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A schematic view of the window support structure is shown in Figure 4.6 . The support structure consists of a rigid circle (28 mm diameter) with 8 equally spaced radial struts. Below this ``rib'' structure there are two mesh systems, a coarse mesh of m tungsten wires with 2 mm spacings and a fine mesh with m tungsten wires with 0.4 mm spacings. The total (energy independent) transmission of this window support structure is 79 percent apart from those areas of the detector which are blocked by the struts. This average transmission is obtained only over a sufficiently large area; over a smaller area the actual transmission may increase or decrease depending on how much of the area of the X-ray beam is occulted by the mesh grid wires. In the vicinity of the center of the field of view the focus of the X-ray beam from a point source is so sharp and consequently the X-ray illuminated area on the PSPC so small that significant shadowing can occur behind the coarse mesh grid. Shadowing can also occur behind the rib structure; however, the resolution of the telescope is already greatly diminished at large off-axis angles (cf., Figure 4.4 ). Since the aspect control system is quite stable, a slow spacecraft ``wobble'' will be performed. A slow motion diagonally to the entrance window wire grids will be carried out by the spacecraft (see § 3.4 ). In this fashion the coarse mesh grid will be smoothed out when projected onto fixed sky coordinates and a permanent occultation of X-ray sources behind the coarse wire mesh grid is not possible. In order to illustrate shadowing due to the window support structure, Figure 4.7 plots PSPC count rate versus off-axis angle for a bright X-ray source, Cygnus X-2. At off-axis angles larger than 40', the shadowing due to the wires is completely negligible and loss of photons is dominated by the mirror vignetting function.