We present X-ray characteristics of X-ray telescopes (XRTs) onboard the Astro-E2 satellite. It is scheduled to be launched in February 2005. We have been performed X-ray characterization measurements of XRTs at Institute of Space and Astronautical Science (ISAS) since January 2003. We adopted a raster scan method with a narrow X-ray pencil beam. Angular resolution of the Quadrants composed of the Astro-E2 XRT was evaluated to be 1'.6-2'.2 (HPD; Half Power Diameter), irrespective of the X-ray energy, while those of the Astro-E XRT was 2'.0-2'.2. The effective area of a telescope is approximately 450, 330, 250, and 170 [cm<sup>2</sup>] at energies of 1.49, 4.51, 8.04, and 9.44 keV, respectively. The field of view (FOV) of the XRTs which is defined as Full Width Half Maximum (FWHM) of the vignetting function is ≈18' at 4.51 keV. We summarize these characters of the XRTs.
We report a ground-based X-ray calibration of the Astro-E2 X-ray
telescope at the PANTER test facility. Astro-E2, to be launched in
February 2005, has five X-Ray Telescopes (XRTs). Four of them focus on
the X-Ray Imaging Spectrometers (XIS) while the other on the X-Ray
Spectrometer (XRS). They are designed with a conical approximation of
Wolter-I type optics, nested with thin foil mirrors to enhance their
throughput. A calibration test of the first Astro-E2 flight XRT for
XIS was carried out at the PANTER facility in August 2003. This
facility has an 130 meter long diverging beam from X-ray generator to
XRT. Owing to the small X-ray spot size of about 2 mm dia., we verified that the focal position of each quadrant unit converged within 10 arcsec. The energy band around Au-M edge structures was
scanned with a graphite crystal. The edge energy (Au M5) is consistent with that listed in Henke et al. 1997. Owing to the large area coverage of the PSPC detector which is a spare of the ROSAT satellite, off-axis images including stray lights at large off-axis angle (up to 6 degree) were obtained with a large field of view. We also compared the results with those measured with the parallel pencil beam at ISAS which is in detail reported in our companion paper by Itoh A. et al..
Astro-E2, to be launched in early 2005, will carry five X-ray Telescopes (XRT). The design of the XRT is the same as the previous original mission Astro-E, that is a conical approximation of Wolter Type-I optics, where about 170 thin-foil reflectors are nested confocally. Some modifications from Astro-E are adopted within the severe constraints due to the policy of "re-build" instruments. One of the major changes is the addition of pre-collimators for the stray light protection. Several modifications on the fabrication processes are also made. The replication glass mandrels are screened carefully, which is expected to reduce the figure error of replicated reflectors. We thus expect better performance than Astro-E especially in imaging capability. In order to qualify the performance of the Astro-E2 XRT, we have started ground calibration program of XRT at 30 meter X-ray beam facility of the Institute of Space and Astronautical Science (ISAS). We have found positive improvements on the telescope performance from the Astro-E, which probably arise from the applied modifications. The on-axis half-power diameter (HPD) has been evaluated to be 1.6-1.7 arcmin, which is improved from the Astro-E (2.0 ~ 2.1 arcmin HPD). The on-axis effective areas of quadrants are larger than the average of Astro-E by about 5%. The on-axis effective areas of the XRT for X-ray Imaging Spectrometers (XIS) are approximately 460, 340, 260, and 190 cm<sup>2</sup> at energies of 1.49, 4.51, 8.04, and 9.44 keV, respectively. The present paper describes the recent results of
the performance of the first flight assembly of the Astro-E2 XRT.
