Jay V. Bixler

High Efficiency Germanium Immersion Gratings

We have fabricated several germanium immersion gratings by single crystal, single point diamond flycutting on an ultra-precision lathe. Use of a dead sharp tool produces groove corners less than 0.1 micron in radius and consequently high diffraction efficiency. We measured first order efficiencies in immersion of over 80% at 10.6 micron wavelength. Wavefront error was low averaging 0.06 wave rms (at 633 nm) across the full aperture. The grating spectral response was free of ghosts down to our detection limit of 1 part in 104. Scatter should be low based upon the surface roughness. Measurement of the spectral line profile of a CO2 laser sets an upper bound on total integrated scatter of 0.5%.

The implications of atmospheric effects for fiber-fed spectroscopy

The effects of atmospheric dispersion in terms of S/N losses are calculated as a function of displacement of an optical fiber with respect to the source image. The results are presented as response curves for a wide variety of seeing conditions and source-to-sky brightness ratios. The limitations of fiber systems for precision spectrophotometry and ways in which these limitations may be overcome are discussed. The effects of differential refraction and field rotation are also calculated, showing that it can be a nonnegligible effect at large zenith angles for observations of more than 1-hour duration.

Reflection grating arrays for the Reflection Grating Spectrometer on board XMM

The reflection grating spectrometer (RGS) on-board the x-ray multi-mirror (XMM) mission incorporates an array of reflection gratings oriented at grazing incidence in the x- ray optical path immediately behind a grazing incidence telescope. Dispersed light is imaged on a strip of CCD- detectors slightly offset from the telescope focal plane. The grating array picks off roughly half the light emanating from the telescope; the other half passes undeflected through the array where it is imaged by the European photon imaging camera (EPIC) experiment. XMM carries two such identical units, plus a third telescope with an EPIC detector, but no RGS. The basic elements of the RGA include: 202 identical reflection gratings, a set of precision rails with bosses that determine the position and alignment of each grating, a monolithic beryllium integrating structure on which the rails are mounted, and a set of three, kinematic support mounts which fix the array to the telescope. In this paper, we review our progress on the fabrication and testing of the RGA hardware, with particular attention to the components comprising the engineering qualification model, a flight-representative prototype which will be completely assembled in September of this year.

Reflectivity and scattering measurements of an Advanced X-ray Astrophysics Facility test coating sample

Reflectivity and scattering profile measurements were made on a gold-coated witness sample produced to evaluate mirror coatings for the Advanced X-ray Astrophysics Facility program. Reflectivity measurements were made at Al K, Ti K, and Cu K energies as a function of incident graze angle. The results are fit to a model that includes the effects of roughness, particulate and organic contamination layers, and gold-coating density. Reflectivities are close to theoretical, with the difference being well accounted for by 4.1 Å of roughness at spatial frequencies above 4 µm(-1), a gold-coating density equal to 0.98 bulk, and a surface contaminant layer 27 Å thick. Scattering measurements extending to ±35 arcmin of the line center were obtained by the use of Al K x rays and incidence angles from 0.75° to 3°. The scattering profiles imply a power spectral density of surface-scattering frequencies that follows a power law with an index of -1.0 and a total surface roughness for the spatial frequency band between 0.05 µm(-1) and 4 µm(-1) of 3.3 Å Combining the roughnesses derived from both the reflectivity and scattering measurements yields a total roughness of 5.3 Å for scattering frequencies between 0.05 µm(-1) and 15,000 µm(-1).

Performance of a variable-line-spaced master reflection grating for use in the reflection grating spectrometer on the x-ray multimirror mission

The X Ray Multimirror Mission will include a spectrometer consisting of two arrays of variable line-spaced reflection gratings for use in the 350 eV to 2.5 keV energy range. Approximately 720 replica gratings will be needed for two flight grating arrays and one spare. Evaluation of potential master gratings to be used in the replication process has begun. Both reflectivity and scattering x-ray measurements for three mechanically ruled prototype master gratings have been reported.

