Abstract
Conical Wolter-I geometry is employed for many x-ray telescopes to lower their cost and fabrication difficulty at the expense of angular resolution. Owing to the conic error, the angular resolution of conical Wolter-I geometry is much worse than that of Wolter-I geometry, especially for the telescopes with large diameter. We optimized the conical Wolter-I geometry to significantly improve the angular resolution. We designed a conical Wolter-I geometry with sectioned secondary mirrors. Based on the normal conical Wolter-I geometry, we divided the secondary mirror into two equal sections along the optical axis. In this case, the collecting area was reduced by 5% because of the interval between the two sections. Meanwhile, the conic error was reduced by about 50%, indicating a great improvement in angular resolution. Regarding our improvement in the thermal slumping technique, it is feasible to fabricate sectioned mirrors, thus improving the angular resolution by 50% at the cost of a 5%-reduction in collecting area. In addition, a hybrid geometry, comprising the sectioned and nonsectioned geometries, is proposed as an alternative for x-ray telescopes with a large amount of nested shells, to obtain both a large collecting area and decent angular resolution.
Highlights
Throughout the history of x-ray astronomy, grazing-incidence telescopes have played very important roles in x-ray observation, and Wolter-I geometry has been considered very crucial in grazing-incidence x-ray observation
We have described a procedure for modifying the parameters of conical Wolter-I geometry, to significantly improve its angular resolution
By dividing the secondary mirror into two sections along the optical axis, the conic error could be decreased by 50%, implying a 50% improvement in angular resolution (i.e., half power diameter (HPD))
Summary
Throughout the history of x-ray astronomy, grazing-incidence telescopes have played very important roles in x-ray observation, and Wolter-I geometry has been considered very crucial in grazing-incidence x-ray observation. Considering the equilibrium between effective area and angular resolution, as well as the time and money spent on x-ray telescope fabrication, Chen et al.[32] presented three x-ray telescope structures based on Wolter-I geometry, which use one conical surface and one quadric surface as the mirror pair, respectively. To balance the performance and fabrication of the x-ray telescope, we designed a conical Wolter-I geometry with sectioned secondary mirrors. The normal rays, reflected by the primary mirror, will only strike either of the two secondary sections, and converge to the focal plane In this case, the sectioned structure was expected to have an improved resolution about 50% better than that of the normal conical Wolter-I geometry, at the cost of a reduction of about 5% in collecting area. A hybrid geometry, consisting of the sectioned and nonsectioned geometries, is introduced as an alternative for x-ray telescopes with a large amount of nested shells
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