ASTRO-H is an astrophysics satellite dedicated for X-ray spectroscopic study non-dispersively and to carry out survey complementally, which will be borne out of US-Japanese collaborative effort. Among the onboard instruments there are four conically approximated Wolter-I X-ray mirrors, among which two of them are soft X-ray mirrors\ of which the energy range is from a few hundred eV to 15 keY, currently being fabricated in the X-ray Optics Lab at Goddard Space Flight Center. The focal point instruments will be a calorimeter (SXS) and a CCD camera (SXI), respectively. The reflectors of the mirror are made of heat-formed aluminum substrate of the thickness gauged of 152 micron, 229 micron, and 305 micron of the alloy 5052 H-19, followed by epoxy replication on gold-sputtered smooth Pyrex cylindrical mandrels to acquire the X-ray reflective surface. The epoxy layer is 10 micron nominal and surface gold layer of 0.2 micron. Improvements on angular response over the Astro-El/Suzaku mirrors come from error reduction on the figure, the roundness, and the grazing angle/radius mismatching of the reflecting surface, and tighter specs and mechanical strength on supporting structure to reduce the reflector positioning and the assembly errors. In this paper, we report the results of calibration of the engineering model of SXT (EM), and project the quality of the flight mirrors

Hahne, Devin

Okajima, Takashi

Serlemitsos Peter J.

