In high energy solar astrophysics, imaging hard X-rays by direct focusing offers higher dynamic range and greater sensitivity compared to past techniques that used indirect imaging. The Focusing Optics X-ray Solar Imager (FOXSI) is a sounding rocket payload that uses seven sets of nested Wolter-I figured mirrors together with seven high-sensitivity semiconductor detectors to observe the Sun in hard X-rays through direct focusing. The FOXSI rocket has successfully flown twice and is funded to fly a third time in summer 2018. The Wolter-I geometry consists of two consecutive mirrors, one paraboloid and one hyperboloid, that reflect photons at grazing angles. Correctly focused X-rays reflect once per mirror segment. For extended sources, like the Sun, off-axis photons at certain incident angles can reflect on only one mirror and still reach the focal plane, generating a background pattern of singly reflected rays (i.e., ghost rays) that can limit the sensitivity of the observation to faint, focused sources. Understanding and mitigating the impact of the singly reflected rays on the FOXSI optical modules will maximize the instruments’ sensitivity to background-limited sources. We present an analysis of the FOXSI singly reflected rays based on ray-tracing simulations and laboratory measurements, as well as the effectiveness of different physical strategies to reduce them.
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment conducts direct imaging and spectral observation of the Sun in hard X-rays, in the energy range 4 to 20 keV. These high-sensitivity observations are used to study particle acceleration and coronal heating. FOXSI is designed with seven grazing incidence optics modules that focus X-rays onto seven focal plane detectors kept at a 2m distance. FOXSI-1 was flown with seven Double-sided Si Strip Detectors (DSSD), and two of them were replaced with CdTe detectors for FOXSI-2. The upcoming FOXSI-3 flight will carry DSSD and CdTe detectors with upgraded optics for enhanced sensitivity. The detectors are calibrated using various radioactive sources. The detector’s spectral response matrix was constructed with diagonal elements using a Gaussian approximation with a spread (sigma) that accounts for the energy resolution of the detector. Spectroscopic studies of past FOXSI flight data suggest that the inclusion of lower energy X-rays could better constrain the spectral modeling to yield a more precise temperature estimation of the hot plasma. This motivates us to carry out an improved calibration to better understand the finer-order effects on the spectral response, especially at lower energies. Here we report our improved calibration of FOXSI detectors using experiments and Monte-Carlo simulations.
High resolution imagery of the Sun's X-ray corona provides an essential clue in understanding dynamics and heating processes of plasma particles there. However, X-ray imagery of the Sun with sub-arcsecond resolution has so far never been conducted due to severe technical difficulty in fabricating precision Wolter mirrors. For future X-ray observations of the solar corona, we are attempting to realize precision Wolter mirrors with sub-arcsecond resolution by adopting advanced surface polish and metrology methods to sector mirrors which consist of a portion of an entire annulus, by direct polishing onto the mirror substrate. Based on the knowledge obtained through fabrication of the first (in 2013) and second (in 2014) engineering Wolter mirrors and subsequent evaluations on their X-ray focusing performance, the third engineering mirror was made in 2015−2016. The primary target of improvement over the second mirror was to suppress figure error amplitude especially for spatial frequencies around 1 mm-1 and to suppress the large astigmatism that was present in the second mirror, by introducing improved deterministic polish and smoothing on the precision mirror surfaces (32.5 mm × 10 mm in area for both parabola and hyperbola segments), as well as by careful characterization of the systematic error in the figure measurement system for the precision polish. Measurements on the focusing performance of thus-fabricated third Wolter mirror at SPring-8 synchrotron facility with 8 keV X-rays demonstrated that the mirror attained sub-arcsecond focusing performance with its HPD (half-power diameter) size reaching as small as ~0.2 arcsec for meridional focusing while ~0.1 arcsec for sagittal focusing. The meridional focusing achieved nearly diffraction limited performance (~0.12 arcsec FWHM for the PSF core). We also confirmed that the large astigmatism noted in the second mirror was correctly removed in the third mirror with the correction of the above-mentioned systematic error.
