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.
High resolution imagery of the solar X-ray corona provides a crucial key to understand dynamics and heating processes of plasma particles there. However, X-ray imagery of the Sun with sub-arcsecond resolution has yet to be conducted due to severe technical difficulty in fabricating precision Wolter mirrors. For future X-ray observations of the Sun's corona, we are attempting to realize precision Wolter mirrors with sub-arcsecond resolution by adopting advanced surface polish and metrology methods based on nano-technology to sector mirrors which consist of a portion of an entire annulus. Following fabrication of the first engineering mirror and subsequent evaluation on the X-ray focusing performance in 2013, the second engineering mirror was made with improvements in both precision polish and metrology introduced. Measurement of focusing performance on the second mirror at SPring-8 synchrotron facility with 8 keV X-rays has demonstrated that the FWHM size of the PSF core reached down to 0.2" while its HPD (Half Power Diameter) size remained at ~3" due to the presence of small-angle scatter just outside of the core. Also, there was notable difference in the focal length between sagittal and meridional focusing which could have been caused by an error in the sag in the meridional direction of <10 nm in the mirror area. Further improvements to overcome these issues have been planned for the next engineering mirror.
The Chromospheric Lyman-Alpha Spectro-Polarimeter (CLASP) is a sounding-rocket instrument currently under development at the National Astronomical Observatory of Japan (NAOJ) as a part of an international collaboration. CLASP’s optics are composed of a Cassegrain telescope and a spectro-polarimeter which are designed to achieve an unprecedentedly accurate polarization measurement of the Ly-α line at 121.6nm emitted from the solar upper-chromosphere and transition region. CLASP’s first flight is scheduled for August 2015. Reaching such accuracy requires a careful alignment of the optical elements to optimize the image quality at 121.6 nm. However Ly-α is absorbed by air and therefore the optics alignment has to be done under vacuum condition which makes any experiment difficult. To bypass this issue, we proposed to align the telescope and the spectrograph separately in visible light. Hence we present our alignment procedure for both telescope and spectro-polarimeter. We will explain details about the telescope preliminary alignment before mirrors coating, which was done in April 2014, present the telescope combined optical performance and compare them to CLASP tolerance. Then we will present details about an experiment designed to confirm our alignment procedure for the CLASP spectro-polarimeter. We will discuss the resulting image quality achieved during this experiment and the lessons learned.
We present overview and development activities of a soft X-ray photon-counting spectroscopic imager for the solar corona that we conceive as a possible scientific payload for future space solar missions including Japanese Solar-C. The soft X-ray imager will employ a Wolter I grazing-incidence sector mirror with which images of the corona (1 MK to beyond 10 MK) will be taken with the highest-ever angular resolution (0.5"/pixel for a focal length of 4 m) as a solar Xray telescope. In addition to high-resolution imagery, we attempt to implement photon-counting capability for the imager by employing a backside-illuminated CMOS image sensor as the focal-plane device. 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 with the soft X-ray imager will provide a noble probe for investigating mechanism(s) of magnetic reconnection and generation of supra-thermal (non-thermal) electrons associated with flares. Ongoing development activities in Japan towards the photon-counting imager is described with emphasis on that for sub-arcsecond-resolution grazing-incidence mirrors.
We present science and development activities of the soft X-ray photon-counting spectroscopic imager for the solar
corona that we conceive as a possible scientific payload for the Japanese Solar-C mission. The imager employs a
grazing-incidence sector mirror of Wolter-I type with which images of the corona are to be taken in a wide temperature
range (1 MK to beyond 10 MK) with the highest-ever angular resolution (0.5"/pixel for a focal length of 4 m) as an Xray
telescope for the Sun. Moreover, by employing a back-thinned CMOS image sensor as the focal-plane device, we
attempmt to implement photon-counting capability 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 magnetic reconnection, generation of supra-thermal electrons
in the reconnecting magnetic structure during flares, and for the generation of hot coronal plasmas (heated beyond a few
MK) which may be responsible for the formation of the hot cores of solar active regions.
Solar-C is the third generation solar observatory led by JAXA. The accepted ‘Plan-B’ payload calls for a radiation-hard
solar-staring photon-counting x-ray spectrometer. CMOS APS technology offers advantages over CCDs for such an
application such as increased radiation hardness and high frame rate (instrument target of 1000 fps). Looking towards the
solution of a bespoke CMOS APS, this paper reports the x-ray spectroscopy performance, concentrating on charge
collection efficiency and split event analysis, of two baseline e2v CMOS APSs not designed for x-ray performance, the
EV76C454 and the Ocean Colour Imager (OCI) test array. The EV76C454 is an industrial 5T APS designed for machine
vision, available back and front illuminated. The OCI test arrays have varying pixel design across the chips, but are 4T,
back illuminated and have thin low-resistivity and thick high-resistivity variants. The OCI test arrays’ pixel variants
allow understanding of how pixel design can affect x-ray performance.
