The heart of the observatory is, of course, the x-ray telescope. Grazing-incidence optics function because x rays reflect efficiently if the angle between the incident ray and the reflecting surface is less than the critical angle. This critical grazing angle is approximately , where is the density in g-cm and E is the photon energy in keV. Thus, higher energy telescopes must have dense optical coatings (iridium, platinum, gold, etc.) and smaller grazing angles. The x-ray optical elements for Chandra and similar telescopes resemble shallow angle cones, and two reflections are required to provide good imaging over a useful field of view; the first CXO surface is a paraboloid and the second a hyperboloid - the classic Wolter-1 design. The collecting area is increased by nesting concentric mirror pairs, all having the same focal length. The wall thickness of the inner elements limit the number of pairs, and designs have tended to fall into two classes: Those with relatively thick walls achieve stability, hence angular resolution, at the expense of collecting area; those with very thin walls maximize collecting area but sacrifice angular resolution. NASA's Einstein Observatory (1978), the German ROSAT (1990), and the CXO optics are examples of the high-resolution designs, while the Japanese-American ASCA (1993) and European XMM mirrors are examples of emphasis upon large collecting area.
The mirror design for CXO includes eight optical elements comprising four paraboloid/hyperboloid pairs which have a common ten meter focal length, element lengths of 0.83-m, diameters of 0.63, 0.85, 0.97, and 1.2-m, and wall thickness between 16-mm and 24-mm. Zerodur, a glassy ceramic, from Schott was selected for the optical element material because of its low coefficient of thermal expansion and previously demonstrated capability (ROSAT) of permitting very smooth polished surfaces.
Figure 
shows the largest optical element being ground at HDOS. Final polishing was performed with a large lap designed to reduce surface roughness without introducing unacceptable lower frequency figure errors. The resulting rms surface roughness over the central 90% of the elements varied between 1.85 and 3.44 in the 1 to 1000-mm band; this excellent surface smoothness enhances the encircled energy performance at higher energies by minimizing scattering.
Figure: The largest optical element being ground.
The mirror elements were coated at OCLI by sputtering with iridium over a binding layer of chromium. OCLI performed verification runs with surrogates before each coating of flight glass; these surrogates included witness samples. The x-ray reflectivities of the witness flats were measured at SAO to confirm that the expected densities were achieved. The last cleaning of the mirrors occurred at OCLI prior to coating, and stringent contamination controls were begun at that time because both molecular and particulate contamination have adverse impacts on the calibration and the x-ray performance. Figure shows the smallest paraboloid in the OCLI handling fixture after being coated.
Figure: The smallest parabaloid after coating.
The final alignment and assembly of the mirror elements into the High Resolution Mirror Assembly (HRMA) was done at, and by, EKC. The completed mirror element support structure is shown in Figure . Each mirror element was bonded near its mid-station to flexures previously attached to the carbon fiber composite mirror support sleeves. The four support sleeves and associated flexures for the paraboloids can be seen near the top of the figure, and those for the outer hyperboloid appear at the bottom. The mount holds more than 1000 kg of optics to sub-arcsecond precision.
Figure: The fixture to which the eight optical elements were mounted.
The mirror alignment was performed with the optical axis vertical in a clean and environmentally controlled tower. The mirror elements were supported to approximate a gravity-free and strain-free state, positioned, and then bonded to the flexures. A photograph taking during the assembly and alignment process is shown in Figure . Despite the huge mass of the system and the stringent environmental controls, the heat produced by a 50 watt light bulb at the top of the facility caused some alignment anomalies until detected and resolved.
Figure: A photograph of the HRMA during assembly and alignment at EKC.
The HRMA was taken to MSFC for pre-launch x-ray calibration (see O'Dell and Weisskopf (1998) and references therein) in the fall of 1996, and then to TRW for integration into the spacecraft. Testing at MSFC took place in the X-Ray Calibration Facility (XRCF), shown in Figure . The calibration facility has a number of x-ray source and detector systems and continues to be used for x-ray tests of developmental optics for such programs as Constellation-X. Details concerning the XRCF may be found in Weisskopf and O'Dell (1997) and references therein.
X-ray testing demonstrated that the CXO mirrors are indeed the largest high-resolution X-ray optics ever made; the nominal effective area (based on the ground calibrations) is shown as a function of energy in the left panel of Figure , along with those of their Einstein and ROSAT predecessors. The CXO areas are about a factor of four greater than the Einstein mirrors. The effective areas of CXO and ROSAT are comparable at low energies because the somewhat smaller ROSAT mirrors have larger grazing angles; the smaller grazing angles of CXO yield more throughput at higher energies. The fraction of the incident energy included in the core of the expected CXO response to 1.49-keV x rays is shown as a function of image radius in the right panel of Figure including early in-flight data. The responses of the Einstein and ROSAT mirrors also are shown. The improvement within 0.5-arcsec is dramatic, although it is important to note that the ROSAT mirrors bettered their specification and were well matched to the principal detector for that mission. The excellent surface smoothness achieved for the CXO (and ROSAT) mirrors result in a very modest variation of the performance as a function of energy; this reduces the uncertainties which accrue from using calibration data to infer properties of sources with different spectra, and improves the precision of the many experiments to be performed.
Figure: An aerial view of the X-ray Calibration Facility at MSFC.
Figure: Effective area and encircled energy comparisons.