Systems engineering analysis of aplanatic Wolter type I x-ray telescopes

It is well known that normal-incidence aplanatic telescope designs perform better at small field angles than ones corrected only for spherical aberrat ion. This is why most large astronomical telescopes fabricated in the past fifty years have been of the Ritchey-Chretien (aplanatic) design rather tha n of the classical Cassegrain design. For the relatively new field of x-ray astronomy, the Welter type I grazing incidence design has been extensively utilized. It consists of a paraboloidal primary mirror coaxial with a conf ocal hyperboloidal secondary mirror. Aplanatic versions of the Welter type I grazing incidence x-ray telescope have been discussed in detail in the li terature, and are widely touted as being superior designs. However, scatter ing effects from residual optical fabrication errors and other practical en gineering error sources prevent these grazing-incidence telescopes from bei ng near diffraction-limited (even on axis) at the very short operational x- ray wavelengths. A systems engineering analysis of these error sources indi cates that they will dominate coma at the small field angles, and of course astigmatism, field curvature, and higher-order aberrations dominate coma a t the large field angles. Hence, there is little improvement in performance when going to an aplanatic design. Comparison of performance predictions f or the classical versus aplanatic Welter type I x-ray telescope are present ed for the special case of the Solar X-Ray Imager (SXI) baseline design. SX I is expected to become a standard subsystem aboard the next generation of NOAA/GOES weather satellites. (C) 2000 society of Photo-Optical Instrumenta tion Engineers. [S0091-3286(00)01606-8].