Karl-Heinz Stephan

Optical filter for x-ray astronomy CCDs

Abstract Charge coupled device (CCD) detectors are sensitive from the X-ray to the NIR (near infrared) region. For X-ray and soft X-ray observations in astronomy, an optical filter has to be placed in front of the CCD to suppress the UV (ultraviolet) and VIS (visible) radiation of stars by more than 6 orders of magnitude. However, this filter must remain highly transparent at photon energies above 100 eV. A prototype filter was developed for a satellite borne CCD X-ray imaging detector. This unsupported filter has an effective size of 30 mm × 10 mm and is composed of multilayered thin films of parylene N, aluminum and carbon with mass densities of 25, 30 and 25 μg/cm2, respectively. The fabrication technique used for the filter and measurements of its optical and mechanical properties are presented.

Characterization of thin films by X-ray transmission measurements

Abstract We have used synchrotron radiation in order to determine the mass densities and the elemental composition of single and multilayered solid films. Soft X-rays were used to measure the spectral transmittance in a certain photon energy range, i.e. between 50 eV and 10 keV, and the local transmission profile at discrete photon energies within this range. To give an example, we present some results obtained for an optical filter which is to be used in the focal plane PN CCD-camera of ESAs XMM (X-ray Multi Mirror) astronomy satellite. Moreover, we established a laboratory set-up for transmittance measurements using the discrete line radiation emitted from a solid target X-ray source. Thus, we are able to control layer thicknesses of thin films produced by vacuum evaporation condensation. We describe the compact X-ray test facility, and give the obtained performance data of the filters.

The boron filter for the ROSAT X-ray telescope

Abstract We have developed multilayered films composed of boron carbide and carbon, which serve as spectral filters in the focal plane of the Wolter type I X-ray telescope on board the X-ray astronomy satellite ROSAT (Rontgensatellit). We describe the manufacturing process and qualification measurements of the filters and present the resulting performance data. Finally the pulse height spectrum of the active star AR Lac observed by ROSAT with and without boron filter will be shown.

Spectral function of an optical filter for the PN-CCD camera on board the german astronomy satellite ABRIXAS

We have provided optical filters developed at the Max-Planck Institut fuer extraterrestrische Physik to the German x-ray astronomy observatory ABRIXAS. Specific Si-PN CCDs will be serving as focal plane camera. Since this detector is sensitive to radiation from the x-ray to the near IR spectral range, for observation in x-ray astronomy it must be protected from visible and UV radiation. This is achieved by an optical filter which combines a high transmittance for photon energies in the soft x-ray range and a high transmittance for photon energies in the soft x-ray range and a high absorptance for UV and visible radiation. With respect to the mission goal in orbit a spectral transmittance function is required attenuating radiation below photon energies of 10 eV by more than 7 orders of magnitude and transmitting soft x-ray photon energies above 1000 eV by more than 90 percent. This was realized by a 0.80 micrometers thick polypropylene film coated with approximately 60 nm aluminum on both sides. The filter has an effective diameter of 73 mm without any support structure. Environmental test have been performed and proved the filters to be resistant against acoustic nose and vibrational load during the launch. Synchrotron radiation in the photon energy range from 60 eV to 2000 eV was used to characterize the spectral transmittance in the center and the transmittance topography across the filter. The thickness of the surface oxide layer was determined by x-ray reflectance measurements on an Al-layer in order to study longterm variations of transmittance caused by oxidation effects. We present the resulting performance data.

Soft x-ray transmission measurements on thin films used for XMM CCD-filters

We have been developing optical filters for ESA’s X-ray astronomy project XMM (X-ray Multi Mirror Mission). Specific CCDs will be used as detectors in the focal plane on board the observatory (1). Since these detectors are sensitive from the X-ray to the near infrared spectral range, X-ray observations require optical filters, which combine a high transparency for photon energies in the soft X-ray region and a high opacity for ultraviolet and visible radiation as well. With respect to the mission goal in orbit three types of flight model filters are designed having different spectral transmittance functions. We report on one of these types, a so-called “thick” filter, which has been realized within the EQM (Electrical Qualification Model)-phase of the project. The filter features a cut-off in the extreme ultraviolet spectral range and suppresses radiation below ∼10 eV photon energy by more than 8 orders of magnitude. It has an effective aperture of 73 mm without any support structure. A 0.35 μm thick poly...

