5. X-Ray Telescopes (XRTs)

Suzaku has five light-weight thin-foil X-Ray Telescopes (XRTs). The XRTs have been developed jointly by NASA/GSFC, Nagoya University, Tokyo Metropolitan University, and ISAS/JAXA. These are grazing-incidence reflective optics consisting of compactly nested, thin conical elements. Because of the reflectors' small thickness, they permit high density nesting and thus provide large collecting efficiency with a moderate imaging capability in the energy range of 0.2-12 keV, all accomplished in telescope units under 20 kg each.

Four XRTs onboard Suzaku (XRT-I) are used on the XIS, and the other XRT (XRT-S) is for the XRS. XRT-S is no more functional. The XRTs are arranged on the Extensible Optical Bench (EOB) on the spacecraft in the manner shown in Figure 5.1. The external dimensions of the 4 XRT-Is, however, are the same (See Table 5.1, which also includes a comparison with the ASCA telescopes).

Figure 5.1: Layout of the XRTs on the Suzaku spacecraft.

Table 5.1: Telescope Dimensions and Parameters of XRT-I

	Suzaku XRT-I	ASCA
Number of telescopes	4	4
Focal length	4.75 m	3.5 m
Inner Diameter	118 mm	120 mm
Outer Diameter	399 mm	345 mm
Height	279 mm	220 mm
Mass/Telescope	19.5 kg	9.8 kg
Number of nested shells	175	120
Reflectors/Telescope	1400	960
Geometric area/Telescope	873 cm$^2$	558 cm$^2$
Reflecting surface	Gold	Gold
Substrate material	Aluminum	Aluminum
Substrate thickness	155 $\mu$m	127 $\mu$m
Reflector slant height	101.6 mm	101.6 mm

The angular resolutions of the XRTs range from $1.8^{\prime}$ to $2.3^{\prime}$, expressed in terms of half-power diameter, which is the diameter within which half of the focused X-ray is enclosed. The angular resolution does not significantly depend on the energy of the incident X-ray in the energy range of Suzaku, 0.2-12 keV. The effective areas are typically 440 cm$^{\rm 2}$ at 1.5 keV and 250 cm$^{\rm 2}$ at 8 keV. The focal lengths are 4.75 m for the XRT-I. Individual XRT quadrants have their component focal lengths deviated from the design values by a few cm. The optical axes of the quadrants of each XRT are aligned within 2$^{\prime}$ from the mechanical axis. The field of view for XRT-Is is about 17$^{\prime}$ at 1.5 keV and 13$^{\prime}$ at 8 keV. (see also Table 2.1)

5.1 Basic Components of XRT

The Suzaku X-Ray Telescopes (XRTs) consist of closely nested thin-foil reflectors, reflecting X-ray at small grazing angles. An XRT is a cylindrical structure, having the following layered components: 1. a thermal shield at the entrance aperture to help maintain a uniform temperature; 2. a pre-collimator mounted on metal rings for stray light elimination; 3. a primary stage for the first X-ray reflection; 4. a secondary stage for the second X-ray reflection; 5. a base ring for structural integrity and interface with the EOB of the spacecraft. All these components, except the base rings, are constructed in 90 segments. Four of these quadrants are coupled together by interconnect-couplers and also by the top and base rings (Figure 5.2). The telescope housings are made of aluminum for an optimal strength to mass ratio. Each reflector consists of a substrate also made of aluminum and an epoxy layer that couples the reflecting gold surface to the substrate.

Including the alignment bars, collimating pieces, screws and washers, couplers, retaining plates, housing panels and rings, each XRT-I consists of over 4112 mechanically separated parts. In total, nearly 7000 qualified reflectors were used and over 1 million cm$^{\rm 2}$ of gold surface was coated.

Figure 5.2: A Suzaku X-Ray Telescope

5.1.1 Reflectors

In shape, each reflector is a 90 segment of a section of a cone. The cone angle is designed to be the angle of on-axis incidence for the primary stage and 3 times that for the secondary stage. They are 101.6 mm in slant length and with radii extending approximately from 60 mm at the inner part to 200 mm at the outer part. The reflectors are nominally 178 m in thickness. All reflectors are positioned with grooved alignment bars, which hold the foils at their circular edges. There are 13 alignment bars at each face of each quadrant, separated at approximately 6.4 apart.

