Exp Astron (2005) 20:131–137
DOI 10.1007/s10686-006-9072-z
ORIGINAL ARTICLE
Development of the HEFT and NuSTAR focusing
telescopes
Fiona A. Harrison · Finn E. Christensen ·
William Craig · Charles Hailey · Wayne Baumgartner ·
C. M. H. Chen · James Chonko · W. Rick Cook ·
Jason Koglin · Kristin-Kruse Madsen ·
Michael Pivavoroff · Steven Boggs · David Smith
Received: 5 July 2006 / Accepted: 17 August 2006
C Springer Science + Business Media B.V. 2006
Abstract Hard X-ray/soft gamma-ray astrophysics is on the verge of a major advance with
the practical realization of technologies capable of efficiently focusing X-rays above 10 keV.
Hard X-ray focusing telescopes can achieve orders of magnitude improvements in sensitivity
compared to the instruments based on coded apertures and collimated detectors that have traditionally been employed in this energy band. Compact focal planes enable high-performance
detectors with good spectral resolution to be employed in efficient, low-background configurations. We have developed multilayer coated grazing incidence optics and solid state Cadmium Zinc Telluride focal plane systems for the High Energy Focusing Telescope (HEFT)
balloon-borne experiment, and for the Nuclear Spectroscopic Telescope Array (NuSTAR)
Small Explorer satellite. In this paper we describe the technologies, telescope designs, and
performance of both experiments.
Keywords X-ray telescopes . X-ray optics . X-ray detectors
F. A. Harrison () · W. Baumgartner · C. M. H. Chen · W. R. Cook
Space Radiation Laboratory, California Institute of Technology
F. E. Christensen · K.-K. Madsen
Danish National Space Centre
W. Craig · J. Chonko · M. Pivavoroff
Lawrence Livermore National Laboratory
C. Hailey · J. Koglin
Columbia Astrophysics Laboratory, Columbia University
S. Boggs
University of California, Berkeley
D. Smith
University of California, Santa Cruz
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Exp Astron (2005) 20:131–137
1 Introduction
The last decade has seen a major technological advance in hard X-ray/soft gamma-ray
astronomy – the ability to focus efficiently – creating the potential for instruments that
improve spatial and spectral resolution by more than two orders of magnitude at energies
above 10 keV. Extending grazing incidence telescopes to operate efficiently in the hard X-ray
band requires shallow graze-angle optics and position sensitive detectors with high quantum efficiency from 10–100 keV. Incorporating these technologies into astrophysical instruments results in greatly enhanced signal to background ratios compared to coded aperture
imagers.
Several research groups have developed focusing hard X-ray systems over the last ten
years, and three balloon experiments have been constructed incorporating them. The InFOCuS experiment [4] extends the segmented aluminum foil mirror technology developed
for Astro-E2 to 40 keV utilizing an optic coated with Pt/C depth graded multilayers. The
InFOCuS focal plane consists of a CdZnTe detector with a 12 × 12 array of 2-mm pixels.
The telescope achieves angular resolution of 2.2′ (HPD) and spectral resolution of 4 keV
at 32 keV, and has flown twice, in 2001 and 2004. The HERO experiment [5] uses shallow
graze-angle iridium-coated replicated nickel optics and xenon gas scintillation proportional
counters to achieve 45′′ (HPD) angular resolution and 1.5 keV (FWHM) energy resultion
at 30 keV. HERO flew in 2001 with a small complement of shells, and in Spring 2005 with
three telescope modules.
In this paper, we describe the High Energy Focusing Telescope (HEFT) balloon payload, as well as the Nuclear Spectroscopic Telescope Array (NuSTAR) Small Explorer mission. Both of these experiments are based on multilayer-coated formed glass segmented
optics and CdZnTe pixel detectors. HEFT flew once in Spring 2005 with a complement of
three telescopes, demonstrating angular resolution of 1.5′ (HPD) and with spectral resolution of 1 keV (FWHM) at 60 keV. Results from the flight are being published elsewhere.
NuSTAR employs demonstrated improvements to the optics to achieve 40′′ (HPD) angular resolution in a high throughput three-module telescope array. In this paper we describe
the optics and detector technologies, and the HEFT and NuSTAR experiment designs and
performance.
2 Depth graded multilayer optics
For HEFT and NuSTAR we developed depth-graded multilayer coated mirrors in a conical approximation to the Wolter I geometry. The optics are based on formed glass; each
shell (full figure of revolution) is assembled from multiple segments [3]. Each segment
is thermally formed from a glass sheet, and is coated using a planar magnetron sputtering system developed at the Danish National Space Centre for these programs [2]. The
mirrors are assembled layer by layer using graphite spacers epoxied to the glass then
machined to accomodate the next layer [1]. Figure 1 shows the assembled HEFT flight
optics.
