Optical EngineeringSystems engineering analysis of aplanatic Wolter type I x-ray telescopes
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It is well known that normal-incidence aplanatic telescope designs perform better at small field angles than ones corrected only for spherical aberration. This is why most large astronomical telescopes fabricated in the past fifty years have been of the Ritchey-Chretien (aplanatic) design rather than of the classical Cassegrain design. For the relatively new field of x-ray astronomy, the Wolter type I grazing incidence design has been extensively utilized. it consists of a paraboloidal primary mirror coaxial with a confocal hyperboloidal secondary mirror. Aplanatic versions of the Wolter type I grazing incidence x-ray telescope have been discussed in detail in the literature, and are widely touted as being superior designs. However, scattering effects from residual optical fabrication errors and other practical engineering error sources prevent these grazing-incidence telescopes from being near diffraction-limited (even on axis) at the very short operational x-ray wavelengths. A systems engineering analysis of these error sources indicates that they will dominate coma at the small field angles, and of course astigmatism, field curvature, and higher-order aberrations dominate coma at the large field angles. Hence, there is little improvement in performance when going to an aplanatic design. Comparison of performance predictions for the classical versus aplanatic Wolter type I x-ray telescope are presented for the special case of the Solar X-Ray Imager (SXI) baseline design. SXI is expected to become a standard subsystem aboard the next generation of NOAA/GOES weather satellites.