three beryllium disks as scatterers and a cylindrical position-sensitive proportional counter as a detector of the scattered X-rays. Its performance has been investigated through computer simulations. From these, it was concluded that the polarimeter can obtain a modulation factor of 34%, a detection efficiency of /spl sim/10%, and a minimum detectable polarization of about 3.0% for Her X-1 in 2 keV bands and a 2/spl times/10/sup 5/ sec observation.

New technology in grazing-incidence mirror fabrication and assembly is necessary to achieve subarcsecond optics for large-area x-ray telescopes. In order to define specifications, an understanding of performance sensitivity to design parameters is crucial. MSFC is undertaking a systematic study to specify a mounting approach, mirror substrate, and testing method. Lightweight mirrors are typically flimsy and are, therefore, susceptible to significant distortion due to mounting and gravitational forces. Material properties of the mirror substrate along with its dimensions significantly affect the distortions caused by mounting and gravity. A parametric study of these properties and their relationship to mounting and testing schemes will indicate specifications for the design of the next generation of lightweight grazing-incidence mirrors. Here we report initial results of this study.

We provide a mathematical formalism for optimizing the mirror nodal positions along the optical axis and the tilt of a commonly employed detector configuration at the focus of a x-ray telescope consisting of nested mirror shells with known mirror surface prescriptions. We adopt the spatial resolution averaged over the field-of-view as the figure of merit M. A more complete description appears in our paper in these proceedings.

Over the past three years NASA Marshall Space Flight Center has been collaborating with Brookhaven National Laboratory to develop a modular Silicon Drift Detector (SDD) X-Ray Spectrometer (XRS) intended for fine surface mapping of the light elements of the moon. The value of fluorescence spectrometry for surface element mapping is underlined by the fact that the technique has recently been employed by three lunar orbiter missions; Kaguya, Chandrayaan-1, and Chang e. The SDD-XRS instrument we have been developing can operate at a low energy threshold (i.e. is capable of detecting Carbon), comparable energy resolution to Kaguya (

X-ray telescopes having a relatively wide field-of-view and spatial resolution vs. polar off-axis angle curves much flatter than the parabolic dependence characteristic of Wolter I designs are of great interest for surveys of the X-ray sky and potentially for study of the Sun s X-ray emission. We discuss the various considerations affecting the design of such telescopes, including the possible use of polynomial mirror surface prescriptions, a method of optimizing the polynomial coefficients, scaling laws for mirror segment length vs. intersection radius, the loss of on-axis spatial resolution, and the positioning of focal plane detectors.

We are fabricating optics for the hard-x-ray region using electroform nickel replication. The attraction of this process, which has been widely used elsewhere, is that the resulting full shell optics are inherently stable and thus can have very good angular resolution. The challenge with this process is to develop lightweight optics, and to keep down the costs of mandrel fabrication. We accomplished the former through the development of high-strength, low-stress nickel alloys that permit very thin, stable, shells without fabrication- and handling-induced deformations. For the latter, we have utilized inexpensive grinding and diamond turning to figure the mandrels and then purpose-built polishing machines to finish the surface. In-house plating tanks and a simple water-bath separation system complete the process. To date we have built shells ranging in size from 5 cm diameter to 50 cm, and with thickness down to 100 micron. For our HERO balloon program, we are fabricating over 200 iridium-coated shells, 250 microns thick, for hard-x-ray imaging up to 75 keV. Early test results on these have indicated half-power-diameters of 15 arcsec. The status of these developments will be reviewed.

The severe degradation in spectrometric performance observed by the AXAF CCD Imaging Spectrometer (ACIS) instrument on-board the Chandra X-ray Observatory (CXO) was associated with a sharp increase in charge transfer inefficiency combined with relatively constant dark current. It was also observed that damage occurred only during passage through the earth's radiation belts, and only when ACIS remained in the focal plane during the passage. Subsequent measurements and analyses support the conjecture that the damaging radiation entered through the Observatory's High-Resolution Mirror Assembly (HRMA) aperture. A mechanism whereby low energy magnetospheric protons and/or heavier ions are scattered through the HRMA and reach the focal plane with just enough energy to stop in the CCD's charge transfer region currently provides a reasonably consistent explanation of all observed phenomena. In this paper, we will describe analyses which support this conclusion. Simulations using standard ion transmission codes were used to generate a bi-directional response function (BDRF) for the mirror surfaces and transmission through the various path elements. The BDRF was convolved with the geometry via ray-trace. Damage estimates using measured proton fluence and ground measurements of ACIS- type CCD damage vs. proton energy will be presented and compared with observed on- orbit damage.

