Optical design and simulations of the soft x-ray telescope for Einstein Probe mission

Abstract

The Einstein Probe (EP) mission, which aims at discovering transients and monitoring variable objects in 0.5-4 keV Xrays, is a small scientific satellite dedicated to time-domain high-energy astrophysics. For this purpose, a large instantaneous field-of-view (60°×60°) X-ray telescope (WXT) is required in this mission, along with moderate spatial resolution (FWHM ~5′) and energy resolution. It will also carry a follow-up observation X-ray telescope (FXT) with a smaller field-of-viewcapable of much larger light-collecting power and better energy resolution than the main survey telescope. In this paper, we present the optical design and simulations of the FXT, which include the optimized structure parameters of the FXT and its focusing capabilities. The optical design and simulations are performed based on MATLAB program. The FXT employs conically-approximated Wolter-I geometry with a focal length of 2 m. And thin glasses with 100 mm in length and 0.3 mm in thickness are utilized as mirrors. To obtain a large collective area, the FXT will consist of 66 shells tightly-nested, whose diameters range from 80 mm to 250 mm based on the center of secondary mirrors. Seven different kinds of coatings will be deposited, designed by IMD. Based on such a kind of configuration, the effective area can be 245 cm2 at 1 keV and 196 cm2 at 4 keV, with an image quality to be approximately 56′′ in half-power diameter (HPD), and with a field of view (FOV) to be approximately 30′, theoretically.