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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; ).