We are developing high-energy grazing-incidence replicated optics for a balloon-borne hard-x-ray telescope. When completed, the instrument will have 170 cm2 of effective collecting area at 40 keV and 130 square cm at 60 keV with

Two monochromator sources were used during the AXAF calibration at Marshall Space Flight Center's X-ray Calibration Facility. Data acquired using these sources require special analysis owing to large spatial variations in the monochromators' beams and to higher-order contributions to the spectral content. A description of the two monochromators is given, followed by a discussion of the spectral and spatial content as monitored by beam- normalization detector flow proportional counters. Emphasis is given to the methodology employed in determining the spatial content through fits to beam uniformity measurements. A model is described which adequately represents the beam spatial properties over the range of source conditions encountered during calibration.

The AXAF X-ray mirrors underwent thorough calibration using the X-ray Calibration Facility (XRCF) at the Marshall Space Flight Center in Huntsville, AL from late 1996 to early 1997. The x-ray calibration made novel use of the x-ray continuum from a conventional electron-impact source. Taking advantage of the good spectral resolution of solid-state detectors, continuum measurements proved advantageous in calibration the effective area of AXAF's High-Resolution Mirror Assembly (HRMA) for the entire AXAF energy band. The measurements were made by comparing the spectrum detected by a SSD at the focal plane with the spectrum detected by a beam normalization SSD. The HRMA effective area was calibrated by comparing the measurements with the HRMA raytrace model. The HRMA on-orbit performance predictions are made using the calibration results.

The Flux Contamination Monitor (FCM) on the Advanced X-ray Astrophysics Facility (AXAF) serves the purposes of transfering the absolute flux calibration from the ground calibration at the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center to operation on orbit and of detecting any changes in molecular contamination of the High Resolution Mirror Assembly (HRMA) between ground calibration and the post-launch activation phase. we describe the design, construction, and characterization of the FCM radioactive sources, and their placement on the Forward Contamination Cover (FCC). We present results from FCM measurements with the AXAF focal plane instruments, particularly the AXAF CCD Imaging Spectrometer (ACIS), during the ground calibration phase at XRCF in 1997. Finally, we describe the plans for FCM on-orbit measurements during observatory activation and the subsequent analysis.

The AXAF High-Resolution Camera (HRC) was calibrated at NASA MSFC's X-Ray Calibration Facility (XRCF) during 1997 March and April. We have undertaken an analysis of the HRC effective area using all data presently available from the XRCF. We discuss our spectral fitting of the beam-normalization detectors (BNDs), our method of removing higher order contamination lines present in the spectra, and corrections for beam non-uniformities. We apply a model of photon absorption depth in order to fit a smooth curve to the quantum efficiency of the detector. This is then combined with the most recent model of the AXAF High-Resolution Mirror Assembly (HRMA) to determine the ensemble effective area versus energy for the HRC. We also address future goals and concerns.

The Stellar X-Ray Polarimeter (SXRP) will be more than an order of magnitude more sensitive than any previous x-ray polarimeter in the 2-15 keV energy band. The SXRP is a focal plane detector for a Danish-Russian SODART telescope, which will be launched on the Russian Spectrum-X-Gamma (SXG) mission. The SXRP exploits the polarization dependence of Bragg reflection from a graphite crystal, and of Thomson scattering from a target of metallic lithium. The SXRP flight model (FM) was calibrated at a facility at Lawrence Livermore National Laboratory (LLNL) equipped with polarized and unpolarized x-ray sources producing x-rays in the band pass for the graphite and the lithium scatterers. By adjusting the orientation of the SXRP with respect to the incident x-ray beam, it was possible to simulate the converging beam from a SODART telescope and to measure the SXRP response to telescope pointing errors. In this paper we describe the SXRP-FM calibration and present results for the graphite polarimeter.

