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The Soft X-Ray Imager (SXI)

The THESEUS Soft X-ray Imager (SXI) comprises 2 DU. Each DU is a wide field lobster eye telescope using the optical principle first described by Angel (1979) with an optical bench as shown in Figure 2. The optics aperture is formed by an array of 8x8 square pore Micro Channel Plates (MCPs). The MCPs are 40x40 mm2 and are mounted on a spherical frame with radius of curvature 600 mm (2 times the focal length of 300 mm). Table 1 summarizes SXI characteristics. The mass of each SXI camera is about 40 kg.


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Energy band (keV)


Telescope type:

Lobster eye

Optics configuration

8x8 square pore MCPs

MCP size (mm2)


Focal length (mm)


Focal plane shape


Focal plane detectors


Pixel Number per device


Number of devices per module


Field of View (square deg)

~0.5 sr

Angular accuracy

<2 arcmin

Power per module [W] ~35
Mass per module [kg] ~40
Figure 2. Optical elements of a SXI module. Table 1. The SXI characteristics.


The left-hand side of Figure 2 shows the SXI optic assembly. The front surface is spherical with radius of curvature 600 mm giving a focal length of 300 mm. The right-hand panels of Figure 3 shows a schematic of a single plate and a micrograph that reveals the square pore glass structure. The focal plane of each SXI module is a spherical surface of radius of curvature 600 mm situated a distance 300 mm (the focal length) from the optics aperture. The detectors for each module comprise a 2x4 array of CMOS detectors, where each device has an active area of about 40x80 mm2. The detectors are tilted to approximate to the spherical focal surface.

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Figure 3. Left: The THESEUS SXI optic assembly. Right: MPO plate and micrograph of a typical plate showing the square packet channel structure


SXI sensitivity

The ensamble of 2 SXI modules has a total field of view of about 0.5 steradian.

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Figure 4. The point spread function of the SXI obtained from a ray-tracing simulation at 1 keV.


The point spread function is shown in Figure 4. The off-axis angle at which the cross arms go to zero is determined by the L/d ratio of the pores and for an optimum performance, we require L/d=60. The central true-focus spot has a FWHM of 5.5 arcminutes and all the true-focus flux is contained by a circular beam of diameter 14.7 arcminutes. Because the angular width of the optics MPO-array is larger than the CMOS-array the field of view is unvignetted at 1 keV and above so the collecting area is constant across the entire field of view. The sensitivity to transient sources as a function of integration time is described on the science requirements web-page.