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The HRI positional resolution is measured by projecting X-rays through a shadow mask and analyzing the resulting image. A test pattern consisting of alternating bars of open and closed area of various widths is used for this purpose. Figure 4.13 shows a region of this mask that was used for high resolution measurements and is an X-ray image of the mask obtained with the ROSAT HRI. The smallest slits have a width of 12.7 m and center-to-center spacing of 50.8 m, while the larger slits have a width of 25 m and 102 m spacing. A projection of the smallest slit data is made and fit to a gaussian resolution function (PSF ) that is convolved with the the true slit along each orthogonal axis. These projections are shown on Figure 4.14 , and the resulting fit yields m for the x axis, and m for the y axis. Taking the mean of these two values, m, and the ROSAT X-ray telescope focal length of 2.4 m, the FWHM ( ) resolution of the HRI corresponds to 1.7''.
The large scale distortion of the HRI has been measured by projecting a mesh pattern on the face of the detector. This mesh is composed of m wide slits, m long, with a center-to-center spacing of m, and is aligned with the detector axes. Figure 4.15 shows the deviations from the average mesh spacing for individual pairs of slits as a function of their X coordinate. The rms deviation from the average is m for the X axis and m for the Y axis. Based on these data, the HRI is capable of mapping large scale structure with an accuracy of better than 1'' over its field of view. However, the image resolution will vary over the field of view due to the off-axis response of the X-ray telescope as discussed in Chapter 11 .
The small scale differential non-linearity of the HRI is determined by measuring the spacing of a regularly spaced dot pattern projected by a UV lamp onto the face of the HRI, and from the spacing of the mesh pattern described above. The rms deviation of the data from the mean is 1.3 m ( ).