Soong, Yang

NASA Technical Reports Server

ASTRO-H Soft X-ray Telescope (SXT) 
Yang Soonga, Peter J. Serlemitsos, Takashi Okajima, and Devin Hahneb 
NASA Goddard Space Flight Center, 20771 
aCRESSTIUSRA, "Bastion Technologies, Inc. 
Abstract 
ASTRO-H is an astrophysics satellite dedicated for X-ray spectroscopic study non-dispersively and to carry out survey 
complementally, which will be borne out of US-Japanese collaborative effort. Among the onboard instruments there are 
four conically approximated Wolter-I X-ray mirrors, among which two of them are soft X-ray mirrors\ of which the 
energy range is from a few hundred eV to 15 keY, currently being fabricated in the X-ray Optics Lab at Goddard Space 
Flight Center. The focal point instruments will be a calorimeter (SXS) and a CCD camera (SXI), respectively. The 
reflectors of the mirror are made of heat-formed aluminum substrate of the thickness gauged of 152 !lm, 229 !lm, and 
305 !lm of the alloy 5052 H-19, followed by epoxy replication on gold-sputtered smooth Pyrex cylindrical mandrels to 
acquire the X-ray reflective surface. The epoxy layer is 10 !lm nominal and surface gold layer of 0.2 !lm. Improvements 
on angular response over the Astro-ElISuzaku mirrors come from error reduction on the figure, the roundness, and the 
grazing angle/radius mismatching of the reflecting surface, and tighter specs and mechanical strength on supporting 
structure to reduce the reflector positioning and the assembly errors. In this paper, we report the results of calibration of 
the engineering model of SXT (EM), and project the quality of the flight mirrors. 
Keywords: X-ray Optics, X-ray Telescope, X-ray Astrophysical Instrument 
1. Introdnction 
Conical approximation ofthe Wolter-I type X-ray imagers has been a major tool for spectroscopic study 
in the last quarter of century since the launch of a sounding rocket experiment to study SN 1987 A. The high 
throughput, essential for the spectroscopic study, of the telescope is provided by using thin-foil reflectors, and 
the continuous improvement of angular response on X-ray imaging is accomplished by switching the 
substrate surface smoothing scheme from earlier lacquer-dipping to recent epoxy replication that reduced the 
mid-frequency waviness2• The low micro-roughness of the surface produces specular reflection of X-rays, 
with high Z material such as Au, Pt, or Ir as the reflecting surface. The energy range coverage can be up to 15 
ke V by lowering the grazing angle. Despite the continuous improvement over the last two decades from 4 to 
1.7 arc-minutes, the demand of even better angular resolution of such thin-foil mirrors is high. The current 
image quality is still far from achieving the conical approximation limit, which is about 0.2 arc-minute. The 
difficulty comes from the relatively weak mechanical integrity of the reflectors and the imprecision in the 
assembly/aligmnent scheme. The improvement ofHPD from the Suzaku mirror of2 arc-minutes to the recent 
test of engineering model of ASTRO-H of 1 arc-minute will be the main subject of this paper. 
Table 1 Design parameters of ASTRO-H and Suzaku 
ASTRO-H Suzaku 
Effective aperture diameter (cm) 11.6-45.0 11.6-40.0 
Focal length (m) 5.6 4.75/4.50 
Reflector height (cm) 10.16 10.16 
# of reflectors 203 175/168 
Mass (kg) 45 20 
# of mirrors (0.4-12 keY) 2 411 
Substrate thickness (!lm) 152,229,305 152 
Epoxy thickness (!lm) 10 25 
https://ntrs.nasa.gov/search.jsp?R=20110022503 2019-08-30T18:05:49+00:00Z
Au thickness (A) 2000 
Grazin an Ie (0) 0.15-0.59 
FOV (arc-minute) 17 
Ex ected effective area cm 562/426 1/6 keY 1.5/8.0 keY (measured 
2. The Scheme 
Error terms of image at the quadrant level can be categorized with quality of reflectors and assembly. For those 
coming from reflectors are axial figure, roundness, radius/grazing angle matching, surface roughness, and micro-
roughness of the reflecting surface. Assembly errors come from precision of the holding structure, alignment, thermal, 
and under the gravity influence/image stability. We shall list the status of the error reduction. 
Table 2 Error tenns at quadrant level. The dominant tenn of error is the reflector positioning (alignment) 
ASTRO-H Suzaku 
Radius/grazing angle mismatching 1.2-2.5% 2.3-9.2% 
Axial figure (arc-minute) 0.4 1 
Mid-frequency, /-tm-mm (A) 4-6 4-6 
Micro-roughness (A) 5 5 
Alignment groove position drift (/-tm) <5 <40 
Alignment groove random (/-tm) 5 12 
Alignment (/-tm) 5 10 
Focal length uncertainty (cm) 0.5 <5 
Image quality HPD (arc-minute) 1.1 1.7-2.2 
2.1 Reflectors 
Forming mandrels were conically shaped with sequentially increasing radius so that to provide the complete set of 
substrate at all design radii. There are 71 pairs of aluminum mandrels for ASTRO-H vs. 20 pairs of quartz mandrels for 
Suzaku. CTE matching with the substrate during forming at 200°C are much improved (23xI0-6;oC vs. 0.59xlO-6/°C). 
Radius/grazing angle matching: related to substrate forming scheme. For all substrate thickness, we stack 11 pieces 
in one forming batch. Only the innermost foil in the batch will have the right radius/grazing angle match, the rest of the 
batch will have the matching error, from inner to outer foils, of 9.2%/2.3% for Suzaku and 2.5%/1.2% for ASTRO-H. 
Axial figure: generally is improved from 4 /-tm to 1-2 /-tm (surface figure error of the substrate forming mandrels was 
reduced). RMS error of the figure slope is about 0.4 arc-minute3• 
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Reference substraction 
o 20 40 60 80 100 
Foil Axial Position (mm) 5/10/2011 09:40:40 
Fig.l Axial figure of a typical reflector. Scan resolution is 200 Ilm 
Roundness: error of 113 arc-minute (substrate size difference ~ +1- 1), which translates to 10 !-tm in reflector edge 
positioning error in the alignment bar grooves. RMS of this term is on an order of 1 arc-minute, which largely a limiting 
factor for reaching a sub-are-minute image quality, 
Mid-frequency roughness: 4-5 A at spatial wavelength 10-3 - 1 mm. Result of Zygo surface profilometer is provided 
below. 
Fig. 2 Surface profile of a typical reflector 
Micro-roughness: no obvious problem was implied by X-ray scattering. 
2.2 Assembly at quadrant level 
Fig. 3 ASTRO-H EM fully-populated primary housing of203 shells. There are 13 alignment bars, on each phase, each with 203 
grooves that hold the 203 shells by the edges 
Holding structure: 
• 
• 
alignment bars, < 5 !lm systematic walk of groove position from end to end; random error of the groove 
position: +/- 5 !lm. 
mechanically strengthen the housing structure to resist distortion in the assembly; 
