The layouts and the FOV coverage of WXT and FXT are below. WXT consists of eight modules, with FXT mounted at the center. WXT employs the MPO Lobster-eye optics, with a focal length of 375 mm. Six of the WXT modules have an aperture size of about 280mm*280mm and subtend 20*20 square degrees per module. Each is composed of 7*7 mosaicking MPO pieces. The other two modules of WXT have an aperture size of about 280*400 mm and cover 20*30 square degrees each, which is mosaicked by 7*10 MPO pieces.
WXT has a large focal plane size, ~140mm*140mm for each of the six 280mm*280mm modules, and ~140mm*200mm for each of the remaining two modules. The baseline choice of the WXT detector is gas detector. Currently, the employment of GEM-based (Gas electron multiplier) detectors is being investigated. The need of high transmission at low energy X-rays requires a very thin entrance window. As a back-up plan for the WXT focal plane detectors, MCP detectors are also considered as a surrogates to gas detectors if necessary.
FXT is a long-focal length focusing X-ray telescope to achieve the largest possible effective area (the current FXT focal length is limited by the satellite platform). The present design of FXT is of a MPO K-B type, narrow field-of-view telescope with a focal length of 1.4 m. FXT has an aperture size of about 240 mm, which is mosaicked by 6*6 MPO pieces. The current consideration for the focal plane detector of FXT is silicon based detector, i.e. CCD to gain better spectral performance. As alternatives, a Wolter-I type MPO telescope (e.g. one similar to MIXS-T to gain a larger effective area at the same focal length) or a Wolter-I type Al foil telescope developed Tongji University in China is under consideration.
Figure 1 Layouts of WXT and FXT
Figure 2 Sketch of the FOVs of WXT and FXT (not to scale)
Figure 3 Effective area of WXT
Figure 4 Effective area of FXT
Figure 5 The red line indicates the effective area obtained by considering the data on the center spot and arms, and the blue line is corresponding to the data just on the centre spot.