Showing papers by "Smithsonian Astrophysical Observatory published in 1993"

A catalog of intracluster gas temperatures

Abstract: We have searched the Einstein Monitor Proportional Counter (MPC) data base for observations of clusters of galaxies. By coadding the MPC spectra obtained during all pointed observations of clusters with IPC count rates greater than 0.1 counts/s, we have obtained sufficient photon statistics to estimate the X-ray temperature of 84 clusters. Combining the MPC results with EXOSAT and Ginga results reported in the literature yields a combined sample of 104 clusters with known X-ray temperatures. One of the best studied X-ray correlations between clusters is that between their X-ray luminosity and gas temperature. We show that the best-fit power-law relation for our combined cluster sample can be explained by the observed increase in the gas-to-stellar mass ratio between low- and high-temperature clusters. The statistical significance of any evolution in our combined X-ray sample has been examined and compared with the statistical properties of clusters culled from optical catalogs. We find that there is strong evidence for a decrease in the X-ray luminosity of optically rich clusters beyond z approximately about 0.06. This result is used to estimate the normalization of the primordial power spectrum of density fluctuations.

Asteroid spectroscopy: Progress and perspectives

Abstract: A broad understanding of meteorite and asteroidal studies is needed for the most dynamic progress of both disciplines, in their common goal of understanding the origin and early evolution of the solar system. Asteroid compositional investigations are currently shifting from the survey mode to focused investigations of selected targets, in pursuit of specific problems. Attention is presently given the compositional nature and diversity of asteroid taxonomic classes, as addressed by the Small Main-Belt Asteroid Spectroscopic Survey.

Stellar coronae at the end of the main sequence: A Rosat survey of the late M dwarfs

Abstract: We present X-ray data, both detections and upper limits, from the ROSAT all-sky survey for most known M dwarfs later than type M5, as well as from selected ROSAT pointed observations of some of these stars. We compare these data with similar data for early M dwarfs in an attempt to probe the nature of the magnetic dynamo and coronal heating mechanism for the very late M dwarfs, which are presumably totally convective. Our results indicate that late M dwarfs can have coronae which are just as active as those for the early M dwarfs and that coronal heating efficiency for 'saturated' stars does not drop at spectral type M6.

A tight correlation between radio and X-ray luminosities of M dwarfs

Abstract: We present results of a survey of nonflare radio and X-ray properties of dM/dMe stars. This survey was obtained during the ROSAT All-Sky Survey and is accompanied by mostly simultaneous VLA observations. We find that the X-ray and radio luminosities are correlated over three orders of magnitude, L(R) is proportional to L(X), irrespective of spectral type. This result improves if strictly simultaneous observations are considered. This correlation points to a physical relation between the particle populations responsible for the two emissions.

The spectrum and variability of radio emission from AE Aquarii

Abstract: The first detections of the magnetic cataclysmic variable AE Aquarii at millimeter wavelengths are reported. AE Aqr was detected at wavelengths of 3.4 and 1.25 mm. These data are used to show that the time-averaged spectrum is generally well fitted by a power law S(nu) varies as nu exp alpha, where alpha is approximately equal to 0.35-0.60, and that the power law extends to millimeter wavelengths, i.e., the spectral turnover is at a frequency higher than 240 GHz. It is suggested that the spectrum is consistent with that expected from a superposition of flarelike events where the frequency distribution of the initial flux density is a power law f (S0) varies as S0 exp -epsilon, with index epsilon approximately equal to 1.8. Within the context of this model, the high turnover frequency of the radio spectrum implies magnetic field strengths in excess of 250 G in the source.

The Einstein Observatory catalog of IPC x ray sources. Volume 3E: Right ascension range 04h 00m to 07h 59m

Abstract: The Einstein Observatory (HEAO-2, launched November 13, 1978) achieved radically improved sensitivity over previous x-ray missions through the use of focusing optics which simultaneously afforded greatly reduced background and produced true images. During its 2.5-yr mission, the Einstein X-Ray Telescope was pointed toward some 5,000 celestial targets, most of which were detected, and discovered several thousand additional 'serendipitous' sources in the observed fields. This catalog contains contour diagrams and source data, obtained with the imaging proportional counter in the 0.16 to 3.5 keV energy band, and describes methods for recovering upper limits for any sky position within the observed images. The main catalog consists of six volumes (numbered 2 through 7) of right ascension ordered pages, each containing data for one observation. Along with the primary documentation describing how the catalog was constructed, volume 1 contains a complete source list, results for merged fields, a reference system to published papers and data useful for calculating upper limits and fluxes.

