Wide-field X-ray Telescope

2019-09-16
1. Lobster-eye MPO X-ray focusing imaging
In general, X-ray focusing instruments are based on multiple grazing incidence reflections on smooth surfaces almost parallel to the direction of incident X-rays. The optics can be arranged in several different configurations.  The conventionally and commonly adopted configuration is the Wolter-I optics (Wolter 1952), which reflects X-rays by a tubular, rotationally symmetric parabolic surface followed by a hyperbolic surface. Such a system is used for many of the past and current X-ray telescopes such as Einstein, ROSAT, Chandra, XMM-Newton, and Swift/XTR. The FoV of such optics is inherently small, typically less than one degree or so.
An alternative configuration is Lobster-eye optics (Angel 1979), which mimics the imaging principle of the eyes of lobsters as shown in Figure 1. Incoming light is reflected off the walls of many tiny square pores arranged on a sphere and pointed towards the co-centric spherical center. The reflection surfaces are configured orthogonal to each other without a specific optical axis, and thus the FOV can in principle subtend the entire solid angle. 

Figure 1 Lobster eye optics: X-rays from a distant source illuminating micro pores are brought onto the focus on a focal plane (sphere) with a characteristic cruciform pointing spread function. 

The FoV of the optical arrangement indicated in Figure 2 is only limited by the size of the optics (the number of MPO pieces) or the size of the detector. The PSF remains almost unchanged over the entire FOV without vignetting of the effective area. Such a wide-field lobster-eye telescope provides the technological basis of the next generation wide-field X-ray monitors to detect faint and short-lived phenomena like high-redshift Gamma-Ray Bursts, distant X-ray novae and tidal disruption events.

  

  Figure 2 A demonstration prototype of a lobster-eye MPO mirror assembly developed at X-ray Imaging Lab, NAOC, CAS.

2. Design of WXT

WXT consists of 12 identical modules with 375 mm focal length, each covering about 300 square degrees. The 12 modules make a total un-vignetted FoV of WXT of about 3600 square degrees (~1.1 steradian). An illustration of the WXT FoV configuration is shown in Figure 4. WXT has a large-format focal plane of approximately 420mm by 420 mm. The baseline choice of the focal plane detectors is CMOS imaging sensors, developed by Gpixel Inc. in China.

  

  Figure 3 Illustration of the field-of-views of the WXT modules and the FXT.