Leon P. Van Speybroeck

Advanced X-Ray Astrophysics Facility (AXAF)

AXAF is an x-ray observatory designed to study x-ray emission from al categories of astronomical objects, from normal stars to quasars. AXAF has broad scientific objectives and outstanding capability to provide high resolution images, spectrometric imaging and high resolution dispersive spectroscopy over the energy bandwidth from 0.1 to 10-keV. This is a significant year in the development of AXAF, to be launched in late 1998. Major elements of the observatory, the optics and the scientific instruments, are now nearing completion in preparation for calibration later this year.

Performance expectation versus reality

The AXAF (Advanced X-ray Astrophysics Facility) high resolution mirror assembly (HRMA) now is complete and has been tested at the NASA Marshall Space Flight Center (MSFC) X-ray Calibration Facility (XRCF). The surface and alignment properties of the mirror were thoroughly measured before the x-ray test, which allowed accurate performance predictions to be performed. The preliminary analysis of the measured x-ray image distributions for all energies tested show excellent agreement with predictions made before the beginning of the test. There is a discrepancy between the measured and predicted effective areas; this typically is less than 5%, and is less than 13% for all energies measured. We present evidence that this discrepancy is due to uncertainties in the calibration of the test instrumentation, and therefore can be expected to be reduced when results from further instrument calibration tests now in progress are incorporated into the analysis. We predict that 65 - 80% (depending upon energy) of the flux from an imaged point source will be contained on a one arc second diameter aperture in flight. We expect the HRMA to more than fulfill the requirements necessary to achieve the AXAF scientific objectives.

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

The Advanced X-ray Astrophysics Facility (AXAF) is the x-ray component of NASAs Great Observatories. To be launched in late 1998, AXAF will provide unprecedented capabilities for high-resolution imaging, spectrometric imaging, and high-resolution dispersive 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.

Observations of X-ray sources in M31

We have observed 69 unresolved X-ray sources and seven diffuse or confused source regions in M31 with the Einstein Observatory. The typical limiting sensitivies in the 0.5--4.5 keV band were 9 x 10/sup 36/ ergs s/sup -1/. There are 21 sources in a compact inner bulge, seven globular clusters, 40 sources we categorize as Population I, and i sources near globular clusters but with position errors large enough so that four chance coincidences would be expected. The Population I tracers. Luminosity distributions of these classes are given. The nucleus of M31 is coincident with a 10/sup 38/ ergs s/sup -1/ X-ray source at approximately the 90% confidence level.

Orbital Measurement and Verification of the Chandra X-Ray Observatory's PSF

We present here results of the on-orbit calibration of the point spread function (PSF), comparing it with our predictions. We discuss how the PSF varies with source location in the telescope field of view, as well as with the spectral energy distribution of the source.

Einstein Observatory (HEAO-B) Mirror Design And Performance

The mirrors for the Einstein Observatory represent a substantial advance in the state of the art x-ray optics, and have been tested more thoroughly than previous telescopes. The image is characterized by a narrow, wavelength independent core, and broad energy dependent wings. The width of the core is about that expected from mechanical and visible light measurements taken during fabrication and assembly. The wings of the response function are caused by microscopic surface scattering, but only a qualitative understanding of this process exists. Improvement in surface texture to reduce the wide angle scatter is the most urgent task in the development of large x-ray optics. The mirrors have survived the experiment integration period and launch, and are performing in orbit as they performed upon the ground.© (1979) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

New method to model X-ray scattering from random rough surfaces

This paper presents a method for modeling the X-ray scattering from random rough surfaces. An actual rough surface is (incompletely) described by its Power Spectral Density (PSD). For a given PSD, model surfaces with the same roughness as the actual surface are constructed by preserving the PSD amplitudes and assigning a random phase to each spectral component. Rays representing the incident wave are reflected from the model surface and projected onto a flat plane, which approximates the model surface, as outgoing rays and corrected for phase delays. The projected outgoing rays are then corrected for wave densities and redistributed onto an uniform grid where the model surface is constructed. The scattering is then calculated by taking the Fast Fourier Transform (FFT) of the resulting distribution. This method is generally applicable and is not limited to small scattering angles. It provides the correct asymmetrical scattering profile for grazing incident radiation. We apply this method to the mirrors of the Chandra X-ray Observatory and show the results. We also expect this method to be useful for other X-ray telescope missions.

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).

Chandra X-ray Observatory mirror effective area

Chandra X-ray Observatory (CXO) -- the third of NASAs Great Observatories -- has now been successfully operated for four years and has brought us fruitful scientific results with many exciting discoveries. The major achievement comparing to previous X-ray missions lies in the heart of the CXO -- the High Resolution Mirror Assembly. Its unprecedented spatial resolution and well calibrated performing characteristics are the keys for its success. We discuss the effective area of the CXO mirrors, based on the ground calibration measurements made at the X-Ray Calibration Facility in Marshall Space Flight Center before launch. We present the derivations of both on-axis and off-axis effective areas, which are currently used by Chandra observers.

Grazing incidence optics for the U.S. High-Resolution X-Ray Astronomy Program.

The major telescopes in this program are the one flown on the Einstein or HEAO -B satellite in 1978 and the one planned for the AXAF which should be launched in late 1995. The x -ray performance of the Einstein mirror is reviewed briefly, and its surface properties are inferred from a combination of its x -ray properties and the information available from the limited fabrication metrology of that era. The improvements necessary for satisfying the AXAF specification are summarized. Much of this technology already has been demonstrated in the TMA program, and this progress is reviewed.The major telescopes in this program are the one flown on the Einstein or HEAO-B satellite in 1978 and the one planned for the AXAF which should be launched in late 1995. The x-ray performance of the Einstein mirror is reviewed briefly, and its surface properties are inferred from a combination of its x-ray properties and the information available from the limited fabrication metrology of that era. The improvements necessary for satisfying the AXAF specification are summarized. Much of this technology already has been demonstrated in the TMA program, and this progress is reviewed.