Heterogeneous reaction probabilities, solubilities, and the physical state of cold volcanic aerosols

Abstract: On 19 January 1992, heterogeneous loss of HNO3, ClNO3, and HCl was observed in part of the Mount Pinatubo volcanic cloud that had cooled as a result of forced ascent. Portions of the volcanic cloud froze near 191 kelvin. The reaction probability of ClNO3 and the solubility of HNO3 were close to laboratory measurements on liquid sulfuric acid. The magnitude of the observed loss of HCl suggests that it underwent a heterogeneous reaction. Such reactions could lead to substantial loss of HCl on background sulfuric acid particles and so be important for polar ozone loss.

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

Abstract: 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).

Polarimetry of H-Lyman α for coronal magnetic field diagnostics

Abstract: We discuss and analyze the possible sources of observational and instrumental uncertainty that can be encountered in measuring magnetic fields of the solar corona through polarimetric observations of the Hanle effect of the coronal Ly-alpha line. The Hanle effect is the modification of the linear polarization of a resonantly scattered line, due to the presence of a magnetic field. Simulated observations are used to examine how polarimetric measurements of this effect are affected by the line-of-sight integration, the electron collisions, and the Ly-alpha geocorona. We plan to implement the coronal magnetic field diagnostics via the Ly-alpha Hanle effect using an all-reflecting Ly-alpha coronagraph/polarimeter (Ly-alphaCoPo) which employs reflecting multilayer mirrors, polarizers, and filters. We discuss here the requirements for such an instrument, and analyze the sources of instrumental uncertainty for polarimetric observations of the coronal Ly-alpha Hanle effect. We conclude that the anticipated polarization signal from the corona and the expected performance of the Ly-alphaCoPo instrument are such that the Ly-alpha Hanle effect method for coronal field diagnostics is feasible.

SCIAMACHY and GOME - The scientific objectives

Abstract: The Global Ozone Monitoring Experiment (GOME) and the Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY), are two European experiments which will fly on satellite platforms in the 1990s. GOME is a small scale version of SCIAMACHY and together they form the SCIAMACHY scientific project. The principal scientific objective is the global determination of the distributions of atmospheric constituents: trace gases, aerosol, and cloud. Special emphasis is placed in this project on stratospheric and tropospheric measurements. GOME observes between 240 and 790 nm in nadir sounding, whereas SCIAMACHY will sound the atmosphere in nadir, limb and solar, and lunar occultation viewing geometries between 240 and 2380 nm.

A CCD-based area detector for X-ray crystallography using synchrotron and laboratory sources

Abstract: The design and characteristics of a CCD-based area detector suitable for X-ray crystallographic studies using both synchrotron and laboratory sources are described. The active area is 75 mm in diameter, the FWHM of the point response function is 0.20 mm, and for Bragg peaks the dynamic range is 900 and the DQE ∼ 0.3. The 1320 × 1035-pixel Kodak CCD is read out into an 8 Mbyte memory system in 0.14 s and digitized to 12 bits. X-ray crystallographic data collected at the NSLS synchrotron from cubic insulin crystals are presented.

Reflectance calibrations of AXAF mirror samples at absorption edges using synchrotron radiation

Abstract: We are developing a system to calibrate reflectances of witness coupons to the AXAF flight mirrors at the National Synchrotron Light Source over the 0.05-12 keV energy range. These witness coupons will be coated in the same process as the AXAF mirror elements. One of the key issues is the accurate determination of mirror efficiencies across the absorption edges of the mirror coating elements. We present a series of reflectance measurements with 2 eV resolution of a nickel-coated flat mirror in the region of the Ni L-II (870 eV) and L-III (853 eV) absorption edges. Scans of reflectance versus grazing angle at fixed energies in this region show distinct interference fringes at grazing angles larger than the critical angle which are extinguished as the photon energy is increased beyond the low point of the L-III edge, indicating total absorption of the evanescent wave within the Ni film. At 51 arc minutes grazing angle, measured reflectance decreases smoothly by 35 percent and then recovers in an 8 eV band at the L-III edge. We have also measured reflectances in the M absorption edge region for gold, platinum, and iridium coated mirrors. We derive optical parameters n and k specific to the film for comparison to the existing data tables.

