A wide field X-ray telescope for astronomical survey purposes: from theory to practice

📝 Abstract

X-ray mirrors are usually built in the Wolter I (paraboloid-hyperboloid) configuration. This design exhibits no spherical aberration on-axis but suffers from field curvature, coma and astigmatism, therefore the angular resolution degrades rapidly with increasing off-axis angles. Different mirror designs exist in which the primary and secondary mirror profiles are expanded as a power series in order to increase the angular resolution at large off-axis positions, at the expanses of the on-axis performances. Here we present the design and global trade off study of an X-ray mirror systems based on polynomial optics in view of the Wide Field X-ray Telescope (WFXT) mission. WFXT aims at performing an extended cosmological survey in the soft X-ray band with unprecedented flux sensitivity. To achieve these goals the angular resolution required for the mission is very demanding ~5 arcsec mean resolution across a 1-deg field of view. In addition an effective area of 5-9000 cm^2 at 1 keV is needed.

💡 Analysis

X-ray mirrors are usually built in the Wolter I (paraboloid-hyperboloid) configuration. This design exhibits no spherical aberration on-axis but suffers from field curvature, coma and astigmatism, therefore the angular resolution degrades rapidly with increasing off-axis angles. Different mirror designs exist in which the primary and secondary mirror profiles are expanded as a power series in order to increase the angular resolution at large off-axis positions, at the expanses of the on-axis performances. Here we present the design and global trade off study of an X-ray mirror systems based on polynomial optics in view of the Wide Field X-ray Telescope (WFXT) mission. WFXT aims at performing an extended cosmological survey in the soft X-ray band with unprecedented flux sensitivity. To achieve these goals the angular resolution required for the mission is very demanding ~5 arcsec mean resolution across a 1-deg field of view. In addition an effective area of 5-9000 cm^2 at 1 keV is needed.

📄 Content

arXiv:0912.5331v2 [astro-ph.IM] 11 Feb 2010 Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 13 June 2018 (MN LATEX style file v2.2) A wide field X–ray telescope for astronomical survey purposes: from theory to practice Paolo Conconi1, Sergio Campana1,⋆, Gianpiero Tagliaferri1, Giovanni Pareschi1, Oberto Citterio1, Vincenzo Cotroneo1, Laura Proserpio1, Marta Civitani1 1 INAF-Osservatorio Astronomico di Brera, Via Bianchi 46, I–23807, Merate (LC), Italy 13 June 2018 ABSTRACT X–ray mirrors are usually built in the Wolter I (paraboloid-hyperboloid) configuration. This design exhibits no spherical aberration on-axis but suffers from field curvature, coma and astigmatism, therefore the angular resolution degrades rapidly with increas- ing off-axis angles. Different mirror designs exist in which the primary and secondary mirror profiles are expanded as a power series in order to increase the angular reso- lution at large off-axis positions, at the expanses of the on-axis performances. Here we present the design and global trade offstudy of an X–ray mirror systems based on polynomial optics in view of the Wide Field X-ray Telescope (WFXT) mission. WFXT aims at performing an extended cosmological survey in the soft X–ray band with unprecedented flux sensitivity. To achieve these goals the angular resolution re- quired for the mission is very demanding ∼5 arcsec mean resolution across a 1-deg field of view. In addition an effective area of 5–9000 cm2 at 1 keV is needed. Key words: Telescopes — X–rays: general — instrumentation: high angular resolu- tion 1 INTRODUCTION Focussing telescopes for X–ray astronomy are usually built in the Wolter I configuration (Wolter 1952a, 1952b) provid- ing, at least theoretically, perfect images for on-axis sources. Wolter I telescopes are made by two mirror segments, shaped as two surfaces of revolution (with a parabolic and hyper- bolic profile, respectively), joining at the intersection plane. Despite theoretically perfect images on-axis, the image qual- ity rapidly degrades far from the optical axis due to the curvature of the best focal plane, spherical and chromatic aberrations, limiting the capabilities of carrying out sur- veys of the X–ray sky. Simple solutions were suggested to improve the off-axis angular response, like shifting slightly out of focus a single detector (e.g. Cash et al. 1979) to get closer to the best focal plane at larger off-axis angles (like with the SWIFT XRT CCD, Burrows et al. 2005) or tilting the detectors (if more than one) to better approximate the (curved) best focal plane (as with the Chandra ACIS-I de- tectors or the XMM-Newton MOS detectors). These simple recipes however provide only mild improvements. In order to sensibly improve the off-axis response of an X–ray telescope it is mandatory to act directly on the mirror design. The Wolter-Schwarzschild telescope eliminates the coma aber- ration for paraxial rays and provides a superior response ⋆E-mail: sergio.campana@brera.inaf.it at large off-axis angles with respect to the Wolter I tele- scope (Chase & VanSpeybroeck 1973). Nariai (1987, 1988) suggested the idea of a telescope made by two hyperboloid surfaces (see also Harvey & Thompson 1999) which pro- vides good performances over a field of view of ∼20′. This was adopted for the Solar X–ray Imager telescope. Mirror shells for X–ray telescopes can be built, with the same de- gree of complexity, with polynomial profiles (Werner 1977; Burrows, Burg & Giacconi 1992, hereafter BBG). Polyno- mial mirror profiles are described usually by forth (or third) grade polynomia and optimization techniques can be easily implemented (BBG; Conconi & Campana 2001, CC here- after). Depending on the a priori optimization specifications one can re-discover the Wolter I design (optimizing for the best on-axis angular response) or other designs optimizing the angular response over a given field of view. Wide field design are commonly used for X–ray observations of the Sun (e.g. Tsuneta et al. 2000; Lemen et al. 2004; DeLuca et al. 2005) and have been proposed for X–ray survey purposes (e.g. BBG, CC; Conconi et al. 2004; Thompson & Harvey 2000). Besides changing the mirror shape, additional improve- ments can be introduced. In particular, as also discussed in CC, an improvement of the optical quality can be achieved by building different shells with a variable mirror length de- pendent upon the radius, giving to the total mirror assem- bly a butterfly-like shape. This solution causes a reduction 2 P. Conconi et al. of the total effective area with respect to shells with equal (longer) length, but it allows to keep the same curvature of the focal plane for the different shells, with the consequent improvement of image quality and sensitivity. In addition, mirror shells have a different plate scale, so that at any off- axis angle the X–rays of each shell are focussed in slightly different positions. To correct for this effect shells have to be built with modified focal lengths and (small) displ

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