The Focusing Optics X-ray Solar Imager (FOXSI) is, in its initial form, a sounding rocket experiment designed to apply the technique of focusing hard X-ray (HXR) optics to the study of fundamental questions about the high-energy Sun. Solar HXRs arise via bremsstrahlung from energetic electrons and hot plasma produced in solar flares and thus are one of the most direct diagnostics of are-accelerated electrons and the impulsive heating of the solar corona. Previous missions have always been limited in sensitivity and dynamic range by the use of indirect (Fourier) imaging due to the lack of availability of direct focusing optics, but technological advances now make direct focusing accessible in the HXR regime (as evidenced by the NuSTAR spacecraft and several suborbital missions). The FOXSI rocket experiment develops and optimizes HXR focusing telescopes for the unique scientific requirements of the Sun. To date, FOXSI has completed two successful flights on 2012 November 02 and 2014 December 11 and is funded for a third flight. This paper gives a brief overview of the experiment, which is sensitive to solar HXRs in the 4-20 keV range, describes its first two flights, and gives a preview of plans for FOXSI-3.
The sounding rocket Chromospheric Lyman-Alpha SpectroPolarimeter (CLASP) was launched on September 3rd, 2015, and successfully detected (with a polarization accuracy of 0.1 %) the linear polarization signals (Stokes Q and U) that scattering processes were predicted to produce in the hydrogen Lyman-alpha line (Lyα; 121.567 nm). Via the Hanle effect, this unique data set may provide novel information about the magnetic structure and energetics in the upper solar chromosphere. The CLASP instrument was safely recovered without any damage and we have recently proposed to dedicate its second flight to observe the four Stokes profiles in the spectral region of the Mg II h and k lines around 280 nm; in these lines the polarization signals result from scattering processes and the Hanle and Zeeman effects. Here we describe the modifications needed to develop this new instrument called the "Chromospheric LAyer SpectroPolarimeter" (CLASP2).
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a sounding-rocket instrument developed at the National Astronomical Observatory of Japan (NAOJ) as a part of an international collaboration. The instrument main scientific goal is to achieve polarization measurement of the Lyman-α line at 121.56 nm emitted from the solar upper-chromosphere and transition region with an unprecedented 0.1% accuracy. The optics are composed of a Cassegrain telescope coated with a "cold mirror" coating optimized for UV reflection and a dual-channel spectrograph allowing for simultaneous observation of the two orthogonal states of polarization. Although the polarization sensitivity is the most important aspect of the instrument, the spatial and spectral resolutions of the instrument are also crucial to observe the chromospheric features and resolve the Ly-α profiles. A precise alignment of the optics is required to ensure the resolutions, but experiments under vacuum conditions are needed since Ly-α is absorbed by air, making the alignment experiments difficult. To bypass this issue, we developed methods to align the telescope and the spectrograph separately in visible light. We explain these methods and present the results for the optical alignment of the CLASP telescope and spectrograph. We then discuss the combined performances of both parts to derive the expected resolutions of the instrument, and compare them with the flight observations performed on September 3rd 2015.
We developed a polarization modulation unit (PMU), a motor system to rotate a waveplate continuously. In polarization measurements, the continuous rotating waveplate is an important element as well as a polarization analyzer to record the incident polarization in a time series of camera exposures. The control logic of PMU was originally developed for the next Japanese solar observation satellite SOLAR-C by the SOLAR-C working group. We applied this PMU for the Chromospheric Lyman‐alpha SpectroPolarimeter (CLASP). CLASP is a sounding rocket experiment to observe the linear polarization of the Lyman‐alpha emission (121.6 nm vacuum ultraviolet) from the upper chromosphere and transition region of the Sun with a high polarization sensitivity of 0.1 % for the first time and investigate their vector magnetic field by the Hanle effect. The driver circuit was developed to optimize the rotation for the CLASP waveplate (12.5 rotations per minute). Rotation non‐ uniformity of the waveplate causes error in the polarization degree (i.e. scale error) and crosstalk between Stokes components. We confirmed that PMU has superior rotation uniformity in the ground test and the scale error and crosstalk of Stokes Q and U are less than 0.01 %. After PMU was attached to the CLASP instrument, we performed vibration tests and confirmed all PMU functions performance including rotation uniformity did not change. CLASP was successfully launched on September 3, 2015, and PMU functioned well as designed. PMU achieved a good rotation uniformity, and the high precision polarization measurement of CLASP was successfully achieved.
