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A practical design for upcoming spaceborne x-ray telescopes with ultrahigh angular resolution is proposed. Particular attention is directed to technological simplicity and robust as well as cheap components. Based on dispersion corrected Fresnel lenses, an optimized arrangement will be identified with respect to the instrumental sensitivity for a given focal spot size. We show that this optical Gamow peak essentially depends on the radial transmission profile of a diffractive--refractive aperture. Examples for energies above 4 keV illustrate astronomical capabilities for large-scale compact and segmented objectives as well. The spectral and spatial resolutions of conventional semiconductor detectors are very well matched to imaging characteristics of those achromatic lenses. The constraints to fabrication techniques using most promising materials like Li, Be, and plastics are discussed. OCIS codes: 050.1970, 110.6770, 110.7440, 350.1260.

Byline: Christoph Braig (1), Peter Predehl (1) Keywords: instrumentation: high angular resolution; techniques: spectroscopic telescopes Abstract: Capabilities of large-scale diffractive X-ray telescopes are discussed. Based on purely transmissive optics, an angular resolution of at least 10.sup.-3 arcsec will be achieved using detection techniques with spectral selectivities in the sub-eV range for short focal distances of few 10.sup.2 km. We use stepped versions of Fresnel apertures made of plastic foils, divided into optically independent segments by two alternative schemes. It is shown that point source sensitivities near 10.sup.3 cm.sup.2 keV require lens diameters up to 30 m. Like monochromatic objectives, properly shaped dual- or multiband telescopes may be tuned over several keV. Such configurations are made of partial Fresnel lenses with coinciding focal distances and similar spot sizes and compete well with single-band analogues. Author Affiliation: (1) Max-Planck-Institute fur Extraterrestrische Physik, D-85748, Garching, Germany Article History: Registration Date: 15/10/2006 Received Date: 01/04/2006 Accepted Date: 12/10/2006 Online Date: 09/01/2007

An optical setup was built for microscopic damage inspection on transmission-grating facets composed of a gold-wire structure. Contrast improvement was achieved by exploiting the polarizing properties of these gratings in the near-infrared region. Spatial filtering yields an additional contrast enhancement and reduces unwanted signals caused by the periodic support structure. An image-processing algorithm is developed that evaluates the number and the size of the faults in a grating facet with high accuracy from only one digital image. Key words: Machine vision, image processing, transmission gratings.

Self-supporting transmission gratings with periods of 1 [mu]m or below are used in combination with grazing-incidence telescopes in celestial x-ray astronomy. They can be produced with sizes up to only a few [cm.sup.2]; therefore, several hundreds or even thousands of individual elements are needed in order to cover the aperture of a telescope. This large number leads to the problem of characterization of the gratings regarding their x-ray performance. We demonstrate that spectrometry in the resonance domain using H polarization is a suitable method for the determination of the grating wire profile and deviations of the grating surface from a plane. Although developed originally for microwave applications it can be shown that the methods of strict solution of the Helmholtz equation are able to explain even small effects related to imperfections of periodic submicrometer structures. Key words: Transmission gratings, x-ray astronomy., The problem of characterizing the transmission gratings incorporated in grazing incidence telescopes for celestial x-ray astronomy applications is presented. A novel method, H-polarized radiation in the resonance domain, is shown to address the problem. Meanwhile, the strict application of HelmHoltz equations used in microwave cases is seen to be applicable for understanding the nonidealities of periodic submicrometer devices.

The x-ray source GX5-1 in he galactic bulge has been observed with the position-sensitive proportional counter onboard the Rontgen satellite (ROSAT) during and after a lunar occultation. Extended emission around the source was unambiguously discovered while the central source was behind the lunar rim. This emission is interpreted as a dust-scattering halo around GX 5-1 that has a fractional intensity of 28 percent, implying a grain column density between GX 5-1 and Earth of 3 X [10.sup.10] per square centimeter. The halo derived from imaging during the ROSAT all-sky survey is identical to that obtained from the lunar occultation, thus demonstrating that the ROSAT x-ray mirror scattering has not changed as compared with the mirror properties as measured in preflight calibrations., In the 1960s, the possible existence of halos, produces by the scattering of x-rays off dust grains in the interstellar medium, around the stronger and moderately absorbed x-ray sources was [...]