Instruments

The driving science of X-ray all-sky monitoring calls for new generation instruments with high sensitivity, good angular resolution (a few arc-minutes or less) and a large sky coverage (field of view of order of thousands square degrees). These requirements can be fulfilled by wide-field X-ray focusing optics—the emerging lobster-eye Micro-Pore Optics (MPO), whose focusing imaging results in enormously enhanced gain in signal to noise, and hence high detecting sensitivity. EP carries a Wide-field X-ray Telescope (WXT) with a large instantaneous FoV, which adopts such a novel lobster-eye MPO technology.  Complementary to this wide-field instrument is a Follow-up X-ray Telescope (FXT) with a large effective area and a narrow field-of-view. Figure 1 shows a possible configuration of the EP payload.

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Figure 1 Preliminary configuration of the EP payloads alignment, with twelve WXT modules surrounding the FXT telescope at the center.

1 Wide-field X-ray Telescope

1.1 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 2. 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 4 . 

 

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Figure 2 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 3 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.

 

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Figure 3 A demonstration prototype of a lobster-eye MPO mirror assembly developed at X-ray Imaging Lab, NAOC, CAS.

1.2 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.

 

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Figure 4 Illustration of the field-of-views of the WXT modules and the FXT at the centre.

 

 

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Figure 5 Design of one module of the wide-field X-ray telescope (WXT), consisting mainly of an optical baffle, MPO plates and focal plane detectors.

Figure 5 shows the layout of one WXT module. An optical baffle is attached at the front end of the MPO assembly to shield optical stray light from the Sun, the Moon and the Earth. The Lobster-eye optics assembly is mounted right below the optical baffle, which is composed of 6 by 6 mosaicking MPO plates glued onto a AL-SiC supporting grid.  A CMOS focal-plane assembly compose 2×2 CMOS sensors. Since the focal-plane of Lobster-eye optics is sphere, each CMOS is tilted at a certain angle to reduce imaging degradation. This new type of detectors are back-side illumination CMOS sensors, which have 4 k by 4 k pixels (of pixel size 15 micro) and 6 cm by 6 cm in size. The CMOS detectors have some advantages over CCDs for their fast read-out speed, as fast as several tens of frames per seconds. They can thus be operated at moderately low temperatures. The focal-plane assembly are active cooled to -30 degrees Celsius. There are two layers of optical blocker, which are one layer of 150 nm Al and 100 nm Polyimide coated directly onto MPO chips and one layer of 50 nm Al coated onto CMOS sensor. All 12 modules are controlled by a single electronics box.

One WXT module weights 17 kg including the MPO mirror assembly, detector and electronics unit, optical baffle, structure and thermal control, with power consumption less than 13 W. The total weight of WXT is about 251 kg and the power consumption is less than 315 W with the electronics box.

2 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 6. 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.

 

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Figure 6 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.

 

 

Requirement

Goal

FOV

≥ 38′in diameter

≥ 38′in diameter

Effective area

≥200 cm2 @1.25keV,on axis

300 cm2 @1.25keV,on axis

HEW

≤2′ FWHM

30″ HPD on axis

FWHM

≤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

≤15″ 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

(TBC)

120 W

120 W

Max size

Φ630mm×2700mm

Φ630mm×2700mm

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.