One of the biggest challenges in heliophysics is to decipher the magnetic structure of the solar chromosphere.
The importance of measuring the chromospheric magnetic field is due to both the key role the chromosphere
plays in energizing and structuring the outer solar atmosphere and the inability of extrapolation of photospheric
fields to adequately describe this key boundary region. Over the last few years, significant progress has been
made in the spectral line formation of UV lines as well as the MHD modeling of the solar atmosphere. It is
found that the Hanle effect in the Lyman-alpha line (121.567 nm) is a most promising diagnostic tool for weaker
magnetic fields in the chromosphere and transition region. Based on this groundbreaking research, we propose
the Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) to NASA as a sounding rocket experiment, for
making the first measurement of the linear polarization produced by scattering processes and the Hanle effect
in the Lyman-alpha line (121.567 nm), and making the first exploration of the magnetic field in the upper
chromosphere and transition region of the Sun. The CLASP instrument consists of a Cassegrain telescope, a
rotating 1/2-wave plate, a dual-beam spectrograph assembly with a grating working as a beam splitter, and
an identical pair of reflective polarization analyzers each equipped with a CCD camera. We propose to launch
CLASP in December 2014.
We report science and development activities of the X-ray/EUV telescope for the Japanese Solar-C mission whose
projected launch around 2019. The telescope consists of a package of (a) a normal-incidence (NI) EUV telescope and (b)
a grazing-incidence (GI) soft X-ray telescope. The NI telescope chiefly provides images of low corona (whose
temperature 1 MK or even lower) with ultra-high angular resolution (0.2-0.3"/pixel) in 3 wavelength bands (304, 171,
and 94 angstroms). On the other hand, the GI telescope provides images of the corona with a wide temperature coverage
(1 MK to beyond 10 MK) with the highest-ever angular resolution (~0.5"/pixel) as a soft X-ray coronal imager. The set
of NI and GI telescopes should provide crucial information for establishing magnetic and gas-dynamic connection
between the corona and the lower atmosphere of the Sun which is essential for understanding heating of, and plasma
activities in, the corona. Moreover, we attempt to implement photon-counting capability for the GI telescope with which
imaging-spectroscopy of the X-ray corona will be performed for the first time, in the energy range from ~0.5 keV up to
10 keV. The imaging-spectroscopic observations will provide totally-new information on mechanism(s) for the
generation of hot coronal plasmas (heated beyond a few MK), those for magnetic reconnection, and even generation of
supra-thermal electrons associated with flares. An overview of instrument outline and science for the X-ray photoncounting
telescope are presented, together with ongoing development activities in Japan towards soft X-ray photoncounting
observations, focusing on high-speed X-ray CMOS detector and sub-arcsecond-resolution GI mirror.
We report instrument outline as well as science of the photon-counting soft X-ray telescope that we have been studying
as a possible scientific payload for the Japanese Solar-C mission whose projected launch around 2019. Soft X-rays (~1-
10 keV) from the solar corona include rich information on (1) possible mechanism(s) for heating the bright core of active
regions seen in soft X-rays (namely, the hottest portion in the non-flaring corona), (2) dynamics and magnetohydrodynamic
structures associated with magnetic reconnection processes ongoing in flares, and even (3) generation of
supra-thermal distributions of coronal plasmas associated with flares. Nevertheless, imaging-spectroscopic investigation
of the soft X-ray corona has so far remained unexplored due to difficulty in the instrumentation for achieving this aim.
With the advent of recent remarkable progress in CMOS-APS detector technology, the photon-counting X-ray telescope
will be capable of, in addition to conventional photon-integration type exposures, performing imaging-spectroscopic
investigation on active regions and flares, thus providing, for example, detailed temperature information (beyond the sofar-
utilized filter-ratio temperature) at each spatial point of the observing target. The photon-counting X-ray telescope
will emply a Wolter type I optics with a piece of a segmented mirror whose focal length 4 meters, combined with a
focal-plane CMOS-APS detector (0.4-0.5"/pixel) whose frame read-out rate required to be as high as 1000 fps.
