Follow-up X-ray Telescope

FXT is a Wolter-I telescope operating in the 0.5-10 keV energy range. It has a narrow field of view (38 arcmin in diameter) and a source localization error of 5-15 arcsec (90% c.l.) depending on the source strength. The FXT is responsible for the quick follow-up observations (within 5 minutes) of the triggered sources from WXT, and will also observe other interested targets during the all sky survey at the rest time.

The design of FXT is shown in Figure 1. The FXT detector is made of two pnCCD modules (the image size 28.8mm×28.8mm, from MPE) and a shifter, yielding a replaceable focal plane detector. The two pnCCD modules are cooled (-110~-80℃) by two helium pulse tube refrigerators, respectively. In front of the pnCCD module, there is a filter wheel with 4 options, including an optical blocking filter, an Fe-55 radioactive, a blanked window and an open window. The mirror module comprises of 54 nested gold coated nickel shells of Wolter-I type, to be fabricated by Media Lario in Italy on contract. The half energy width (HEW) is about 30 arcsec (on axis) at energy of 1keV. The temperature of the mirror module is controlled by several heaters and thermal filters inside the thermal baffle. The thermal baffle can protect the mirror system and detector against the potential optical stray-light. The X-ray baffle at the entrance of mirror can protect against the X-ray stray-light from single reflections in the Wolter-I type mirror.

The electronic box under the detector performs clock generation, data acquisition, high voltage supply and mode switch for FXT.



  Figure 1 Preliminary design of the Follow-up X-ray Telescope FXT. The CCD camera contains a pnCCD module and a camera electronics box.

The main scientific characteristics and source requirements of this instrument resulting from the design described above are summarized in Table 1. 





≥ 38′in diameter

≥ 38′in diameter

Effective area

≥100 cm@1.25keV,on axis

≥300 cm@1.25keV,on axis


≤2′ FWHM

30″ HPD on axis


≤170 eV@1.25keV(MgKα)

≤120 eV@1.25keV(MgKα)

≤160 eV@5.9keV(MnKα)

Energy range

0.5~8.0 keV

0.5~10.0 keV

Source position error

≤20″ in detector coordinate system

90% c.l., observation time more than 100 second 

and x-ray counts more than 100

≤4″ in detector coordinate system

90% c.l., observation time more than 100 second

 and x-ray counts more than 100

Frame rate

≥1 Hz

≥20 Hz

Mass (TBC)

146 kg

146 kg

Power consumption


120 W

120 W

Max size



Mission duration

3 years

5 years


  Table 1. The performance and requirements of FXT.

FXT has five operation modes in orbit, including image mode, timing mode, small windows mode, calibration mode and diagnostic mode. The image mode is the normal mode. The advantage of timing mode is the good time resolution, and the events are readout continuously without position information. The small window mode is a special mode, in which only a small imaging area is active so as to achieve good time resolution. The calibration mode is designed to calibrate the detector with the Fe-55 radioactive source or the readout noise of each pixel. Under the diagnostic mode, we will check all the parts of FXT with a very low data rate. These five operation modes of FXT could be chosen through sending orders from ground.