Design optimization and trade-off study of WFXT optics Paolo Conconi a , Giovanni Pareschi a , Sergio Campana a , Oberto Citterio a , Marta Civitani a , Vincenzo Cotroneo a , Laura Proserpio a , Gianpiero Tagliaferri a , Giancarlo Parodi b a INAF-Osservatorio astronomico di Brera, Via Bianchi 46, Merate (LC), I-23807 Italy; b BCV Progetti, Via Orsola 1, Milano, I-20123, Italy ABSTRACT X–ray mirrors are usually built in the Wolter I (paraboloid-hyperboloid) configuration which provides, in princi- ple, perfect on-axis images. 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. Differ- ent 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. 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, of 5 arcsec HEW resolution goal to be achieved across a 1-deg field of view, in addition an effective area of 5–9000 cm 2 at 1 keV is needed. Keywords: Missions, X–ray astronomical instrumentation, X–ray telescopes, wide–field X–ray optics, X–ray astronomical surveys 1. INTRODUCTION Exploring the high-redshift Universe to the era of galaxy and cluster formation requires an X–ray survey that is both sensitive and extensive, which complements current and planned deep, high-resolution, wide-field surveys in the optical, radio, and infrared bands. To this end, the Wide-Field X–Ray Telescope (WFXT), is designed to be two orders of magnitude more effective than previous and planned X–ray missions in carrying out surveys (see Table 1). WFXT is the only X–ray survey mission that will match, in area and sensitivity, the next generation of wide-area optical, IR and radio surveys, as well as that will support contemporaneous and planned ground-based giant telescopes and ALMA, and on-orbit facilities, including planned and proposed missions, such as JWST and IXO. WFXT will address many outstanding cosmological and astrophysical objectives. Unlike previous X–ray astronomy telescopes, WFXT is optimized for performance over its full field of view (rather than mainly on- axis), which makes it an ideal survey instrument. Focussing telescopes for X–ray astronomy are usually built in the Wolter I configuration 1,2 providing, at least theoretically, perfect images along the telescope’s optical axis. Wolter I telescopes are made by two mirror segments (usually called parabola and hyperbola) joining at the intersection plane. The image quality rapidly degrades for observations far from the optical axis. In order to sensibly improve the off-axis response of an X–ray telescope one has to act on the mirror design. Mirror shells for X–ray telescopes can be built, with the same degree of complexity, with polynomial profiles 3,4 . Polynomial mirror profiles are described usually by forth (or third) grade polynomia and optimization techniques can be easily implemented 4,5 . Depending on the optimization criteria one can find again 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 4,5,6,7 . To optimize a mirror assembly for X–ray surveys further improvements can be introduced. As discussed in ref. 5, mirrors can be build with self-similar (homothetic) shapes giving to the mirror assembly a “butterfly”-like shape. This will guarantee that each mirror have the same curvature of the focal plane. In addition, in order to Further author information: (Send correspondence to SC.) SC: E-mail: sergio.campana@brera.inaf.it, Telephone: 39 039 5971018 Optics for EUV, X-Ray, and Gamma-Ray Astronomy IV, edited by Stephen L. O'Dell, Giovanni Pareschi, Proc. of SPIE Vol. 7437, 74370D · © 2009 SPIE CCC code: 0277-786X/09/$18 · doi: 10.1117/12.827468 Proc. of SPIE Vol. 7437 74370D-1