The direct replication of W/Si multilayers and the effect of separating layer thickness on the performance of the multilayer before and after replication are investigated systematically. Platinum separating layers with different layer thicknesses were first deposited onto different supersmooth mandrels and then W/Si multilayers with the similar structure were deposited onto these Pt-coated mandrels by using a high vacuum DC magnetron sputtering system. After the deposition, these multilayers were replicated onto the commercially available float glass substrates by epoxy replication technique. These multilayers before and after replication are characterized by grazing-incident X-ray reflectance measurement and atomic force microscope. The measured results show that before and after replication, the reflectivity curves are much similar to those calculated and the surface roughness of each sample is close to that of the mandrel, when the separating layer thickness is larger than 1.5 nm. These results reveal that the W/Si multilayer with the separating layer thickness larger than 1.5 nm can be successfully replicated onto a substrate without modification of the structure, significant increase of surface roughness or apparent change of reflectivity.
The direct replication of a periodic W/Si multilayer was investigated systematically. The W/Si multilayer was deposited by a high vacuum dc magnetron sputtering system. After deposition, the multilayer was transferred from the supersmooth mandrel onto a commercially available float glass substrate by the epoxy replication technique. The multilayer was characterized by the grazing incident x-ray reflectance (GIXR) measurement, atomic force microscope (AFM), and Zygo GPI interferometer before and after replication. The measured results showed that the multilayer structure and reflectivity were almost the same, and the surface roughness was 0.22 and 0.23 nm before and after replication, respectively. It was demonstrated that the W/Si multilayer was successfully replicated without modification of the structure, a significant increase of surface roughness, or apparent change of reflectivity. The optical figure of the substrate after replication experienced significant changes of many waves but was actually improved for this specific test. Future studies will focus on learning how to control the resulting optical figure of our replication process on a thin substrate. C 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).
Optical design of nested conical Wolter I X-ray telescope covering energy band from 1 to 30 keV is investigated systematically. Recurrence relation of the nested structure is deduced, and the impact of the initial parameters on the performance is analyzed. Due to the need for hard X-ray astronomical observations in China, the initial structure is presented, for which six groups of W/B4C aperiodic multilayer coatings between the innermost and the outermost shell of the mirror are designed. The effective area, resolution, and field of view are calculated in the simulation. The results show that the effective area can achieve 71 cm 2 and the field of view can achieve 13 ′ at 30 keV. The resolution is estimated to be ∼10 ′′ in half-power diameter.OCIS codes: 340.7440, 340.7460. doi: 10.3788/COL201210.103401.The cosmic X-Ray background (CXB) resides in the 0.1 to 100 keV energy range. Deep X-ray surveys have resolved CXB around 1 keV into a population of discrete sources nearly entirely composed of active galactic nuclei (AGN) [1] . In addition, CXB in the 2 to 10 keV range has been completely resolved by Chandra and XMM-Newton telescopes when the sky flux is dominated by extragalactic emission. The main contributors are thought to be absorbed and unabsorbed AGNs with a mixture of quasars and narrow emission line galaxies [2,3] . The Chandra telescope, which is composed of four nested Wolter-I mirror pairs coated with a single iridium layer, could achieve the effective area of 800/400 cm [2] at 0.25/5 keV and resolution of 0.5 ′′ [4,5] . The XMM-Newton telescope, which consists of 58 gold-coated nested Wolter-I mirrors, could achieve the effective area of 1 475/580 cm 2 at 1.5/8 keV and resolution of 16 ′′[6] . Many X-ray sources above 10 keV are available, such as the peak of CXB at ∼ 30 keV [7] , which cannot be observed by currently in-orbit telescopes due to the severe decline in the effective area caused by low reflectivity of the single metal layer beyond the total external reflection. Aperiodic multilayer mirrors are preferred for high throughput in next-generation hard X-ray telescopes [8,9] . Hard X-ray telescopes are being developed, including IXO and Astro-H. IXO uses the nested Wolter-I structure with aperiodic tungsten and silicon multilayer coating. By using aperiodic multilayer coating, IXO is able to achieve an effective area of 0.6 m 2 /150 cm 2 at 6/30 keV and resolution of 5 ′′ [10,11] . To reduce the cost and the difficulty in fabrication of highly aspherical mirrors, conical structure is suggested instead of Wolter-I structure in Astro-H. Astro-H, with its coating of aperiodic platinum and carbon multilayer, is able to achieve the effective area of 300 cm 2 at 30 keV and resolution of 1.7 ′ [12] . Conical structures, which utilize conical mirrors instead of parabolic and hyperbolic mirrors, cannot image perfectly [13] , but only at a lower resolution of a few arcsec. Actually, resolution at the arcmin level is affected by the mirror fabrication that uses the epoxy replication method. The ASCA [1...
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