Charles M. Fleetwood

Mirror Technology Development for the International X-ray Observatory Mission

The International X-ray Observatory mission is a collaborative effort of NASA, ESA, and JAXA. It will have unprecedented capabilities in spectroscopy, imaging, timing and polarization measurement. A key enabling element of the mission is a flight mirror assembly providing unprecedented large effective area (3 m2) and high angular resolution of (5 arcseconds half-power diameter). In this paper we outline the conceptual design of the mirror assembly and development of technology to enable its construction.

Long-duration orbital effects on optical coating materials

We flew specimens of eight different optical coating materials in low earth orbit as part of the Long Duration Exposure Facility manifest to determine their ability to withstand exposure to the residual atomic O and other environmental effects at those altitudes. We included samples of Al, Au, Ir, Os, Pt, Al + MgF(2), Al + SiO(x), and chemical-vapor-deposited SiC, representing reflective optical applications from the vacuum ultraviolet through the visible portions of the spectrum. We found that the majority of the materials suffered sufficient reflectance degradation to warrant careful consideration in the design of future space-flight instrumentation.

Optical coating technology for the EUV

Abstract Advances in optical coating and materials technology have been one of the key motivators for the development of missions such as the Far Ultraviolet Spectroscopic Explorer recently selected by NASA for an Explorer class mission in the mid 1990s. We review the performance of a range of candidate coatings for normal incidence and glancing incidence applications, and discuss strengths and problem areas for their use in space. The importance of recent developments in multilayer films, chemical vapor deposited SiC (CVD-SiC) mirrors, and SiC films are discussed in the context of EUV instrumentation design. For example, the choice of optical coatings is a design driver for the selection of the average glancing angle for the FUSE telescope, and impacts efficiency, short wavelength cut-off, and physical size.

ASTRO-E/XRS blocking-filter calibration

We describe the transmission calibration of the Astro-E XRS blocking filters. The XRS instrument has five aluminized polymide blocking filters. These filters are located at thermal stages ranging from 200 K to 60 mK. They are each about 1000 angstrom thick. XRS will have high energy resolution which will enable it to see some of the extended fine structure around the oxygen and aluminum edges of these filters. Thus, we are conducting a high spectral resolution calibration of the filters near these energies to resolve out extended fine structure and absorption lines.

An approach for alignment, mounting, and integration of IXO mirror segments

The telescope on the International X-ray Observatory (IXO) comprises nearly 15 thousand thin glass mirror segments, each of them is capable of reflecting board-band soft x-rays at grazing angles. These mirror segments form densely packed, two-staged shells, in a Wolter type I optical design, in which each pair of the mirrors focus x-ray onto the focal plane in two reflections. The requirement in angular resolution of the IXO telescope is 5 arc-seconds. This requirement places severe challenges in forming precisely shaped mirror segments as well as in aligning and mounting these thin mirrors, which are 200 to 400 mm in size and 0.4 mm in thickness. In this paper, we will describe an approach for aligning and mounting the IXO mirror segments, in which no active adjustment is made to correct for any existing figure errors. The approach comprises processes such as suspension of a mirror under gravity to minimize gravity distortion, temporary bonding onto a strongback, alignment and transfer to a permanent structure and release of mirror from the temporary mount. Experimental results and analysis in this development are reported.

Chemical vapor deposited silicon carbide mirrors for extreme ultraviolet applications

Advances in optical coating and materials technology have made possible the development of instruments with substantially im- proved efficiency in the extreme ultraviolet (EUV). For example, the de- velopment of chemical vapor deposited (CVD) SiC mirrors provides an opportunity to extend the range of normal-incidence instruments down to 60 nm. CVD SiC is a highly polishable material yielding low-scattering surfaces. High UV reflectivity and desirable mechanical and thermal properties make CVD SiC an attractive mirror and/or coating material for EUV applications. The EUV performance of SiC mirrors, as well as some strengths and problem areas, is discussed.

Contamination-induced degradation of optics exposed to the Hubble Space Telescope interior

After the first Hubble Space Telescope (HST) servicing mission, the WFPC-1 and HSP instruments were returned to earth. Three optical surfaces from these instruments were analyzed in detail. They were the WFPC-1 pickoff mirror, the WFPC-1 aperture window, and the HSP filter assembly, all of which faced the central hub area of the HST. Hub-facing optics were of particular interest because any degradation in their performance might indicate a changed environment within the telescope itself. The pickoff mirror reflectance and aperture window transmittance were both found to be severely degraded in the far UV. The cause of the reflectance loss was contamination; the pickoff mirror was covered with a contaminant film about 450 angstrom thick, and the aperture window and HSP filters each had about 150 angstrom. The contamination contained multiple chemical species, some of which had been photopolymerized by exposure to earth-albedo UV. A UV-stimulated deposition and polymerization mechanism was posited. This contamination process is not expected to happen, however, for current and future instruments in HST. The HST components outgassed for 3.5 years before the first servicing mission, so the contaminants are no longer present in any appreciable quantity. Steps are being taken to ensure that any new equipment installed in the HST will not outgas. Over 2.5 years of operation, neither the WFPC-2 instrument nor the corrective optics module (COSTAR) has shown performance degradation in the UV There is also no evidence that the primary or the secondary mirror of the HST has changed.

Optical metrology for the segmented optics on the Constellation-X soft x-ray telescope

We present the metrology requirements and metrology implementation necessary to prove out the mirror technology for the Constellation-X (C-X) soft x-ray telescope (SXT). This segmented, 1.6m diameter highly nested Wolter-1 telescope presents many metrology and alignment challenges. A variety of contact and non-contact optical shape measurement, profiling and interferometric methods are combined to test the forming mandrels, some of the replication mandrels, the formed glass substrates before replication and the replicated mirror segments. The mirror segments are tested both stand-alone and in-situ in mirror assemblies. Some of these methods have not been used on prior x-ray telescopes and some are feasible only because of the segmented approach used on the SXT. Methods to be discussed include axial interferometric profiling, azimuthal circularity profiling, midfrequency error profiling, and axial roughness profiling. The most critical measurement is axial profiling, and we compare the method in use to previous methods such as the long trace profilometer (LTP). A companion paper discusses the method of non-contact 3D profiling using a laser sensor and distance measuring interferometers.

CVD silicon carbide mirrors for EUV applications

Advances in optical coating and materials technology have made possible the development of instruments with substantially improved efficiency in the extreme ultraviolet (EUV). For example, the development of chemical vapor deposited (CVD) SiC mirrors provides an opportunity to extend the range of normal incidence instruments down to 60 nm. CVD-SiC is a highly polishable material yielding low scatter surfaces. High UV reflectivity and desirable mechanical and thermal properties make CVD-SiC an attractive mirror and/or coating material for EUV applications. The EUV performance of SiC mirrors as well as some strengths and problem areas are discussed.

PERFORMANCE OF HIGH-DENSITY CAST SILICON CARBIDE IN THE EXTREME ULTRAVIOLET

The normal-incidence reflectance of high-density cast silicon carbide (SiC) is evaluated in the extreme ultraviolet (EUV) spectral region. High reflectivity in the EUV is achieved. High reflectivity and the relatively low-cost manufacturing process make high-density cast SiC a promising mirror material for EUV applications.