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The X-ray mirror assembly on ROSAT is a grazing incidence four-fold nested Wolter type I configuration. All of the 8 mirror shells are made out of Zerodur, a glass ceramic with an almost negligible thermal expansion coefficient, and are coated with a thin gold layer in order to increase X-ray reflectivity. Typical grazing angles are between 1 and 2 degrees, depending on the mirror subshell considered. Performance figures for the X-ray mirror assembly are summarized in Table 4.1 . The performance of the XMA in terms of collecting area and spatial resolution varies with off-axis angle; guest observers wishing to utilize the full field of view for mapping extended regions of sky should therefore consider the effects of diminishing sensitivity, resolving power and the complicated structure of the point response function with off-axis angle (see § 4.1.2; ).