Engineering topics

Elsner, R. F.

Publications and source records attributed to Elsner, R. F..

At least 55 records · Page 3

NASA NTRS2000

Ion Transmission to the Focal Plane of the Chandra X-Ray Observatory

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.

Kolodziejczak, J. J.↗

NASA NTRS2000

HERO: High Energy Replicated Optics for a Hard-X-Ray Balloon Payload

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 <= 30 arc seconds half power diameter. This payload will offer unprecedented sensitivity in the hard-x-ray region, with around 250 microCrab sensitivity on long-duration flights and 50-100 microCrab on ultra- long-duration balloon missions The payload consists of 16 mirror modules, each with 14 nested mirrors made from a high-strength nickel alloy, and a corresponding array of 16 focal plane detectors. An engineering demonstration flight is scheduled for the Spring of 2000, using just two mirror modules each with 3 shells, above a pair of gas-scintillation-proportional counters. This flight is intended to test a newly designed gondola pointing and aspect system and the stability of the optical bench design. The first scientific flight of the full payload is scheduled for the Fall of 2002. Full details of the payload and its capabilities will be presented together with data from various mirror-module tests. If available data from the first flight will also be presented.

Ramsey, B.↗

NASA NTRS2000

Radiation Environment of the Chandra X-Ray Observatory

The Chandra X-ray Observatory lies in a highly elliptical orbit which dips into the outer radiation belt. During the initial stages of orbital operations, Chandra's front-illuminated CCD's (but not the back-illuminated ones) experienced an unanticipated degradation of the charge-transfer efficiency. The subsequent anomaly investigation determined that moderately low-energy protons (or other ions) propagated through the mirror assembly, causing the observed damage. This paper summarizes the effort to determine the radiation environment at the Chandra focal plane and the steps taken to prevent further performance degradation by magnetospheric and solar-wind protons and other ions.

ODell, S. L.↗

NASA NTRS2000

Discovery of Spatial and Spectral Structure in the X-Ray Emission from the Crab Nebula

The Chandra X-ray Observatory observed the Crab Nebula and Pulsar During orbital calibration. Zeroth-order images with the High-Energy Transmission Grating (HETG) read-out by the Advanced CCD Imaging Spectrometer spectroscopy array (ACIS-S) show a striking richness of X-ray structure, at a resolution comparable to that of the best ground-based visible-light observations. The HETG-ACIS-S images reveal, for the first time, an X-ray knots along the inner ring and (perhaps) along the inward extension of the X-ray jet. Although complicated by instrumental effects and the brightness of the Crab Nebula, the spectrometric analysis shows systematic variations of the X-ray spectrum throughout the Nebula.

Weisskopf, M.↗

NASA NTRS1998

Progress in Using Continuum radiation for AXAF Calibration

X-ray calibration of the AXAF observatory at MSFC's 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. Further verification of AXAF response models will be presented.

Kolodziejczak, J. J.↗

NASA NTRS1998

Calibration Results for the AXAF Flux Contamination Monitor

The AXAF Flux Contamination Monitor (FCM) serves the purposes of transferring the absolute flux calibration from the ground calibration at the X-Ray Calibration Facility (XRCF) 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. 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 the XRCF in 1997. Finally, we describe the plans for FCM on-orbit measurements during observatory activation and for the subsequent analysis.

Elsner, R. F.↗

NASA NTRS1997

Uses of continuum radiation in the AXAF calibration

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.

Kolodziejczak, J. J.↗

NASA NTRS1997

Graded Multilayers not Required for Hard-X-Ray Imaging

For a given x-ray optic, a graded-multilayer coating extends the energy response to higher energies. However, this does not imply that graded multilayers are required for efficient hard-x-ray focusing. Indeed, smaller x-ray optics using conventional coatings provide advantages - namely, greater effective area per unit mass, less diffractive scattering by surface micro-roughness, and demonstrated feasibility - over larger optics coated with multilayers.

Weisskopf, M. C.↗

NASA NTRS1997

Grazing Incidence Nickel Replicated Optics for Hard X-ray Telescopes

The requirements for future hard x-ray (up to 50 keV) telescopes are lightweight, high angular resolution optics with large collecting areas. Grazing incidence replicated optics are an excellent candidate for this, type of mission, providing better angular resolution, comparable area/unit mass, and simpler fabrication than multilayer-coated foils. Most importantly, the technology to fabricate the required optics currently exists. A comparison of several hard x-ray telescope designs will be presented.

