Naoki Ishida

Hard x-ray telescopes to be onboard ASTRO-H

The new Japanese x-ray astronomy satellite, ASTRO-H, will carry two identical hard x-ray telescopes (HXTs), which cover the energy range of 5 to 80xa0keV. The HXT mirrors employ tightly nested, conically approximated thin-foil Wolter-I optics, and the mirror surfaces are coated with Pt/C depth-graded multilayers to enhance the hard x-ray effective area by means of Bragg reflection. The HXT comprises foils 120-450xa0mm in diameter and 200xa0mm in length, with a focal length of 12xa0m. To obtain a large effective area, 213 aluminum foils 0.2xa0mm in thickness are tightly nested confocally. The requirements for HXT are a total effective area of >300u2009u2009cm2 at 30xa0keV and an angular resolution of <1.7 in half-power diameter (HPD). Fabrication of two HXTs has been completed, and the x-ray performance of each HXT was measured at a synchrotron radiation facility, SPring-8 BL20B2 in Japan. Angular resolutions (HPD) of 1.9 and 1.8 at 30xa0keV were obtained for the full telescopes of HXT-1 and HXT-2, respectively. The total effective area of the two HXTs at 30xa0keV is 349u2009u2009cm2.

Recent results of hard x-ray characterization of ASTRO-H HXT at SPring-8

We present recent results of hard X-ray characterization of ASTRO-H HXT at SPring-8. The HXT onboard ASTRO-H is thin-foil, multi-nested conical optics similar to the Suzaku X-ray telescope. To reflect hard X-rays efficiently, reflector surface is coated with depth-graded Pt/C multilayer. The integraion of the HXT-1 mirror module has been completed. This mirror module has been characterized at a synchrotron radiation facility, SPring-8 beamline BL20B2. We have adopted, newly, an active tuning procedure with piezoelectric actuator to improve a focused image confocality. We have measured point spread function and effective area at 30, 40, 50, and 60 keV. An angular resolution of 1.9 arcmin (HPD) at 30 keV was obtained in the full telescope. The effective area of HXT-1 at 30 keV meets the requirements of HXT.

Current status of hard x-ray characterization of ASTRO-H HXT at SPring-8

We present the current status of hard X-ray telescope developments of ASTRO-H. ASTRO-H is Japans 6th Xray satellite mission following to Suzaku. It will be launched in 2014. The HXT onboard ASTRO-H is thin-foil, multi-nested conical optics as well as Suzaku XRT. To reflect hard X-rays efficiently, reflector surfaces are coated with depth-graded Pt/C multilayer. Reflectors are fabricated by the epoxy-replication method. Currently, we have finished the preparation of mirror production facility at Nagoya University, and started test production of reflectors for HXT. The selected 22 pairs of multilayer reflectors have been characterized at the SPring-8 beamline BL20B2.

The current status of the reflector production for ASTRO-H/HXT

Japans 6th X-ray satellite mission ASTRO-H, which is planed to be launched in the fiscal year 2013, will carry two hard X-ray telescopes (HXT) using depth-graded multilayer reflectors which provide us the capability of hard X-ray imaging observation up to 80 keV. ASTRO-H/HXT is the light-weight hard X-ray telescope using Pt/C depth-graded multilayer and high-throughput thin-foil optics. The basic technology for fabricating ASTROH /HXT has been established through the balloon borne experiments, InFOCμS and SUMIT mission. The HXT consists of about 1300 foil reflectors of which a size of the 200 mm mirror length and the diameter range of 120-450 mm which is much larger that those for the balloon borne experiments. To clear the requirements of the angular resolution and the effective photon collecting area for ASTRO-H/HXT, we should produce twice the total number of reflectors and select them. Therefore we need to produce more than 5000 foil reflectors for the two flight telescopes. The installation of the production line and optical evaluation system dedicated to the ASTRO-H/HXT has been almost done. We are testing and improving the production line through productions of several sizes of reflectors. The mass production of the reflectors for the flight model is scheduled to start from July 2010.

Studies of the moisture absorption of thin carbon fiber reinforced plastic substrates for x-ray mirrors

Abstract. We study a lightweight x-ray mirror with a carbon fiber reinforced plastic (CFRP) substrate for next-generation x-ray satellites. For tightly nested x-ray mirrors, such as those on the Suzaku and ASTRO-H telescopes, CFRP is the suitable substrate material because it has a higher strength-to-weight ratio and forming flexibility than those of metals. In flat CFRP substrate fabrication, the surface waviness has a root mean square (RMS) of ∼1u2009u2009μm in the best products. The RMS approximately reaches a value consistent with the RMS of the mold used for the forming. We study the effect of moisture absorption using accelerated aging tests in three environments. The diffusivity of the CFRP substrate at 60°C and at relative humidity of 100% is ∼9.7×10−4u2009u2009mm2·h−1, and the acceleration rate to the laboratory environment was 180 times higher. We also develop co-curing functional sheets with low water-vapor transmissivity on the CFRP substrate. Co-curing the sheets successfully reduced the moisture absorption rate by 440 times compared to the un-co-cured substrate. Details of the CFRP substrate fabrication and moisture absorption tests are also reported.