The angular resolution achieved using this method is currently dictated by the substrate quality, and depends on the segment size as well as the degree to which substrates are preselected based on figure. The HEFT optics achieved 1.5′ HPD with
no selection and 20 segments per shell. Several prototype optics have demonstrated
that we can achieve 40′′ HPD with 32 segments/shell and 50% preselection on glass
segments.
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Fig. 1 Photo of the three HEFT flight optics. Each unit is fabricated from formed, multilayer-coated glass
Fig. 2 Photo showing a HEFT focal plane detector system. Two CdZnTe/ASIC hybrid pixel sensors are
mounted side-by-side
3 Cadmium zinc telluride pixel detectors
We employ CdZnTe pixel detectors for the HEFT and NuSTAR focal planes. A focal plane
is comprised of two hybrid sensors (see Figure 2). An individual sensor consists of a
1.2 × 2.4× cm, 2 mm thick CdZnTe crystal, with the anode contact segmented into pixels with 500 µm pitch. Gold stud/epoxy interconnects couple the sensor pixel contacts to
a custom low-noise readout chip. The ASIC, designed at Caltech for HEFT, contains one
circuit for each pixel, laid out on a grid exactly matching the detector pixel array.
The state of the art design achieves excellent imaging and spectral performance, as well
as the ability to measure the interaction depth of the event in the detector. Figure 3 shows an
241
Am spectrum from 100 pixels summed together (somewhat larger than, but not atypical
of the number to be included in a point source reconstruction) taken at −5◦ C. The spectrum
includes all detected X-ray events, including those where charge is shared among multiple
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Fig. 3 Spectrum of an 241Am source illuminating 100 pixels of the HEFT detector. All events, including
those where charge is split among multiple pixels, are included. For comparison, the blue line shows the same
spectrum convolved through the response of an NaI detector
pixels. The FWHM of the 60 keV line is 900 eV, and of the 14 keV line is 800 eV (FWHM).
The depth sensing ability, enabled by the ASIC design, allows events occuring in the back
portion of the detector to be rejected as background, resulting in a factor 2–3 additional
background rejection.
4 HEFT design and performance
HEFT is a balloon payload developed by a collaboration among Caltech, Columbia, the
Danish National Space Center (DNSC), Lawrence Livermore National Laboratories (LLNL)
and Stanford University. Figure 4 shows a photo of the HEFT payload, indicating the major
components. The telescope contains three co-aligned conical-approximation Wolter I mirror
assemblies, each of which focuses hard X-rays/soft gamma-rays onto a shielded, solid-state
Cadmium Zinc Telluride (CdZnTe) focal plane. The focal plane modules are housed inside
a kevlar pressure vessel. The instrument focal length is 6 m.
Table 1 provides an overview of the performance characteristics and instrument parameters. Each of the three optics has 70 shells, with shell radii ranging from 4–12 cm.
The reflectors are coated with W/Si multilayers, which provide good reflectance up to the
W K-edge at 69.5 keV. The lower energy limit is determined by atmospheric attenuation
at balloon altitudes. The total instrument collecting area on-axis is 100 cm2 at 30 keV,
not accounting for atmospheric attenuation. Each detector is surrounded by a Pb/Sn/Cu
graded −Z passive well housed inside a 2-cm thick plastic scintillator. This arrangement
attenuates cosmic and atmospheric backgrounds, and vetos charged particles that create
local background in the passive shield. While not as effective as an alkali halide shield
(NaI or CsI), this configuration provides a cost-effective approach to reducing detector
background.
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Table 1 Instrument performance characteristics and configuration. Sensitivity is listed for 3σ threshold, 20 ksec integration (corresponding to two source transits)
Instrument performance characteristics
Energy range
Angular resolution (HPD)
FOV (20 keV)
Sensitivity (mCrab)
Line sensitivity (γ /cm2 /s)
Energy Resolution (fwhm)
Effective area (3.5 g/cm−2 )
Aspect reconstruction (rms)
Pointing stability (rms)
Time resolution
20–70 keV
1.5′
17′
2 (20 - 60 keV)
2 ×10−5 (68 keV)
980 eV (68 keV)
50 cm2 (40 keV)
6′′
30′′
1 m sec
Instrument parameters
Num. modules
Focal Length
Optics
Mirror substrates
Multilayer
Detector
Shielding
Envelope
Weight
Power
3
6m
Conical Wolter-I
Formed glass
W/Si
2 mm thick CdZnTe
Graded-Z/plastic
6.5 m × 1.25 m diam.