Design and fabrication of the all-reflecting H-Lyman alpha coronagraph/polarimeter

Abstract: We have designed, analyzed, and are now fabricating an All-Reflecting H-Lyman-alpha Coronagraph/Polarimeter for solar research. This new instrument operates in a narrow bandpass centered at 215.7 A - the neutral hydrogen Ly-alpha line. It is shorter and faster than the telescope which produced solar Ly-alpha images as a part of the MSSTA payload that was launched on May 13, 1991. The Ly-alpha line is produced and linearly polarized in the solar corona by resonance scattering, and the presence of a magnetic field modifies this polarization according to the Hanle effect. The Lyman-alpha Coronagraph/Polarimeter instrument has been designed to measure coronal magnetic fields by interpreting, via the Hanle effect, the measured linear polarization of the coronal Ly-alpha line. Ultrasmooth mirrors, polarizers, and filters are being flow-polished for this instrument from CVD silicon carbide substrates. These optical components will be coated using advanced induced transmission and absorption thin film multilayer coatings to optimize the reflectivity and polarization properties at 1215.7 A. We describe some of the solar imaging results obtained with the MSSTA Lyman-alpha coronagraph. We also discuss the optical design parameters and fabrication plans for the All-Reflecting H-Lyman-alpha Coronagraph/Polarimeter.

Internal laser metrology for POINTS

Abstract: We present the designs for laser distance gauges to be used in the POINTS instrument, and preliminary performance data. For the target 5 micro-arcsecond astrometric accuracy, we must hold or monitor some critical internal dimensions of the POINTS instrument with 2 picometer (pm, 10-12 m) accuracy for a few hours. The POINTS architecture makes good use of these gauges, minimizing the number and range of dimensions that must change during operation, and maximizing the similarity of the starlight and metrology measured paths. Gauge designs have been developed for both optical-path-differencing (Michelson) and point-to-point measurements (Fabry-Perot). The Michelson fringes have been measured in a differential (comparison) test; the root-two-point variance (analogous to the Allan variance) in the difference of two measurements over essentially identical 1-meter paths was about 2 pm for averaging times between 40 seconds and 6 hours. A second design for the point-to-point measurements incorporates cornercube retro-reflectors in a resonant cavity. We discuss the new problems anticipated in this design, including the problem of maintaining laser alignment in these point-to-point gauges over the +/- 3 degree range of instrument articulation.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Partition functions of rare-earth halide plasmas.

Abstract: The partition functions are calculated for gases consisting of Ln and [ital X] atoms and Ln[ital X] molecules and their singly charged ions (Ln[equivalent to]Dy,Ho,Tm and [ital X][equivalent to]I,Br).

VETA-I x-ray detection system

Abstract: The alignment and X-ray imaging performance of the Advanced X-ray Astrophysics Facility (AXAF) Verification Engineering Test Article-I(VETA-I) was measured by the VETA-I X-Ray Detection System (VXDS). The VXDS was based on the X-ray detection system utilized in the AXAF Technology Mirror Assembly (TMA) program, upgraded to meet the more stringent requirements of the VETA-I test program. The VXDS includes two types of X-ray detectors: (1) a High Resolution Imager (HRI) which provides X-ray imaging capabilities; and (2) sealed and flow proportional counters which, in conjunction with apertures of various types and precision translation stages, provide the most accurate measurement of VETA-I performance. Herein we give an overview of the VXDS hardware including X-ray detectors, translation stages, apertures, proportional counters and flow counter gas supply system and associated electronics. We also describe the installation of the VXDS into the Marshall Space Flight Central (MSFC) X-Ray Calibration Facility (XRCF). We discuss in detail the design and performance of those elements of the VXDS which have not been discussed elsewhere; translation systems, flow counter gas supply system, apertures and thermal monitoring system.

Intensity distribution of the x-ray source for the AXAF VETA-I mirror test

Abstract: Intensity distribution measurements of the X-ray source for the AXAF VETA-I mirror test are reported. During the VETA-I test, microscope pictures were taken for each used anode immediately after it was brought out of the source chamber. The source sizes and the intensity distribution structures are shown. They are compared and shown to agree with the results from pinhole camera measurements. It is demonstrated that under operating conditions characteristic of the VETA-I test, all the source sizes have an FWHM of less than 0.45 mm. For a source of this size at 528 m away, the angular size to VETA is less than 0.17 arcsec, which is small compared to the on-ground VETA angular resolution. These results were crucial for VETA data analysis and for obtaining the on-ground and predicted in-orbit VETA point response function.