X-Ray Spectrometer (XRS) is the microcalorimeter onboard the X-ray astronomy satellite Astro-E2 which is scheduled to be launched
early in 2005. For the XRS to achieve its best energy resolution
of 6 eV at 6 keV, X-ray intensity should be limited up to several c s<sup>-1</sup> pixel<sup>-1</sup>. The filter wheel (FW) is the instrument to reduce incident X-ray intensity on the XRS using extinction filters. The FW consists of a stepping motor, extinction filters, and a filter disk which has six mounting positions for the extinction filters. Among the six mounting points, two are used for Neutral Density (ND) filters, another two are for Beryllium (Be) filters, and the other two are remained open. The biggest modification from Astro-E is that we attach radioisotopes of
<sup>55</sup>Fe and <sup>41</sup>Ca on the filter disk, which illuminate the XRS pixels to monitor the gain in orbit. We present here the mechanical design of the FW especially on improvements from Astro-E, and the results of our calibration measurements on X-ray transmission of the extinction filters.
Astro-E2 XRTs adopt Wolter Type-I optics and have nested thin foil structure to enhance their throughput. But this structure allows stray X-rays to come from the sky outside of the XRT field of view. Stray lights contaminate focal plane images, especially in the case of extended source observations. We intend to mount pre-collimators on top of the ASTRO-E2 XRTs to intercept stray lights. According to the success for the engineering model pre-collimator to protect the stray lights efficiently, we proceeded to product flight model pre-collimators. Some improements are made for the flight model (FM) pre-collimator: the introduction of heat forming to make slats accurate cylindrical shape, the change of the groove shape of alignment plates and the change of the housing design. We also established the method of pre-collimator mounting. In X-ray measurements, stray light images and the flux of each stray component at any off-axis angles are measured with/without FM pre-collimator. The secondary only reflection component is reduced down to 3% at a larger off-axis angle than 30', and the backside reflection component becomes more remarkable. On the other hand, X-ray measurement of the effective area at on-axis with/without FM pre-collimator verifies that pre-collimator does not interfere the telescope aperture. In addition, the decrease of XRT field of view is ≤8%, which is the same as the ray-tracing simulations.
We report a new calibration system for large size X-ray optics at
ISAS. We adapted a 'dynamical' pencil beam collimated from an X-ray
generator, the maximum voltage for which is 50 kV. By combining two
stage systems for the X-ray generator and a collimator, the pencil
beam dynamically sweeps across a circular region of a telescope with
the radius of 60 cm at maximum. In this case, the X-ray telescope and
the focal plane detector are both statically fixed. A 4.4~m long rail
for detector stage and two positions of the telescope stage provide
focal lengths from 4.5 to 12 m, while the previous system can
accommodate 4.5 or 4.75 m focal length. The preliminary performance of
this system is summarized in this paper. For the post-Astro-EII
satellite, a hard X-ray multi-layer supermirror with an unprecedented
sensitivity up to 80~keV is strongly expected. This beam facility is of importance because the hard X-ray mirrors always require a long focal length of 8-12 m due to the small reflection angle (about 0.3 degree). Focal length and diameter of future telescopes are always decided by the boundary conditions of the mission at the last moment of the design freeze. Our new X-ray beam facility is designed to match with any kind of X-ray telescope parameters.
X-ray characterization measurements of capillary (X-ray Optical Systems Inc) were carried out at ISAS (Japan) X-ray beam facility. Since capillary system has a capability of collecting x-rays effectively by the small angle reflections in the narrow tubes, it is expected to apply it for the observation of astronomical x-ray objects. The depth of focus on the on-axis is 6 mm, and the image size at the focal plane is 0.4 mm (FWHM) with Cu-K. By using continuum x-ray (2 to 15 keV) beam, the on-axis efficiency was evaluated to be 20 %. The efficiency decreases gradually toward low energy range, this could be explained by the absorption effect of Si which is one of the constituent element of capillary tubes. The field of view is defined as the off-axis angle at which the efficiency becomes half of the on-axis value. The diameter of the field of view was 22', 19' at 4.51 keV and 8.04 keV, respectively. Capillary has no imaging capability, in other words, the light-concentrating direction is independent from the incident angle of x-ray beam. When the x-ray incident angle varies from -15' to +15', the focused images are distributed within 3'. From our measurements, we could confirm the performance of capillary and its potential for astronomical applications.