On the performance of optical filters for the XMM focal plane CCD-camera EPIC

Abstract Optical filters have been developed for the X-ray astronomy project XMM (X-ray Multi Mirror Mission) [1] of ESA, where specific CCDs will serve as focal plane cameras on board the observatory. These detectors are sensitive from the X-ray to the NIR (near infrared) spectral region. For observations in X-ray astronomy an optical filter must be placed in front of the CCD, suppressing visible and UV (ultraviolet) radiation of stars by more than 6 orders of magnitude while being highly transparent at photon energies above 100 eV. The flight model filter is designed to have an effective area of 73 mm diameter without making use of a supporting grid. Efforts have been made to utilize plastic foils to tailor filters meeting these specific requirements. It was found, that a typical filter could be composed, e.g., of a polypropylene foil of 20 μg/cm 2 thickness serving as a carrier, coated with metallic films of Al or Al and Sn of about 20–25 μg/cm 2 thickness. Other possible carriers are polycarbonate (Lexan, Macrolon) and poly-para-xylylene (Parylene N) films of similar thicknesses. The preparation and characterization of these three types of carrier foils as well as of two sample filters is described, including mechanical tests as well as optical transmission measurements in the photon energy range from 1 eV to 2 keV.

Transmittance performance of polypropylene, poly-para-xylylene and polycarbonate films in the photon energy range from 1 eV to 10 keV

We have been developing optical filters for ESAs X-ray astronomy project XMM (X-Ray Mirror Mission) where specific CCDs will serve as focal plane cameras on board the observatory. These detectors are sensitive from the X-ray to the near infrared spectral region.

Laboratory soft x-ray source with foil target

We have developed a comparatively small soft x-ray source for application in our test facilities, which are used at present to support the developments of the astrophysical space projects XMM and AXAF. The instrument comprises a commercially available color television tube for generation of the electron beam, which is focused on exchangeable metal films serving as targets. The x rays are taken off after having transversed the foil target and have a sufficient spectral purity with regard to the experimental requirements. The maximum electric operating parameters correspond to an emission current of 100 (mu) A generated by a filament heating power of 6.6 watt at an accelerating voltage of 25 kV. The technical advantages of the instrument are lightweight construction, no water cooling, small size electric supply, cost efficient manufacturing, small sized focus, and quick access to the desired characteristic spectral line by exchange of a complete tube. We describe the measurements on the local x-ray intensity profile of the focus, the spectral features of the beam, and present the resulting performance data. A special development could be used as calibration sources in x-ray telescopes.

Radiation entrance windows for CCD detectors

A prototype spectral filter was developed to be used as a radiation entrance window for a satellite borne CCD (Charge coupled device) X-ray imaging detector, which is presently under development within the EOBB (Electro-Optical-Breadboard) phase of ESAs XMM project. The unsupported filter has an effective size of 30 mm X 10 mm and is composed of multi-layered thin films of parylene N, aluminum, and carbon with mass densities of 25, 30 and 25 (mu) g/cm2 respectively. CCDs have a broad spectral response ranging from the X-ray to the NIR (Near Infrared) range. For X-ray observations the filter being located in front of the detector must suppress the radiation of VIS (Visible) and UV (Ultraviolet) stars by more than 6 orders of magnitude and be highly transparent at energies above 0.1 keV. The technique to manufacture the windows and the measurements of the spectral transmittance as well as the environmental testing is described. The performance data are given.

Measurements of the X-ray transmission topography on radiation entrance windows for the position sensitive proportional counters on board the german as

We have been developing relatively large area radiation entrance windows for the position sensitive proportional counters (PSPC) on board the X-ray astronomy satellite ROSAT. A method is described to measure the transmission topography in the soft X-ray energy range 1.75 >E > 0.1 keV across the effective area. The results are presented as contour plots in a gray scale.