Particle sightings by the infrared telescope on Spacelab 2

Abstract: Part of the objective of the cooled infrared telescope on Spacelab 2 was to determine the particle environment around the Space Shuttle The telescope scanned the sky in six wavelength bands ranging from 2 to 100 microns with high time resolution Dust particles could be identified from their particular signature in the data stream Particle data from about 4 h early in the mission were analyzed in terms of size, color temperature, velocity, and time The 1100 particles that were seen were slow moving and ranged in color temperature from 190 to 350 K with a few much hotter The minimum detectable diameter is between 5 and 13 microns, depending on temperature The size distribution resembles sample distributions collected at a Shuttle preparation facility Although particle detection rates varied widely with time, no specific events were identified to be associated with particle production It was not possible to determine the particle composition, although it was probably not ice

The Einstein Observatory catalog of IPC x ray sources. Volume 1E: Documentation

Abstract: The Einstein Observatory (HEAO-2, launched November 13, 1978) achieved radically improved sensitivity over previous x-ray missions through the use of focusing optics, which simultaneously afforded greatly reduced background and produced true images. During its 2.5-yr mission, the Einstein X-Ray Telescope was pointed toward some 5,000 celestial targets, most of which were detected, and discovered several thousand additional 'serendipitous' sources in the observed fields. This catalog contains contour diagrams and source data, obtained with the imaging proportional counter in the 0.16 to 3.5 keV energy band, and describes methods for recovering upper limits for any sky position within the observed images. The main catalog consists of six volumes (numbered 2 through 7) of right ascension ordered pages, each containing data for one observation. Along with the primary documentation describing how the catalog was constructed, volume 1 contains a complete source list, results for merged fields, a reference system to published papers, and data useful for calculating upper limits and fluxes.

POINTS: the first small step

Abstract: POINTS, an astrometric optical interferometer with a nominal measurement accuracy of 5 microarcseconds for the angle between a pair of stars separated by about 90 deg. is presently under consideration by two divisions of NASA-OSS. Based on a preliminary indication of the observational needs of the two missions, we find that a single POINTS mission will meet the science objectives of both TOPS-1 and AIM. The instrument detects a dispersed fringe (channelled spectrum) and therefore can tolerate large pointing errors. In operation, the difficult problem of measuring the angular separation of widely spaced star pairs is reduced to two less difficult problems: that of measuring the angle between the two interferometers and that of measuring interferometrically the small offset of each star from the corresponding interferometer axis. The question of systematic error is the central theme of the instrument architecture and the data-analysis methods. Stable materials, precise thermal control, and continuous precise metrology are fundamental to the design of the instrument. A preliminary version of the required picometer laser metrology has been demonstrated in the laboratory. Post-measurement detection and correction of time-dependent bias are the essential elements in data analysis. In that post-measurement analysis, individual star-pair separations are combined to determine both the relative positions of all observed stars and several instrument parameters including overall time-dependent measurement bias. The resulting stellar separation estimates are both global and bias-free at the level of the uncertainty in the reduced (i.e., combined and analyzed) measurements.

Status of local oscillators for operating ultra-high resolution frequency discriminators as frequency standards

Abstract: The operation of new improved frequency standards based on new ultra-high-resolution frequency discriminators requires high stability local, or 'flywheel' oscillators We review the spectral density of phase fluctuations of existing flywheel oscillators and the related time domain frequency stability of new and proposed cryogenically cooled oscillators suitable for this application Presently used devices include the quartz crystal oscillator, the room-temperature actively oscillating atomic hydrogen (H) maser, and the superconducting maser oscillator Future devices include the cryogenic H-maser and other cryogenic devices using resonators of superconducting metal or solid crystalline sapphire The relation of the phase spectral density of these devices to the characteristics of present and proposed frequency discriminators based on trapped cooled ions and cold atoms is discussed in terms of their operation as frequency standards