Reflection Grating Spectrometer on board XMM

The x-ray multi-mirror (XMM) mission is the second of four cornerstone projects of the ESA long-term program for space science, Horizon 2000. The payload comprises three co- aligned high-throughput, imaging telescopes with a FOV of 30 arcmin and spatial resolution less than 20 arcsec. Imaging CCD-detectors (EPIC) are placed in the focus of each telescope. Behind two of the three telescopes, about half the x-ray light is utilized by the reflection grating spectrometer (RGS). The x-ray instruments are co-aligned and measure simultaneously with an optical monitor (OM). The RGS instruments achieve high spectral resolution and high efficiency in the combined first and second order of diffraction in the wavelength range between 5 and 35 angstrom. The design incorporates an array of reflection gratings placed in the converging beam at the exit from the x-ray telescope. The grating stack diffracts the x-rays to an array of dedicated charge-coupled device (CCD) detectors offset from the telescope focal plane. The cooling of the CCDs is provided through a passive radiator. The design and performance of the instrument are described below.

Performance of multilayer coated gratings at near normal incidence in the extreme ultaviolet

We report on the normal incidence performance of multilayer coated concave gratings in spectral regions where near normal incidence reflectivities of conventional coatings are calculated to be 10−4 to 10−5. These concave gratings were designed for use in a McPherson 225 Spectrometer and were tested for resolution in this instrument using a Garton flash tube source and a film detector. Efficiency measurements were obtained for both the gratings and flat multilayers fabricated at the same time using a Penning source and an incident beam monochromator to obtain spectral purity. The incident and diffracted beams were measured using the same microchannel plate based two dimensional photon counting detector. Experimental results are compared with scalar theory.

Development, fabrication, and metrology of the electro-optical breadboard model for the reflection grating array of the XMM Grating Spectrometer

A prototype array consisting of eight diffraction gratings has been fabricated for the XMM Reflection Grating Spectrometer. A component of the full spectrometer is an array of approximately 200 diffraction gratings. The diffraction gratings were produced using lightweight silicon carbide substrates and a replication technique. The prototype array was developed and assembled using the same tolerances as the flight arrays which have typical tolerances of 3 micrometers in translation and sub-arc seconds in rotation. The metrology applied during inspection and assembly included precision linear measurements, full aperture figure measurements, and angular interferometry.

Design and analysis of the reflection grating arrays for the X-Ray Multi-Mirror Mission

The Reflection Grating Spectrometer Experiment (RGS), which has been selected for flight on the European Space Agencys X-Ray Multi-Mirror Mission (XMM), includes two arrays of reflection gratings that are placed in the X-ray optical path behind two separate grazing incidence X-ray telescopes. Each of the grating arrays picks off roughly half the X-ray light emanating from its telescope and diffracts it to a dedicated strip of charge-coupled device (CCD) detectors offset from the telescope focal plane. The arrays contain 224 100 mm X 200 mm gratings, each mounted at a graze angle of 1.58° to the incident beam. The gratings are produced by epoxy replication of a common master onto very thin substrates. Both the gratings and the detectors are mounted on a Rowland circle which also includes the telescope focus. In this paper, we review the current state of both the engineering and the optical designs for the grating arrays.

Multilayer coated concave diffraction grating resolution and efficiency in the extreme ultraviolet

Recent advances in multilayer structures for the extreme ultraviolet now make it possible to construct diffraction gratings in the wavelength range below 350 A, which can be used for precision measurements using normal incidence spectrometers. We report results from two such gratings, one with conventionally ruled blazed facets and the other with a holographically ruled sinusoidal surface. Both gratings are 1 m in radius, 1200 lines/mm coated with a molybdenum-silicon multilayer for use in the 150 A wavelength region. A 1 m normal incidence spectrometer with a Garton flash tube source and a film detector was used to test the spectral resolution. Relative efficiency measurements were obtained by comparison to spectra produced by an osmium coated grating. Emission lines in the wavelength region of interest (characteristic of the source) are easily detected and well resolved with both multilayer coated gratings. Quantitative efficiency measurements were obtained using a Penning source coupled to a 1 m grazing incidence monochromator and an imaging photon counting detector.