1854 kg
400 Watts
Fig. 4 Photograph of the HEFT payload indicating the major components of the experiment
HEFT was launched on May 18, 2005 from Ft. Sumner, NM. During this flight we used
observations of three bright point sources; the Crab, Cygnus X-1, and GRS +1915 to verify
the instrument pointing stability, imaging, and throughput. Results from this flight are being
published elsewhere.
5 NuSTAR design
The NuSTAR Small Explorer satellite is based on the technologies developed for the HEFT
experiment, combined with an extendible optical bench developed by ABLE Engineering
and the Jet Propulsion Laboratory for the Shuttle Radar Topography Mission. NuSTAR was
selected for a Phase A study in November, 2003 as one of four missions to compete for two
launch opportunities. In January 2005 NuSTAR was selected for a launch date of February
2009, and was placed into an extended technical development phase.
NuSTAR consists of three co-aligned telescope modules with a 10-m focal length. The
optics and detectors are placed at either end of an extendible mast. For launch the mast is
stowed inside a canister, and the entire payload fits inside a Pegasus XL shroud. NuSTAR
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Fig. 5 The NuSTAR payload,
with each end shown separately
will be placed into an Equatorial circular 550 km orbit. Figure 5 shows a view of the two
ends of the NuSTAR payload after deployment. The bench housing the three optics modules
is extended outward from the spacecraft end, which houses the three focal plane modules.
The optics consist of 130 shells each, with radii ranging from 5.5 to 16.9 cm. The shells
are coated with a combination of W/SiC and Pt/SiC multilayers, with the coating dependent
on the shell graze angle. This, combined with the relatively low graze angles, provides
reflectance with a smooth response extending to 80 keV. NuSTAR will utilize smaller glass
segments (32 per shell) compared to HEFT, and in addition the substrates will be preselected
for figure quality. We have demonstrated through several prototypes that by accepting the
best 50% of the glass segments, NuSTAR will achieve angular resolution (HPD) of 40′′ .
The NuSTAR focal plane detectors have the same dimensions as the HEFT focal planes,
but they are surrounded by an active CsI shield rather than the graded-Z/plastic shield used
on HEFT. This, combined with the depth-sensing capability of the detectors, reduces the
internal detector background to very low levels (<10−4 cts cm−1 , s−1 from 10–80 keV). In
addition to the active well-shaped shield, an aperture stop consisting of rings of graded-Z
material reduces diffuse cosmic background. The aperture stop deploys simultaneously with
the mast after launch.
6 NuSTAR performance
Table 2 summarizes the NuSTAR performance. Compared to coded aperture instruments
that have operated in the same band, NuSTAR achieves more than two orders of magnitude
improvement in continuum flux sensitivity. The excellent energy resolution of the detectors
will allow measurement of velocity shifts in the 44 Ti lines at 68 and 78 keV of ±1000 km s−1 .
Using the combination of sensitivity, spectral and spatial resolution, NuSTAR will be able
to undertake investigations not previously possible in the hard X-ray band. In extragalactic
survey fields NuSTAR can resolve 50% of the hard X-ray background at 30 keV, and will
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Table 2 NuSTAR instrument
performance.
137
Energy range
Angular resolution (HPD)
FOV
Source positions
Spectral resolution
Timing resolution
Line sensitivity (106 s, 68 keV)
Continuum sensitivity
(106 s, 3σ , E/E) = 0.5
Background in HPD/module (40 keV)
Effective area (20 keV)
ToO response
6–80 keV
40′′
8.4 × 8.4′
5′′
900 eV 68 keV
0.1 ms
10−7 ph/cm2 /s
0.7 µCrab (20 keV)
6 µCrab (60 keV)
1.1 × 10−5 cts/s/keV
500 cm2
<24 h
detect several hundred obscured AGN in its 2 square degree survey fields. NuSTAR can also
spatially resolve the 44 Ti line in Cas A, mapping the line doppler shifts with 1000 km s−1
resolution in 40′′ spatial bins. In surveys of the Galactic center region, NuSTAR can resolve
the hard source populations discovered by Integral and XMM, and study the distribution in
latitude of any truly diffuse component.
7 Summary
The HEFT balloon experiment has demonstrated hard X-ray focusing optics and solid state
pixel detectors by imaging cosmic sources in this band. These observations have fully verified
the expected throughput, resolution, and sensitivity of the HEFT instrument. NuSTAR, which
is based on the HEFT technologies, is ready to proceed to phase B, and will realize the
potential of hard X-ray focusing in space.
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