The Focusing Optics x-ray Solar Imager (FOXSI) is a sounding rocket payload funded under the NASA Low Cost
Access to Space program to test hard x-ray (HXR) focusing optics and position-sensitive solid state detectors for
solar observations. Today's leading solar HXR instrument, the Reuven Ramaty High Energy Solar Spectroscopic
Imager (RHESSI) provides excellent spatial (2 arcseconds) and spectral (1 keV) resolution. Yet, due to its use of
an indirect imaging system, the derived images have a low dynamic range (typically <10) and sensitivity. These
limitations make it difficult to study faint x-ray sources in the solar corona which are crucial for understanding
the particle acceleration processes which occur there. Grazing-incidence x-ray focusing optics combined with
position-sensitive solid state detectors can overcome both of these limitations enabling the next breakthrough in
understanding impulsive energy release on the Sun. The FOXSI project is led by the Space Sciences Laboratory
at the University of California, Berkeley. The NASA Marshall Space Flight Center is responsible for the grazingincidence
optics, while the Astro-H team at JAXA/ISAS has provided double-sided silicon strip detectors. FOXSI
is a pathfinder for the next generation of solar hard x-ray spectroscopic imagers. Such observatories will be able
to image the non-thermal electrons within the solar flare acceleration region, trace their paths through the
corona, and provide essential quantitative measurements such as energy spectra, density, and energy content in
The Focusing Optics X-ray Solar Imager (FOXSI) is a rocket experiment scheduled for January 2011 launch.
FOXSI observes 5 - 15 keV hard X-ray emission from quiet-region solar flares in order to study the acceleration
process of electrons and the mechanism of coronal heating. For observing faint hard X-ray emission, FOXSI uses
focusing optics for the first time in solar hard X-ray observation, and attains 100 times higher sensitivity than
RHESSI, which is the present solar hard X-ray observing satellite. Now our group is working on developments
of both Double-sided Silicon Strip Detector (DSSD) and read-out analog ASIC "VATA451" used for FOXSI.
Our DSSD has a very fine strip pitch of 75 μm, which has sufficient position resolution for FOXSI mirrors
with angular resolution (FWHM) of 12 arcseconds. DSSD also has high spectral resolution and efficiency in
the FOXSI's energy range of 5 - 15 keV, when it is read out by our 64-channel analog ASIC. In advance of the
FOXSI launch, we have established and tested a setup of 75 μm pitch DSSD bonded with "VATA451" ASICs.
We successfully read out from almost all the channels of the detector, and proved ability to make a shadow
image of tungsten plate. We also confirmed that our DSSD has energy resolution (FWHM) of 0.5 keV, lower
threshold of 5 keV, and position resolution less than 63 μm. These performance satisfy FOXSI's requirements.
The Focusing Optics x-ray Solar Imager (FOXSI) is a sounding rocket payload funded under the NASA Low
Cost Access to Space program to test hard x-ray focusing optics and position-sensitive solid state detectors
for solar observations. Today's leading solar hard x-ray instrument, the Reuven Ramaty High Energy Solar
Spectroscopic Imager (RHESSI) provides excellent spatial (2 arcseconds) and spectral (1 keV) resolution. Yet,
due to its use of indirect imaging, the derived images have a low dynamic range (<30) and sensitivity. These
limitations make it difficult to study faint x-ray sources in the solar corona which are crucial for understanding
the solar flare acceleration process. Grazing-incidence x-ray focusing optics combined with position-sensitive
solid state detectors can overcome both of these limitations enabling the next breakthrough in understanding
particle acceleration in solar flares. The FOXSI project is led by the Space Science Laboratory at the University
of California. The NASA Marshall Space Flight Center, with experience from the HERO balloon project, is
responsible for the grazing-incidence optics, while the Astro H team (JAXA/ISAS) will provide double-sided
silicon strip detectors. FOXSI will be a pathfinder for the next generation of solar hard x-ray spectroscopic
imagers. Such observatories will be able to image the non-thermal electrons within the solar flare acceleration
region, trace their paths through the corona, and provide essential quantitative measurements such as energy
spectra, density, and energy content in accelerated electrons.
