Image Quality Criteria For Wide-Field X-Ray Imaging Applications

For staring, wide-field applications, such as a solar X-ray imager, the severe off-axis aberrations of the classical Wolter Type-I grazing incidence X-ray telescope design drastically limits the `resolution\u27 near the solar limb. A specification upon on-axis fractional encircled energy is thus not an appropriate image quality criterion for such wide-angle applications. A more meaningful image quality criterion would be a field-weighted-average measure of `resolution\u27. Since surface scattering effects from residual optical fabrication errors are always substantial at these very short wavelengths, the field-weighted-average half-power radius is a far more appropriate measure of aerial resolution. If an ideal mosaic detector array is being used in the focal plane, the finite pixel size provides a practical limit to this system performance. Thus, the total number of aerial resolution elements enclosed by the operational field-of-view, expressed as a percentage of the number of ideal detector pixels, is a further improved image quality criterion. In this paper we describe the development of an image quality criterion for wide-field applications of grazing incidence X-ray telescopes which leads to a new class of grazing incidence designs described in a following companion paper