X-ray survey missions require nested grazing incidence telescopes with a large field of view (1° or larger) and moderately high resolution across the field of view. This cannot be accomplished using flat focal plane detectors. The telescopes have excellent on-axis resolution, but unacceptable resolution at the edge of the field of view. The aberrations need to be optimized and correctly balanced to improve the telescope optical performance across the field of view.
Wolter-Schwarzschild telescopes provide an excellent baseline. Aberrations are fairly balanced since they do not have coma aberration; thus, its off-axis images are fairly symmetric. This effect led to the investigation of ways to improve its off-axis image quality.
A new class of x-ray grazing incidence telescopes was invented. These telescopes have improved off-axis optical performance. The improvement is realized by adding a small amount of axial second order sag, either to the primary mirror or the secondary mirror of a Wolter-Schwarzschild type-1 x-ray telescope. This additional sag term improves the off-axis image quality at the expense of on-axis image quality, leading to improved overall image quality over the field of view. A segmented focal plane detector consisting of several detectors is required to take full advantage of these designs.
A design program was developed to optimally place the nested telescopes inside each other. The program is based on two design criteria: (1) principal surfaces of the nested telescopes have to match and (2) the nested telescopes do not obstruct the field-of-view of the telescope. The first requirement optimally matches the off-axis images of the nested telescopes.
A ray-tracing program developed for an earlier NASA project was modified. This program allowed addition of small secondorder axial sag terms to the primary or secondary mirror surfaces of the telescope. Using this program, sag values were found that balance the off-axis aberrations optimally across the field of view on the best focal surface of the telescope. In the current program, the same sag value is used for every nested mirror pair. The studied telescope design consists of 148 telescopes nested inside each other, and the outer diameter of the telescope is 1.3 m.
Nested telescopes designed this way provide nearly constant resolution across the field of view. Conventionally, the off-axis aberrations of x-ray telescopes are balanced by defocusing the detector or using polynomial telescope designs as the basis for optimization. Balancing the aberrations by adding and optimizing primary or secondary mirror axial sag of the Wolter-Schwarzschild x-ray telescope offers a simpler approach. Simple design principles can be used, and no complex optimization procedures are required.
This work was done by Timo Saha of Goddard Space Flight Center. GSC-17286-1