X-ray calibration of the Advanced X-ray Astrophysics Facility (AXAF) observatory at the MSFC X-Ray Calibration Facility (XRCF) made novel use of the x-ray continuum from a conventional electron-impact source. Taking advantage of the good spectral resolution of solid-state detectors, continuum measurements proved advantageous in calibrating the effective area of AXAF's High-Resolution Mirror Assembly (HRMA) and in verifying its alignment to the XRCF's optical axis.

The prelaunch calibration of AXAF encompasses many aspects of the telescope. In principle, all that is needed is the complete point response function. This is, however, a function of energy, off-axis angle of the source, and operating mode of the facility. No single measurement would yield the entire result. Also, any calibration made prior to launch will be affected by changes in conditions after launch, such as the change from one g to zero g. The reflectivity of the mirror and perhaps even the detectors can change as well, for example by addition or removal of small amounts of material deposited on their surfaces. In this paper, we give a broad view of the issues in performing such a calibration, and discuss how they are being addressed in prelaunch preparation of AXAF. As our title indicates, we concentrate here on the total throughput of the observatory. This can be thought of as the integral of the point response function, i.e. the encircled energy, out to the largest practical solid angle for an observation. Since there is no standard x-ray source in the sky whose flux is well known to the approximately 1% accuracy we are trying to achieve, we must do this calibration on the ground. We also must provide a means for monitoring any possible changes in this calibration from prelaunch until on-orbit operation can transfer the calibration to a celestial x-ray source whose emission is stable. In the paper, we analyze the elements of the absolute throughput calibration, which we call the effective area. We review the requirements for calibrations of components or subsystems of the AXAF facility, including the mirror, detectors, and gratings. We show how it is necessary to have an absolute calibrated detection system available during the prelaunch calibrations to measure the flux in the x-ray beam used for calibrating AXAF. We show how it is necessary to calibrate this ground-based detection system at standard man-made x-ray sources, such as electron storage rings. We present the status of all these calibrations, with indications of the measurements remaining to be done, even though the measurements on the AXAF flight optics and detectors will have been completed by the time this paper is presented. We evaluate progress toward the goal of making 1% measurements of the absolute x-ray flux from astrophysical sources, so that comparisons can be made with their emission at other wavelengths, in support of observations such as the Sunyaev-Zeldovitch effect, which can give absolute distance measurements independent of the traditional distance measuring techniques in astronomy.

The Advanced X ray Astrophysics Facility (AXAF) contains two focal-plane science instruments -- the microchannel-plate high-resolution camera (HRC) and the AXAF CCD imaging spectrometer (ACIS). Each of these instruments provides two low-internal- background detectors, one for imaging and the other for reading out AXAF's objective transmission gratings. Taking maximum advantage of the low internal background of these instruments requires shielding against external electromagnetic and particle radiation. To optimize the shielding within weight constraints, we performed extensive numerical simulations of the photon transport and of the particle transport and activation within the AXAF. We discuss the simulations and report the results of the shielding study. With the adopted shielding design, the particle-induced background and the x-ray-induced background are each comparable to or less than the anticipated internal background of AXAF's imaging detectors (HRC and ACIS). Thus, the predicted total non-imaged background is very low -- less than 0.2 counts arcsec-2 day-1 for the HRC and even lower for the ACIS half-arcsec angular resolution, this low background has negligible effect upon point-source detection, for reasonable observation durations.© (1995) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

In preparation for calibrating the Advanced X-ray Astrophysics Facility, a multicomponent x- ray source system has been assembled for use at the x-ray calibration facility at MSFC. The system consists of an electron impact point source with filters, a penning gas discharge source, and two monochromators fed by rotating anode x-ray generator. The purpose and predicted performance characteristics of these elements are described as they apply to the AXAF calibration. The planned source characterizations, performed in June 1995 through June 1996 time period, are also described.