Precision optical interferometry in space

Abstract: POINTS, an astrometric Optical interferometer with a nominal measurement accuracy of 5 microarcseconds for the angle between a pair of stars separated by about 90 deg, is presently under consideration by two divisions of NASA-OSSA It will be a powerful new multi-disciplinary tool for astronomical research If chosen as the TOPS-1 (Toward Other Planetary Systems) instrument by the Solar-System Exploration Division, it will perform a definitive search for extra-solar planetary systems, either finding and characterizing a large number of them or showing that they are far less numerous than now believed If chosen as the AIM (Astrometric Interferometry Mission) by the Astrophysics Division, POINTS will open new areas of astrophysical research and change the nature of the questions being asked in some old areas In either case it will be the first of a new class of powerful instruments in space and will prove the technology for the larger members of that class to follow Based on a preliminary indication of the observational needs of the two missions, we find that a single POINTS mission will meet the science objectives of both TOPS-1 and AIM The instrument detects dispersed fringe (channel led spectrum) and therefore can tolerate large pointing errors

Optic-misalignment tolerances for the POINTS interferometers

Abstract: We present the results of two parallel studies of the sensitivity of astrometric measurements to misalignment of each of the major optical assemblies in the POINTS instrument. Tilt and displacement of the optics lead to tilt, displacement, and defocussing of the starlight and metrology beams, giving rise to systematic errors. In one method, we derive analytic expressions for the lowest order dependence of the error on the misalignment, and evaluate them in the present interferometer design. In the second method, we use a commercial numerical ray tracing program to calculate the overall optical path travelled through the misaligned starlight and metrology paths; from those results, our own software determines the dependence of the residual error on the original misalignment. These sensitivities are compared to the analytic results for mutual verification. We also discuss the impact these results have had on the design of the instrument.

NIXT High Resolution Observations

Abstract: Progress in x-ray optics, beginning in the 1960’s and continuing to the present day, has been intimately tied to progress in our understanding of the coronae of the Sun and of solar-like stars. Innovative instrumentation, first used on sounding rockets and then flown on satellites, has been the most productive route for progress. This paper presents an overview of the status of coronal x-ray imaging research at the time that Skylab was launched in 1973, and of the contributions made by Skylab and by the Einstein Observatory. We then discuss the new technique of multilayer imaging for soft x-rays and the contributions which the NIXT experiment is making to our understanding of the structuring and heating of the corona.

Spectral evidence of size dependent space weathering processes on asteroid surfaces

Abstract: Most compositional characterizations of the minor planets are derived from analysis of visible and near-infrared reflectance spectra However, such spectra are derived from light which has only interacted with a very thin surface layer Although regolith processes are assumed to mix all near-surface lithologic units into this layer, it has been proposed that space weathering processes can alter this surface layer to obscure the spectral signature of the bedrock lithology It has been proposed that these spectral alteration processes are much less pronounced on asteroid surfaces than on the lunar surface, but the possibility of major spectral alteration of asteroidal optical surfaces has been invoked to reconcile S-asteroids with ordinary chondrites The reflectance spectra of a large subset of the S-asteroid population have been analyzed in a systematic investigation of the mineralogical diversity within the S-class In this sample, absorption band depth is a strong function of asteroid diameter The S-asteroid band depths are relatively constant for objects larger than 100 km and increase linearly by factor of two toward smaller sizes (approximately 40 km) Although the S-asteroid surface materials includes a diverse variety of silicate assemblages, ranging from dunites to basalts, all compositional subtypes of the S-asteroids conform to this trend The A-, R-, and V-type asteroids which are primarily silicate assemblages (as opposed to the metal-silicate mixtures of most S-asteroids) follow a parallel but displaced trend Some sort of textural or regolith equilibrium appears to have been attained in the optical surfaces of asteroids larger than about 100 km diameter but not on bodies below this size The relationships between absorption band depth, spectral slope, surface albedo and body size provide an intriguing insight into the nature of the optical surfaces of the S-asteroids and space weathering on these objects

Correcting x-ray spectra obtained from the AXAF VETA-I mirror calibration for pileup, continuum, background, and deadtime

Abstract: The VETA-I mirror was calibrated with the use of a collimated soft X-ray source produced by electron bombardment of various anode materials. The FWHM, effective area and encircled energy were measured with the use of proportional counters that were scanned with a set of circular apertures. The pulsers from the proportional counters were sent through a multichannel analyzer that produced a pulse height spectrum. In order to characterize the properties of the mirror at different discrete photon energies one desires to extract from the pulse height distribution only those photons that originated from the characteristic line emission of the X-ray target source. We have developed a code that fits a modeled spectrum to the observed X-ray data, extracts the counts that originated from the line emission, and estimates the error in these counts. The function that is fitted to the X-ray spectra includes a Prescott function for the resolution of the detector a second Prescott function for a pileup peak and a X-ray continuum function. The continuum component is determined by calculating the absorption of the target Bremsstrahlung through various filters, correcting for the reflectivity of the mirror and convolving with the detector response.