distortion was minimized with 3-point mount scheme at two plane interface. The rest ofthe connecting screw 
positions will be dabbed with epoxy as liquid shims. These set of screws will be tightened after the epoxy is 
fully cured. 
Alignment: anyone of the two systems below will do the alignment. The EM alignment was done with the machine 
in the upper panel. It consists of a rotation table that carries the quadrant housings (see fig. 4) and a linear translation 
stage that carries a 500x magnification microscope which monitor the radial position of the alignment bars in the 
housing. After center of rotation is set within +/- I !lm, each alignment bar can be adjusted to its design radial position. 
Focal length determination shall be within +/- 5 mm. The focal length is determined by the angle extending between 
primary and secondary housings, which can be achieved by adjusting thickness of the shims on three points in between 
the two housings to the accuracy of2 !lm. 
Fig. 4 Two alignment setups in polar/Cartesian coordinate, respectively. Precision and repeatability of the 
translation stages are at I !-Lm, and wobble of the rotation stage is at Il-trad 
Stabilize the image by staking reflectors to the alignment bars: once tuning is final, apply Arathane 
(Uralane) 5753A1B in line along the bars. Staking reflectors onto alignment bars while the quadrant is in 
the upright orientation. This is to eliminate free-play of the reflectors in the grooves, and thus gravity-
oriented instability, so that the ground calibration of the mirror can be preserved in the space operation. The 
image of the quadrant has been checked in the horizontal and vertical optical beams, and no difference in 
the two orientation was observed. 
Thermal: EM mirror quadrant was tested for thermal dependence of HPD. At 22°C +/- lOoC the HPD 
changed within 1.1 +f- 0.05 arc-minute. 
2.3 Assembly at telescope level 
Features so designed to ensure the confocality of the four quadrants and optical axes alignment are 
kept in the assembly and environmental tests 
• Rings at the inner radii at topibottom ofthe mirror and a ring at the outer radius to connect the 
four quadrants at secondary bottom phase 
• Stainless tabs on 8 locations at outer flange of the quadrant boundaries to keep the relative 
positions between the quadrants 
• Stycast FT2850/24L V was used as liquid pins in between the quadrants in 8 places 
• All elements were held with 3-point mounts and liquid shims, to achieve minimal stress in the 
assembled system 
• Mounting pre-collimator (PC) onto top of the primary stage 
• Placing thermal shield (TS), O.2-micron polyimide supported by stainless wire mesh (94% 
transmission), onto the PC 
• Match-drilled, in four groups of interface foot (IfF as seen in the above picture at outer flange 
of the housings), of2 points a group, of 90° apart at the outer rim at interface of the primary 
and secondary housings, to be anchoring onto the top plate of the spacecraft 
Fig. 5 ASTRO-H EM integrated mirror of one optical quadrant, three quadrants with mass simulators 
(provided by JAXAlISAS), one pre-collimator (PC, from ISAS) on the optical quadrant, and one thermal 
shield (TS, from Nagoya University) on one of the quadrant with mass simulator 
2.4 X-ray Test Results 
EM quadrant results: a fully loaded EM quadrant 
lO=!05.N=2930 
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radius (aresee) 
radius(_) 
!D= 121 ,N""2930 
projection to Y a>is 
Fig.6 ASTRO-H EM quadrant X-ray test result, clockwise from top left: X-ray image on CCO, Point Spread 
Function (PSF) of the image, Profile of the image projected to the abscissa, and Encircled Energy Function (EEF). 
HPO of the EM image is 1.1 arc-minute 
Sector image and the sector centroid distribution. Sector image is ~ 50-60 arc-seconds. The centroid of 
the sector image spread ~ 0.6 mm on the focal plane, and the RMS of the error is ~ 0.2 arc-minute. 
4 6 10 12 14 
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X [pixel I 
Fig. 7 The upper panel, sector image HPD. 13 alignment bars divided the quadrant aperture into 12 sector in between and 2 half-
sectors at quadrant ends; lower panel, sector centroid location on the focal plane, pixel size 20 /lID x 20 /lID. 
3 Discussion 
Quality of the flight mirror will be similar to that of the EM quadrant. 
Vibration test: the EM went through the vibration test at the qualification level. The execution was nearly complete 
but for the last axis due to a design flaw for the mass simulator. The image quality of the EM optical quadrant was 
verified after the vibration. The change was insignificant and there were no observable damages of the reflectors and the 
supporting structure. 
As we demonstrated in the study, roundness error of the reflectors, which in tum contribute to the image - 1 arc-
minute RMS, is the dominant one. The axial figure and surface roughness can be temporarily ignored until the dominant 
one being sufficiently reduced. Beyond the quadrant level, assembly focuses on the confocality of the quadrants and the 
parallelism of the four quadrants optical axes, which have little impact on the image quality at current level. 
• 
• 
• 
4 Conclusion 
High throughput X-ray mirrors with image quality at 1 arc-minute provide an excellent tool to study 
spectroscopy of the celestial X-ray sources 
Thin-foil X-ray mirror technology made a 2-fold improvement on angular resolution over the Suzaku 
mirrors 
The integrated mirror with four optical quadrants has not been assembled and tested yet, but the error in the 
integration is expected to be minimal 
• As the reflector design/fabrication and assembly scheme keep on improving, the future of this low cost 
imagers may achieve HPD at 10-20 arc-seconds level 
Acknowledgement 
Authors would like to thank the ASTRO-H production crews, including Cliff Brown, Dominic Contino, John 
Kearney, Larry Lozzipone, Ai Numata, Paul Pless, Mark Schofield, Emory Toomey III, for their great contribution on 
fabricating/logging the reflectors, Lawrence Olsen who coordinates the lab operation, Y. Hayashi, Joseph Sytwu and 
Nagoya group led by Dr. Y. Tawara on integrating the EM, and NASAlMSFC and Harvard SAO X-ray optics groups on 
technical exchanges. 
References 
[1] Peter J. Serlemitsos, Yang Soong, Takashi Okajima, and Devin J. Hahne, "Foil X-ray Mirrors for Astronomical 
Observations: Still an Evolving Technology", Proc. SPIE, 7732, 77320A, 2010, and references therein 
[2] Peter J. Serlemitsos, Yang Soong, "Foil X-ray Mirrors", Astrophysics and Space Science, 239, 177, 1996 
[3] Gubarev, Mikhail V., MSFC group for SXT-I, private communication, 2010 


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