Peturzzo, J. J., III↗

NASA NTRS1997

Graded Multilayers not Required for Hard X-ray Imaging!

For a given X ray optic, a graded-multilayer coating extends the energy response to higher energies. However, this does not imply that graded multilayers are required for efficient hard X ray focusing. Indeed, smaller X ray optics using conventional coatings provide advantages -- namely, greater effective area per unit mass, less diffractive scattering by surface micro-roughness, and demonstrated feasibility - over larger optics coated with multilayers.

Weisskopf, M. C.↗

NASA NTRS1995

Advanced X-ray Astrophysics Facility (AXAF): An overview

The Advanced X-ray Astrophysics Facility (AXAF) is the x-ray component of NASA's Great Observatories. To be launched in late 1998, AXAF will provide unprecedented capabilities for high-resolution imaging, spectrometric imaging, and high-resolution disperse spectroscopy, over the x-ray band from about 0.1 keV to 10 keV. With these capabilities, AXAF observations will address many of the outstanding questions in astronomy, astrophysics, and cosmology.

Weisskopf, M. C.↗

NASA NTRS1993

Molecular contamination and the calibration of AXAF

By the end of this decade, the two missions comprising the Advanced X-ray Astrophysics Facility (AXAF) will become the X-ray component of NASA's Great Observatories. In order to meet some of the ambitious scientific objectives, the AXAF team plans to calibrate the telescopes and instruments to an accuracy of a few percent. Particulate and molecular contamination potentially limit this precision, because they would likely change between ground calibration and orbital operation. To avoid this requires careful control over procedures affecting the coated optical surfaces, to ensure that neither particulate nor molecular contamination compromise the calibration. In particular, the fractional areal coverage by particulates or condensed droplets must be less than 0.005 projected onto the aperture plane; the thickness of any molecular film must be less than a few tens of angstrom.

Elsner, R. F.↗

NASA NTRS1993

Evidence for dust contamination on the VETA-1 mirror surface

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.

Kolodziejeczak, J. J.↗

NASA NTRS1993

X-ray evidence for particulate contamination on the AXAF VETA-1 mirrors

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).

O'Dell, S. L.↗

NASA NTRS1993

The X-ray reflectivity of the AXAF VETA-I optics

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.

Kellogg, E.↗

NASA NTRS1992

The x ray reflectivity of the AXAF VETA-I optics

The x-ray reflectivity of the VETA-I optic, the outermost shell of the AXAF x-ray telescope, with a bare Zerodur surface, is measured and compared 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. The data were obtained at the x-ray calibrations facility at Marshall Space Flight Center with an electron impact x-ray source located 528 m from the grazing incidence mirror. The source used photoelectric absorption filters to eliminate bremsstrahlung continuum. The mirror has a diameter of 1.2 m and a focal length of 10 m. The incident and reflected x-ray flux are detected using two proportional counters, one located in the incident beam of x-rays at the entrance aperture of the VETA-I, and the other in the focal plane behind an aperture of variable size. Results on the variation of the reflectivity with energy as well as the absolute value of the reflectivity are presented. We also present a synchrotron reflectivity measurement with high energy resolution over the range 0.26 to 1.8 keV on a flat Zerodur sample, done at NSLS. We present evidence 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 percent and 10 percent, depending on which model for the surface composition is adopted. 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. We also discuss the approach to the final preflight calibration of the full AXAF flight mirror.

Kellogg, Edwin M.↗

NASA NTRS1991

Effective area of the AXAF X-ray telescope - Dependence upon dielectric constants of coating materials

This study examines the dependence of the effective area of the AXAF X-ray telescope upon the complex dielectric constants of possible mirror coatings, over the energy range 0.1-10 keV. At energies near and above the astrophysically important iron-line complex near 6.7 keV, the effective area is very sensitive to the coating density on the three innermost of the telescope's six mirror pairs. Thus, it is desirable to achieve as high a density as feasible. The telescope's spectral response exhibits sharp features at absorption edges of the coating materials. In view of the exceptional energy resolution of the AXAF spectrometers and uncertainties in reflectivities (especially near absorption edges), the AXAF goal of 1-percent accuracy requires both the precise X-ray calibration of the telescope and improved modeling of the telescope and calibration sources. Presented here in the form of plots, the results are also available (on floppy disks) in tabular form.

Elsner, R. F.↗

NASA NTRS1991

Predicted performance of the lithium scattering and graphite crystal polarimeter for the Spectrum-X-Gamma mission

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.

Weisskopf, M. C.↗