We are developing imaging Cadmium Telluride (CdTe) pixel detectors optimized for astrophysical hard X-ray
applications. Our hybrid detector consist of a CdTe crystal 1mm thick and 2cm × 2cm in area with segmented
anode contacts directly bonded to a custom low-noise application specific integrated circuit (ASIC). The CdTe
sensor, fabricated by ACRORAD (Okinawa, Japan), has Schottky blocking contacts on a 605 micron pitch in a
32 × 32 array, providing low leakage current and enabling readout of the anode side. The detector is bonded
using epoxy-gold stud interconnects to a custom low noise, low power ASIC circuit developed by Caltech's
Space Radiation Laboratory. We have achieved very good energy resolution over a wide energy range (0.62keV
FWHM @ 60keV, 10.8keV FWHM @ 662keV). We observe polarization effects at room temperature, but they
are suppressed if we operate the detector at or below 0°C degree. These detectors have potential application for
future missions such as the International X-ray Observatory (IXO).
We have developed a Compton camera with a double-sided silicon strip detector (DSSD) for hard X-ray and
gamma-ray observation. Using a DSSD as a scatter detector of the Compton camera, we achieved high angular
resolution of 3.4° at 511 keV. Through the imaging of various samples such as two-dimentional array sources and
a diffuse source, the wide field-of-view (~ 100°) and the high spatial resolution (at least 20 mm at a distance of
60 mm from the DSSD) of the camera were confirmed. Furthermore, using the List-Mode Maximum-Likelihood
Expectation-Maximization method, the camera can resolve an interval of 3 mm at a distance of 30 mm from the
The Hard X-ray Imager (HXI) is one of three focal plane detectors on board the NeXT (New exploration X-ray
Telescope) mission, which is scheduled to be launched in 2013. By use of the hybrid structure composed of
double-sided silicon strip detectors and a cadmium telluride strip detector, it fully covers the energy range of
photons collected with the hard X-ray telescope up to 80 keV with a high quantum efficiency. High spatial
resolutions of 400 micron pitch and energy resolutions of 1-2 keV (FWMH) are at the same time achieved with
low noise front-end ASICs. In addition, thick BGO active shields compactly surrounding the main detection
part, as a heritage of the successful performance of the Hard X-ray Detector (HXD) on board Suzaku satellite,
enable to achive an extremely high background reduction for the cosmic-ray particle background and in-orbit
activation. The current status of hardware development including the design requirement, expected performance,
and technical readinesses of key technologies are summarized.
A semiconductor Compton camera for a balloon borne experiment aiming at observation in high energy astrophysics
is developed. The camera is based on the concept of the Si/CdTe semiconductor Compton Camera,
which features high-energy and high-angular resolution in the energy range from several tens of keV to a few
MeV. It consists of tightly packed double-sided silicon strip detectors (DSSDs) stacked in four layers, and a
total of 32 CdTe pixel detectors surrounding them. The Compton reconstruction was successfully performed and
gamma-ray images were obtained from 511 keV down to 59.5 keV. The Angular Resolution Measure (ARM) at
511 keV is ~ 2.5 degrees, thanks to the high energy resolution in both the DSSD and CdTe parts.
We developed Schottky CdTe detectors using Al as an anode electrode and measured their performances. We
first fabricated monolithic detectors with four different thicknesses of 0.5, 0.75, 1.0, and 2.0 mm. An Al anode
electrode was implemented with a guard-ring structure. For the 0.5 mm thick CdTe detector, an energy resolution
of 1.2 keV (FWHM) at 122 keV was achieved at a temperature of −20 °C and a bias voltage of 400 V. Using
the same technology, we next developed 8 × 8 pixel CdTe detectors, again with the four different thicknesses.
The Al anode electrode was pixelated and the Pt cathode was made as a single plate. Signals from all pixels
were successfully obtained and an energy resolution of 1.3 keV and 1.9 keV (FWHM) for 59.5 keV and 122 keV
gamma-rays, was achieved at a temperature of −20 °C and a bias voltage of 400 V using the 0.5 mm thick CdTe
detector. The energy resolution was nearly the same in each pixel.