The pre-launch HRI positional resolution was measured in the lab using a shadow mask with a test pattern consisting of alternating bars of open and closed area of various widths, as shown in Figures 4.13 and 4.14 of Appendix F. Mean values from such measurements gives = 8.4m, which at the ROSAT X-ray telescope focal length of 2.4 m, yields an intrinsic FWHM resolution of 1.7'' for the HRI.
Large scale geometrical distortions of the HRI were measured by projecting a mesh pattern on the face of the detector (cf. Appendix F and its Figure 4.15 for details) and found to be less than 1'' over its field of view. Note, however, that the image resolution varies over the field due to the off-axis response of the X-ray Telescope. Small-scale differential non-linearity was also determined to have an RMS deviation from the mean of less than 0.1''.