X-ray telescopes (XRTs) of nested thin foil mirrors were developed for
Astro-E, the fifth Japanese x-ray astronomy satellite. Although the
launch was not successful, the re-flight of Astro-E mission is approved as Astro-E2 and will carry the same XRTs. Ground-based calibration of Astro-E XRT revealed that its image quality and effective area are somewhat worse than what are expected from the original design. Conceivable causes of these defects of the XRT performance (i.e., surface roughness, waviness, misalignment of
reflectors, and so on) are examined by X-rays and optical microscopic
measurements. In this paper, we distinguish quantitatively these
causes to limit the performance of the Astro-E XRT. Using the detail
measurements, we can attribute both degradation of the image quality
and a deficit of the effective area from the design values mainly to a
slope error with a mm scale in each reflector and shadowing effects of
neighboring reflectors due to various factors. There is still room
for improvement in the support system of reflectors (i.e., alignment
bars) in the XRT. One of the main aims of the mirror system calibration is to construct response function. Therefore, it is important that the development of a representative numerical model and its validation against extensive ground-based calibration. Taking account of the results of the pre-flight calibration and the microscopic measurements, we develop and tune a ray-tracing simulator which constructs the XRT response function for a point source at an arbitrary off-axis angle and spatial distributions of celestial X-ray sources.
ASTRO-E2 XRTs adopt Wolter Type-I optics and have nested thin
foil structure to enhance their throughput. But this structure allows
stray X-rays to come from the sky out of the XRT field of view. Stray light contaminates focal plane images, especially in the case of extended source observations. We intend to mount pre-collimators on top of the ASTRO-E2 XRTs to intercept stray light. On the other hand, reflection by the pre-collimator itself newly creates secondary stray light. To decrease these additional stray light as possible, the mil finish aluminium with its roller mark normal to the incident X-ray beam has been used for the slat material, whose reflectivity is reduced down to 1/20 of ideal specular reflection. Optical profilers tell us these samples have very rough surfaces, whose height varies with Σ; = 1-2 μm. According to the design parameters as are described in the related paper in this symposium (Paper I), an engineering model pre-collimator is fabricated with 46 slats out of 175. Before EM pre-collimator is mounted onto XRT, alignment plates are adjusted to align slats to the same position of XRT primary reflectors. In x-ray measurements, stray light images and the flux
of each stray light component at 30' off axis are measured
with/without EM pre-collimator. The secondary only reflection component is reduced down to 3.6%, and the backside reflection component becomes more remarkable. On the other hand, X-ray measurement of the effective area at on axis with/without EM pre-collimator verifies that pre-collimator does not interfere the telescope aperture. In addition, the decrease of XRT field of view is ~10%, which is the same as the ray-tracing simulations. As a whole, EM pre-collimator reduces stray light to 27% level with only ~10% decrease of the XRT F.O.V.
Next Japanese ASTRO-E2 satellite carries five X-ray telescopes (XRTs),
and the pre-collimators for the stray light protection will be
installed on them. The pre-collimator is composed of the vertical
foil cylinders which line up at the top of the primary reflectors of
the XRT. This configuration is effective to reject the stray light of
the 'secondary only' reflection, which is the most of the stray light
from large off-axis angles up to 70' to the focal plane detector
within 18' x 18'. If the height of the pre-collimator is 30 mm (15%
of the total height of XRT), we can protect all the "secondary only"
component at > 30' off-axis. The field of view will become only about
10% less due to the collimation effect of the 30 mm high
pre-collimator. However, new stray light component is generated by the
reflection and scattering by the pre-collimator itself, especially at
small off-axis angles. As a result, we estimate the total flux of the
stray light to be about 10% at 30' off-axis and 5% at 60' off-axis,
compared with the case without pre-collimators.