We have completed extensive synchrotron reflectivity measurements on several iridium mirrors which were intentionally coated with thin layers (100 angstroms or less) of polyethylene, a hydrocarbon contaminant. The purpose was to verify theoretical predictions of alterations in reflection efficiency of an iridium surface for various thicknesses of hydrocarbon contamination, and to evaluate the acceptability of attainable upper limits of such contamination for the mirrors aboard NASA's Advanced X-ray Astrophysics Facility (AXAF). Although the deposition of such thin layers is problematic with no systematic guarantee of uniform thickness or density, successful analysis by modeling the contaminant as a uniform surface layer may be done, within a limited X-ray energy range. The M-edges of iridium are significantly affected by the polyethylene layers. For the most part, contamination increases the reflectance in the M-edge range over that of bare iridium, although cross-over points between contaminated and uncontaminated mirrors occur at several angles relevant to AXAF. However, calibratability of the reflectance is a more significant issue than X-ray mirror efficiency. We present the modeling results for three thicknesses of polyethylene, and discuss the implications for the performance of AXAF mirrors and their calibratability.

Grazing-incidence optics are quite sensitive to molecular and particulate contamination. For NASA's Advanced X-ray Astrophysics Facility-Imaging (AXAF-I), the sensitivities of the flux scale to contamination on the mirrors' surfaces are typically 0.4% per angstrom of a hydrocarbon film and 0.4% per 10-5 fractional areal coverage by particulates. In view of the goal of calibrating the flux-scale to a few-percent accuracy, it is clearly essential to adopt rigorous procedures to control and to monitor both molecular and particulate contamination. Traditional approaches to monitoring contamination for optical surfaces--using optical witness samples in conjunction with facility airborne monitors--are essential during ground operation. However, such techniques do not directly monitor changes in x-ray reflectance or even in contamination of the mirrors themselves; nor can they determine differences in contamination between ground calibration and orbital operations. In order to transfer the AXAF-I flux scale from ground to orbit, AXAF-I will incorporate x-ray- fluorescing radionuclide sources in the forward contamination cover. Comparison of the telescope's throughput, for these x rays, at ground calibration with that at the commencement of orbital operations will validate the flux-scale calibration. If ground-to-orbit changes were to occur, such a comparison would facilitate correction of the flux scale.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

The performance of the engineering prototype Stellar X-Ray Polarimeter (SXRP) has been evaluated. One hundred percent polarized monochromatic x rays at 2.6 keV and 9.7 keV were used to measure the response of the instrument in the energy bands of the graphite and lithium polarizing elements, respectively. On-line analysis showed that the respective depths of modulation are 96% ad 70% as expected. Irradiating SXRP with broadband unpolarized x rays in the energy band 2 - 17 keV demonstrated that the level of spurious modulation inherent in the instrument is less than 3%. Up-to-date results are presented and compared to the predictions of Monte Carlo simulations.

The study measures the X-ray reflectivity of the AXAF VETA-I optic and compares it with theoretical predictions. Measurements made at energies of 0.28, 0.9, 1.5, 2.1, and 2.3 keV are compared with predictions based on ray trace calculations. Results on the variation of the reflectivity with energy as well as the absolute value of the reflectivity are presented. A synchrotron reflectivity measurement with a high-energy resolution over the range 0.26 to 1.8 keV on a flat Zerodur sample is also reported. Evidence is found for contamination of the flat by a thin layer of carbon on the surface, and the possibility of alteration of the surface composition of the VETA-I mirror, perhaps by the polishing technique. The overall agreement between the measured and calculated effective area of VETA-I is between 2.6 and 10 percent. Measurements at individual energies deviate from the best-fitting calculation to 0.3 to 0.8 percent, averaging 0.6 percent at energies below the high energy cutoff of the mirror reflectivity, and are as high as 20.7 percent at the cutoff.