The Einstein Observatory catalog of IPC x ray sources. Volume 6E: Right ascension range 16h 00m to 19h 59m

Abstract: The Einstein Observatory (HEAO-2 launched November 13, 1978) achieved radically improved sensitivity over previous x-ray missions through the use of focusing optics, which simultaneously afforded greatly reduced background and produced true images. During its 2.5-yr mission, the Einstein X-Ray Telescope was pointed toward some 5,000 celestial targets, most of which were detected, and discovered several thousand additional 'serendipitous' sources in the observed fields. This catalog contains contour diagrams and source data, obtained with the imaging proportional counter in the 0.16 to 3.5 keV energy band, and describes methods for recovering upper limits for any sky position within the observed images. The main catalog consists of six volumes (numbered 2 through 7) of right ascension ordered pages, each containing data for one observation. Along with the primary documentation describing how the catalog was constructed, volume 1 contains a complete source list, results for merged fields, a reference system to published papers, and data useful for calculating upper limits and fluxes.

Fringe-tracking filters for space-based interferometers

Abstract: The purpose of this research is to explore, from the point of view of nonlinear filtering, the feasibility of microarcsecond astrometry using space-based optical interferometers in typical disturbance environments. Two nonlinear estimators are developed to enable high precision estimates of the optical path difference (OPD) between the two starlight paths in a spaceborne optical interferometer. Focal plane fringe data measurements by either CCD (charge coupled device) or photon counting (PC) cells are processed by an Extended Kalman Filter (EKF)- based algorithm to yield the OPD estimate. Whereas the filter based on CCD cell measurements results in a straightforward implementation of an EKF, the filter utilizing PC cells requires some innovations before yielding to the EKF framework. Instead of using the photon arrival events as measurements, the filter measurements are taken to be the interarrival time between photons. The excellent results obtained with the PC filter are backed by intuition based on linear analysis results. Simulation studies show that as the CCD readout time is decreased, the performance of the CCD filter approaches that of the PC filter.© (1993) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

AXAF VETA-I mirror encircled energy measurements and data reduction

Abstract: The AXAF VETA-I mirror encircled energy was measured with a series of apertures and two flow gas proportional counters at five X-ray energies ranging from 0.28 to 2.3 keV. The proportional counter has a thin plastic window with an opaque wire mesh supporting grid. Depending on the counter position, this mesh can cause the X-ray transmission to vary as much as +/- 9 percent, which directly translates into an error in the encircled energy. In order to correct this wire mesh effect, window scan measurements were made, in which the counter was scanned in both horizontal (Y) and vertical (Z) directions with the aperture fixed. Post VETA measurement of the VXDS setup were made to determine the exact geometry and position of the mesh grid. Computer models of the window mesh were developed to simulate the X-ray transmission based on this measurement. The window scan data were fitted to such mesh models and corrections were made. After this study, the mesh effect was well understood and the final results of the encircled energy were obtained with an uncertainty of less than 0.8 percent.

Surface finish quality of the outer AXAF mirror pair based on x-ray measurements of the VETA-I

Abstract: We employ the X-ray measurements of the VETA-I taken at the X-Ray Calibration Facility (XRCF) of the Marshall Space Flight Center (MSFC) to extract information about the surface finish quality of the outermost pair of AXAF mirrors. The particular measurements we consider are 1D scans of the core of the point response function (PRF) (FWHM scans), the encircled energy as a function of radius, and 1D scans of the wings of the PRF. We discuss briefly our raytrace model which incorporates the numerous effects present in the VETA-I test, such as the finite source distance, the size and shape of the X-ray source, the residual gravitational distortions of the optic, the despace of the VETA-I, and particulate contamination. We show how the data constrain the amplitude of mirror surface deviations for spatial frequencies greater than about 0.1/mm. Constraints on the average amplitude of circumferential slope errors are derived as well.