X-ray Study of Low-mass Young Stellar Objects in the ρ Ophiuchi ...

4.2. HRMA – HIGH RESOLUTION MIRROR ASSEMBLY 234.2 HRMA – High Resolution Mirror AssemblyIn spite of their high penetrating power, most X-rays can be reflected if the incident angle is smallerthan a critical value of 1 ◦ . HRMA consists of four pairs of Wolter Type-I mirrors, a combinationof parabolic and hypabolic shells, which realize small incident angles (Figure 4.2). The mirrors aremade of Zerodur glass polished and coated with iridium. The diameters of the mirrors are from0.65 to 1.23 m and the focal length is 10.066 m. In order to keep high accuracy of the mirrorsurface, the glass is very thick with the total mass of ∼1,500 kg.Fig. 4.2.— (left) Schematic view of The HRMA mirrors. (right) Wolter type-I optics( c○NASA/CXC/SAO).Effective areaFigure 4.3 shows the HRMA effective area, which is ∼800 cm 2 at the photon energy of 1 keV and∼100 cm 2 at 8 keV. It gradually decreases as the source off-axis angle becomes larger. These valuesare almost comparable to those of ASCA and slightly smaller than those of XMM-Newton.Point spread function and encircled energy fractionFigure 4.4 shows the shape of the point spread function (PSF) of HRMA and the fraction of encircledenergy at the on-axis position. The point source image is nearly circular and very sharp. The halfpower diameter (HPD) is ∼0. ′′ 5 (at 1.49 keV) and about 90 % of the photons are concentrated ina 2 ′′ diameter. The PSF gets elongated in an elliptical shape as the off-axis angle becomes larger(Figure 4.5), which is due to the aberrations of Wolter type I optics and the different shapes infocal surfaces among the four mirrors. The HPD thus becomes large as a function of the off-axis