Relative surface brightness data from the VETA-1 test displayed an energy dependence which was inconsistent with models which assume surface roughness is the only cause of scattering. A means of separating an in-plane component of scattering caused by surface roughness from an azimuthally symmetric component, which is expected from dust, was afforded by images taken while only single quarters of the mirror surface were exposed. In this case, in-plane scattered X-rays should populate only opposing 90 deg azimuthal quadrants of an image centered on the focal point. As such, this approach is a novel, high sensitivity test for detecting extremely small (about 10 exp -5) fractional dust coverage on X-ray optics. Comparison of in-plane and out-of-plane quadrants in these tests indicated that most of the scattered data were azimuthally symmetric, and that the symmetric component was enhanced at lower energies. Both results support a model which invokes the wing scan results with a combination of dust and surface roughness induced scattering. The extent to which parameters such as the mirror surface roughness, dust size distribution, spatial distribution, and density may be determined using this approach is also discussed.

X-ray testing of the AXAF outer mirror pairs, in the VETA-1 configuration, reveals a point spread function (PSF) with unexpectedly large wings at low energies. Although the angular dependence in the wings of the PSF is close to that expected for diffractive scattering from surface roughness, the energy dependence differs substantially. Analyses of the observed X-ray PSF, images near ring focus, and single-quadrant images at conjugate focus suggest that the excess scattering observed at low X-ray energies results from diffractive scattering by relatively small grains (as small as a few tenths micrometer in radius). We develop a simple model for the contribution of scattering by particulates to the PSF. Merging this model with that for scattering by surface roughness, we fit the combined model to the observed energy-dependent PSF, in order to estimate parameters and associated uncertainties characterizing the grain-size distribution and the surface-roughness power spectral density. In particular, we find that the fractional coverage of the mirrors by particulates is approximately 1 x 10 exp -4 (for grain radii between 0.1 and 10 microns), and that the rms surface-roughness is approximately 0.7 nm (for spatial frequencies between 1/mm and 1000/mm).

A Bragg crystal polarimeter for the focal plane of the SODART telescope on the Spectrum-X-Gamma mission is being designed. A mosaic graphite crystal will be oriented at 45 deg to the optic axis of the telescope, thereby preferentially reflecting those X-rays which satisfy the Bragg condition and have electric vectors that are perpendicular to the plane defined by the incident and reflected photons. The reflected X-rays will be detected by an imaging proportional counter with the image providing direct X-ray aspect information. The crystal will be about 50 microns thick to allow X-rays with energies of 4 keV or greater to be transmitted to a lithium block mounted below the graphite. The lithium is used to measure the polarization of these high energy X-rays by exploiting the polarization dependence of Thomson scattering. The development of thin mosaic graphite crystals is discussed and recent reflectivity, transmission, and uniformity measurements are presented.

X-ray scattering from a lithium disk and Bragg reflection from a mosaic graphite crystal can be exploited to measure the linear polarization of radiation emitted from cosmic X-ray sources. The sensitivity is greatly enhanced if these polarimeters are placed at the focus of an X-ray telescope. Such devices form two of the three components of the Stellar X-Ray Polarimeter experiment scheduled to fly on the Spectrum-X-Gamma mission. The experiment will reside at the focus of one of the SODART X-ray telescopes. The expected on-axis performance of these two components of the Stellar X-Ray Polarimeter experiment based on detailed Monte-Carlo simulation is described. Various systematic effects, both external and internal to the experiment, that must be considered in order to properly design and utilize the experiment are also discussed.

X-ray scattering from a lithium disk and Bragg reflection from a mosaic graphite crystal can be exploited to measure the linear polarization of radiation emitted from cosmic X-ray sources. The sensitivity is enhanced if the polarimeters are placed at the focus of an X-ray telescope. Such devices form two of the components of the Stellar X-ray Polarimeter experiment scheduled to fly on the Spectrum-X-Gamma mission. The expected on-axis performance of the two components is described based on detailed Monte Carlo simulations. The polarimetry experiment is expected to provide sensitive measurements of linear polarization for many cosmic X-ray sources. The nature and utility of such observations is described for pulsing X-ray sources such as the Crab pulsar and Her X-1.

Scattering from bound electrons in a suitable material placed at the focus of an X-ray telescope can be exploited to measure the linear polarization of radiation emitted from cosmic X-ray sources. Among the factors that affect the performance of such an instrument is a spurious polarization signal from unpolarized sources that lie within the field of view but are offset from the telescope pointing direction. This paper presents the results of analytical and Monte Carlo studies of this effect and provides means of evaluating its impact on realistic polarimeters for X-ray astronomy.