Studies of print-through and reflectivity of x-ray mirrors using thin carbon-fiber-reinforced plastic

Abstract. We fabricated x-ray mirrors from carbon-fiber-reinforced plastic with a tightly nested design for x-ray satellites, using a replication method for the surfaces. We studied the effects of print-through on the mirror surface as a function of curing temperature. With room temperature curing, the root-mean-square value of the surface error was 0.8 nm. The reflectivity was measured using 8-keV x-rays, and the roughness was calculated as 0.5 nm by model fitting—comparable to that of the ASTRO-H/HXT mirror. We verified the long-term stability of the mirror surface over 6 months. We fabricated Wolter type-I quadrant-shell mirrors with a diameter of 200 mm and performed x-ray measurements at BL20B2 in the SPring-8 synchrotron radiation facility. We obtained reflection images of the mirrors using a 20-keV x-ray spot beam with a slit size of 10×1u2009u2009mm in the radial and circumferential directions, respectively. The averaged half-power diameter (HPD) of the images in one mirror was 1.2 arc min in the circumferential center of the mirror and 3.0 arc min at the edge. In the spot images with a smaller slit size of 10×0.2u2009u2009mm, we achieved an HPD of 0.38 arc min in the best case.

Studies of lightweight x-ray telescope with CFRP

We studies lightweight X-ray mirror with Carbon Fiber Reinforced Plastic (CFRP) substrate for next generation X-ray satellites.nCFRP is suitable material as substrate for thin-foil highly nested X-ray mirrors like telescope of Suzaku, ASTRO-H since it has properties of higher strength-to-weight ratio and flexibility of forming than that of metals.nIn the current year we made flat panels for basic research and full/partial shell substrates by quasi-isotropic laminate with 8 ply prepregs, and performed reflector replication based on technique for the HXT mirror.

First result from a ground calibration of the Hard X-ray Telescope (HXT) onboard ASTRO-H satellite

We report a first result from a ground-based X-ray calibration of the ASTRO-H Hard X-ray Telescope (HXT) at a synchrotron radiation facility SPring-8. ASTRO-H, to be launched in 2015, is Japan’s sixth X-ray satellite mission following to Suzaku satellite. One of the features of ASTRO-H is a simultaneous observation between 0.3 keV to 600 keV with several instruments. ASTRO-H will carry two HXTs to cover hard x-rays up to 80 keV. HXT, which is one of the key instruments in ASTRO-H, is the conically approximated Wolter-I grazing incidence optics similar to the Suzaku X-ray telescope. Reflector surfaces are coated with depth-graded Platinum and Carbon multilayer to reflect hard X-rays efficiently. The integrations of the flight optics of HXT-1 and HXT- 2 were completed, and we performed a ground calibration of HXT-1 at a synchrotron facility, SPring-8 beamline BL20B2 to build a response function of HXT. We use a raster scan method with a pencil beam at the baseline length of 215m. A point spread function and effective area were measured at 30, 40, 50, 60, 70keV. From a preliminary analysis of the data, an angular resolution of 1.5 - 1.9 arcmin. was obtained at five energy band in the full telescope. The effective area is 170 cm2 at 30 keV and 82 cm2 at 50 keV, respectively. The effective area at 30 and 50 keV are about 13 % and 50 % larger than expected, respectively. We also measured the stray light from outside of field of view at 12’ and 20’ of-axis angle. We confirmed the effectiveness of pre-collimator to reduce the stray lights.

Development of a lightweight x-ray mirror using thin carbon-fiber-reinforced plastic (CFRP)

We fabricated X-ray mirrors from carbon-fiber-reinforced-plastic (CFRP) with a tightly nested design for X-ray satellites. The mirror shape is Wolter type-I quadrant shell geometry with a diameter of 200 mm and a focal length of 12 m. The mirror substrates were successfully formed with a rms error of about 1 μm. Through a replication process, a smooth surface was obtained on the CFRP substrate. We are developing a positioning method of thin mirrors in a telescope housing. It is found that a piezo-linear motor is very useful to adjust the mirror position with accuracy of sub μm. The CFRP mirrors were evaluated by using 20 keV X-ray pencil beam at BL20B2 in SPring-8 synchrotron radiation facility. The HPD of the mirrors was estimated to be about 2.3 arc-minutes. The spread of X-ray image would be caused by small waviness on the mirror surface after replication.

Development of an x-ray telescope using the carbon fiber reinforced plastic (CFRP)

We are developing an X-ray mirror using the carbon fiber reinforced plastic (CFRP) as a substrate in order to improve the angular resolution of tightly-nested thin-foil Wolter-I X-ray mirrors. We found that curing of the epoxy used in the replication process at the room temperature is effective to suppress the print through. We were able to make mirrors whose shape accuracy is 3 - 5 μm. Characterization at the synchrotron facility SPring-8 using the X-ray pencil beam of 20 keV showed that the angular resolution was 3 - 5 arcmin as a whole, but can reach to 20 arcsec locally.