To properly reflect and focus X-ray at grazing incidence, the precision of the reflector figure and the smoothness of the reflector surface are important aspects. Since polishing of thin reflectors is both impractical and expensive, reflectors in Suzaku XRTs acquire their surface smoothness by a replication technique and their shape by thermo-forming of aluminum. In the replication method, metallic gold is deposited on extrusion glass mandrel (``replication mandrel''), of which the surface has sub-nanometer smoothness over a wide spatial frequency, and the substrate is subsequently bonded with the metallic film with a layer of epoxy. After the epoxy is hardened, the substrate-epoxy-gold film composite can be removed from the glass mandrel and the replica acquires the smoothness of the glass. The replica typically has 0.5 nm rms roughness in the mm or smaller spatial scale, which is sufficient for excellent reflectivity at incident angle less than the critical angle. The Suzaku XRTs are designed with on-axis reflection at less than critical angle, which is approximately inversely proportional to X-ray energy.

In the thermo-forming of the substrate, pre-cut, mechanically rolled aluminum foils are pressed onto a precisely shaped ``forming mandrel'', which is not the same as the replication mandrel. The combination is then heated until the aluminum softened. The aluminum foils acquire the figure of the properly shaped mandrel after cooling and release of pressure. In the Suzaku XRTs, the conical approximation of the Wolter-I type geometry is used. This approximation fundamentally limits the angle resolution achievable. More significantly, the combination of the figure error in the replication mandrels and the imperfection in the thermo-forming process (to about 4 micrometers in the low frequency components of the figure error in the axial direction) limits the angular resolution to about 1 minute of arc.

5.1.2 Pre-collimator

The pre-collimator, which blocks off stray light that otherwise would enter the detector at a larger angle than intended, consists of concentrically nested aluminum foils similar to that of the reflector substrates. They are shorter, 22 mm in length, and thinner, 120 micrometers in thickness. They are positioned in a fashion similar to that of the reflectors, by 13 grooved aluminum plates at each circular edge of the pieces. They are installed on top of their respective primary reflectors along the axial direction. Due to their smaller thickness, they do not significantly reduce the entrance aperture in that direction more than the reflectors already do. Pre-collimator foils do not have reflective surfaces (neither front nor back). The relevant dimensions are listed in Table 5.2.

Table 5.2: Design Parameters for Pre-collimator

	XRT-I
Number of Collimators	4
Height	32 mm
Blade Substrate	Aluminum
Blade Thickness	120 $\mu$m
Blade Height	22 mm
Height from Blade Top to Reflector Top	30 mm
Number of nested shells	175
Blade/Telescope	700
Mass/Collimator	2.7 kg

5.1.3 Thermal Shields

The Suzaku XRTs are designed to function in a thermal environment of 20 7.5 C. The reflectors, due to its composite nature and thus its mismatch in coefficients of thermal expansion, suffer from thermal distortion that degrades the angular resolution of the telescopes in temperature outside this range. Thermal gradient also distorts the telescope in a larger scale. Even though sun shields and other heating elements on the spacecraft help in maintaining a reasonable thermal environment, thermal shields are integrated on top of the pre-collimator stage to provide the needed thermal control.

Figure 5.3: A thermal shield.

5.2 XRT-I Performance in Orbit

5.2.1 Focal Positions and Angular Resolutions

A point-like source MCG$-$6$-$30-15 were observed at the XIS aimpoint during August 17-18. We subtracted a constant value, evaluated from source-free corner regions as a background, from all the pixels. We used the data taken only during the star-tracker calibration is on. In Fig. 5.4, we show the images and the point spread functions (PSFs) of all the XRT-I$+$XIS modules. The preliminary HPD, with a typical statistical error of $\sim$$0.\!'1$, ranges from $1.\!'8\sim2.\!'3$.

Figure: Images and PSFs are shown in the upper, middle, and lower panels for the XRT-I0 through XRT-I3 from left to right. In each image drawn are ten contours in logarithmic spacing with the outermost contour being 1% surface brightness of the peak. The position of the maximum surface brightness is written as a caption in each panel in a unit of arcmin. Its typical error is $\pm 0.\!'1$. Each PSF is normalized by the number of total photons collected over the entire XIS aperture.