A Bragg crystal oriented at 45 deg to an incoming beam of X-rays acts as a polarization analyzer. This crystal geometry preferentially reflects those X-rays that satisfy the Bragg condition and whose electric vectors are perpendicular to the plane defined by the incident and reflected photons. X-rays with electric vectors parallel to this plane of incidence are photoelectrically absorbed. The energy bandwidth of nearly perfect crystals is extremely small, which makes them very inefficient X-ray polarimeters. This limitation is particularly acute for observations of the relatively weak X-ray continuum of stellar sources. The bandwidth can be greatly increased by employing mosaic or ideally imperfect crystals. Mosaic crystals possess a high integrated reflectivity, which results in a large increase in the reflection of continuum radiation. A review of the theory and performance characteristics of crystal polarimeters designed for observations of cosmic X-ray sources is presented.

It has been suggested that one may build an X-ray polarimeter by exploiting the polarization dependence of the angular distribution of L-shell fluorescence photons. In this paper we examine, theoretically, the sensitivity of this approach to polarimetry. We apply our calculations to several detection schemes using imaging proportional counters that would have direct application in X-ray astronomy. We find, however, that the sensitivity of this method for measuring X-ray polarization is too low to be of use for other than laboratory applications.

A polarimeter utilizing the polarization dependence of x-ray scattering from the low Z metal Lithium will be flown on the SPECTRUM-X-Gamma missionl. The instrument will reside at the focus of one of the SODART x-ray telescopes. An important design consideration is the spurious polarization signature which occurs when unpolarized incident flux is focused a relatively small distance away from the precise axis of symmetry. In this paper we present the results of analytical and Monte-Carlo studies of this effect.

SPIE grants to authors (and their employers) of papers, posters, and presentation recordings published in SPIE Proceedings or SPIE Journals on the SPIE Digital Library (hereinafter "publications") the right to post an author-prepared version or an official version (preferred version) of the publication on an internal or external server controlled exclusively by the author/employer or the entity funding the research, provided that (a) such posting is noncommercial in nature and the publication is made available to users without charge; (b) an appropriate copyright notice and citation appear with the publication; and (c) a link to SPIE's official online version of the publication is provided using the item's DOI. This authorization does not extend to third-party web sites not owned and maintained by the author/employer such as ResearchGate, Academia.edu, YouTube, etc. The Imaging X-ray Polarimetry Explorer (IXPE) will add polarization to the properties (time, energy, and position) observed in x-ray astronomy. A NASA Astrophysics Small Explorer (SMEX) in partnership with the Italian Space Agency (ASI), IXPE will measure the 2-8-keV polarization of a few dozen sources during the first 2 years following its 2021 launch. The IXPE Observatory includes three identical x-ray telescopes, each comprising a 4-m-focal-length (grazingincidence) mirror module assembly (MMA) and a polarization-sensitive (imaging) detector unit (DU), separated by a deployable optical bench. The Observatory's Spacecraft provides typical subsystems (mechanical, structural, thermal, power, electrical, telecommunications, etc.), an attitude determination and control subsystem for 3-axis stabilized pointing, and a command and data handling subsystem communicating with the science instrument and the Spacecraft subsystems.

The pulsing X-ray source 4U 1626-67 was observed with the Monitor Proportional Counter (MPC) on board the Einstein X-Ray Observatory on three occasions in early 1979. The MPC is a sealed, argon-filled proportional counter with a 1.5 mil beryllium window and is coaligned with the X-ray telescope on board the observatory. The spectral data spanning the energy range from 1.1 to 21 keV are divided into eight logarithmically spaced energy channels which integrate for 2.56 s. The Time Interval Processor (TIP) circuitry of the MPC measures time intervals between events to within 1 microsecond or 1.6%, whichever is larger, for a count rate dependent fraction of all events in all eight energy channels. It is found that in the energy range from 1 to 21 keV, the spectrum and 7.7 s pulse shape of 4U 1626-67 are variable on a time scale of minutes. The pulse shape variations correlate in a complex way with the intensity variations in this source.