Figure 5.5 shows the focal position of the XRT-Is, that the source is focused when the satellite points at the XIS aimpoint. The focal positions locate roughly within $0.\!'5$ from the detector center with an deviation of $\sim0.\!'3$. This implies that the fields of view of the XIS coinsides each other within $\sim0.\!'3$.

Figure 5.5: Focal positions at the XISs when the satellite points MCG$-$6$-$30-15 at the XIS aimpoint.

5.2.2 Optical Axes, Effective Area and Vignetting Functions

A series of offset observations of the Crab observations were carried out in August and Septemper at various off-axis angles of $0'$, $3.\!'5$, $7'$. The intensity of the Crab nebula is evaluated for each pointing and for each XIS module separately. By finding the maximum throughput angle, we also have obtained a direction of the optical axis of each telescope. The result is shown in Fig. 5.6 The optical axes locate roughly within $1'$ from the XIS aim point. This implies that the efficiency of all the XRT-Is is more than 97 % even at 10 keV when we observe a point source on the XIS aimpoint.

By assuming the detector efficiency is constant over the field of view, we determined the vignetting function as shown in Figure 5.7. The vignetting function is narrower in higher energy. The averaged effective area over the detector size of XIS (17.8$^\prime$x17.8$^\prime$) is 60%, 60% and 50% of the E.A on axis at 1.5, 4.5 and 8.0 keV, respectively.

Figure: Optical axis directions of the XIS-S0 through S3. The optical axis of the XRT-I0 (XIS-S0), for example, locates at $(1.\!'0,\;-0.\!'2)$, which implies that the maximum throughput is achieved for XRT-I0 when the satellite points at the XIS aimpoint.

Figure 5.7: Vignetting curves of XRT-I at three different energies of 1.5, 4.5 and 8.0 keV. The three solid lines in the plots correspond to a parameter of ray-tracing program while the crosses are the preliminary XRT-I effective area "inferred" from the Crab pointings with some assumptions. The XRT-I effective area shown here does not includes either the quantum efficiency of the detector or transmissivity of the thermal shield and the optical blocking filter.

5.2.3 Stray Light

In-flight stray-light observations were carried out with Crab at off-axis angles of $20'$ (4 pointings), $50'$ (4 pointing) and $120'$ (4 pointing) in August and September. We show an example of $20'$-off image of XRT-I3 together with simulation results of the same off-axis angle for the cases with and without the pre-collimator in Fig. 5.8. It is seen that the pre-collimator works for reducing the stray light in orbit.

Figure 5.9 shows angular responses of the XRT-I at 1.5 and 4.5 keV up to 2 degrees. The effective area is normalized at on-axis. The integration area is corresponding to the detector size of XIS ( $17'.8\times 17'.8$). The plots are necessary to plan observations of diffuse sources or faint emissions near bright sources, such as outskirts of cluster of galaxies, diffuse objects in the Galactic plane, SN 1987A, etc.

The three solid lines in the plots correspond to different parameters of ray-tracing program while the crosses are the normalized effective area using the Crab pointings. For example, the effective area of the stray lights at 1.5 keV is $\sim$10$^{-3}$ at angles smaller than 70 arcmin off axis and $<10^{-3}$ at angles larger than 70 arcmin off. The measured flux of stray lights are in good agreement with that of raytracing within an order. This ray-tracing routine will be incorporated in the ARF generator before the AO1 deadline.

Figure 5.8: left: A $-20'$-off image of the Crab nebula taken with XIS3. middle: A simulated image of a point source at $-20'$ off with the pre-collimator. right: The same as the middle panel but without the pre-collimator. The pre-collimator properly works in orbit.

Figure 5.9: Angular responses of the XRT-I at 1.5 (left) and 4.5 keV (right) up to 2 degrees. The effective area is normalized at on-axis. The integration area is corresponding to the detector size of XIS ( $17'.8\times 17'.8$). The three solid lines in the plots correspond to different parameters of ray-tracing program while the crosses are the normalized effective area using the Crab pointings.