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.
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 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.
Two mission concepts (plan A: out-of-ecliptic mission and plan B: high resolution spectroscopic mission) have been
studied for the next Japanese-led solar mission Solar-C, which will follow the scientific success of the Hinode mission.
The both mission concepts are concluded as equally important and attractive for the promotion of space solar physics. In
the meantime we also had to make efforts for prioritizing the two options, in order to proceed to next stage of requesting
the launch of Solar-C mission at the earliest opportunity. This paper briefly describes the two mission concepts and the
current status on our efforts for prioritizing the two options. More details are also described for the plan B option as the
first-priority Solar-C mission. The latest report from the Solar-C mission concept studies was documented as "Interim
Report on the Solar-C Mission Concept."
The solar chromosphere is an important boundary, through which all of the plasma, magnetic fields and energy in the
corona and solar wind are supplied. Since the Zeeman splitting is typically smaller than the Doppler line broadening in
the chromosphere and transition region, it is not effective to explore weak magnetic fields. However, this is not the case
for the Hanle effect, when we have an instrument with high polarization sensitivity (~ 0.1%). "Chromospheric Lyman-
Alpha SpectroPolarimeter (CLASP)" is the sounding rocket experiment to detect linear polarization produced by the
Hanle effect in Lyman-alpha line (121.567 nm) and to make the first direct measurement of magnetic fields in the upper
chromosphere and lower transition region. To achieve the high sensitivity of ~ 0.1% within a rocket flight (5 minutes) in
Lyman-alpha line, which is easily absorbed by materials, we design the optical system mainly with reflections. The
CLASP consists of a classical Cassegrain telescope, a polarimeter and a spectrometer. The polarimeter consists of a
rotating 1/2-wave plate and two reflecting polarization analyzers. One of the analyzer also works as a polarization beam
splitter to give us two orthogonal linear polarizations simultaneously. The CLASP is planned to be launched in 2014
We present scientific as well as engineering overview of the X-Ray Telescope (XRT) aboard the Japanese Solar-B mission to be launched in 2006, with emphasis on the focal plane CCD camera that employs a 2k x 2k back-thinned CCD. Characterization activities for the flight CCD camera made at the National Astronomical Observatory of Japan (NAOJ) are discussed in detail with some of the results presented.
The X-ray observations from the Yohkoh SXT provided the greatest step forward in our understanding of the solar corona in nearly two decades. We believe that the scientific objectives of the Solar-B mission can best be achieved with an X-ray telescope (XRT) similar to the SXT, but with significant improvements in spatial resolution and in temperature response that take into account the knowledge gained from Yohkoh. We present the scientific justification for this view, discuss the instrumental requirements that flow from the scientific objectives, and describe the instrumentation that will meet these requirements. XRT is a grazing-incidence (GI) modified Wolter I X-ray telescope, of 35 cm inner diameter and 2.7 m focal length. The 2048 X 2048 back-illuminated CCD has 13.5 (mu) pixels, corresponding to 1.0 arcsec and giving full Sun field of view. This will be the highest resolution GI X-ray telescope ever flown for Solar coronal studies, and it has been designed specifically to observe both the high and low temperature coronal plasma.
This paper describes the design and prelaunch performance of the tip-tilt mirror (TTM) system developed for the XUV Cassegrain telescope aboard the ISAS sounding rocket experiment. The spatial resolution of the telescope is about 5 arcsec, whereas the rocket pointing is only controlled to be within +/- 0.5 degree around the target without stability control. The TTM is utilized to stabilize the XUV image on the focal planes by tilting the secondary mirror with two-axes fixed-coil type actuators. The two position- sensitive detectors in the telescope optics and in the TTM mechanical structure from the normal and local closed-loop modes. The TTM has four grain modes with automatic transition among the modes. The low gain mode is used in the initial acquisition, and in case the TTM loses the tracking. The high gain mode is used in the normal tracking mode. This arrangement provides us with the wide initial acquisition angle with single TTM system as well as the high pointing accuracy once the tracking is established. The TTM has a launch-lock mechanism against the launch vibration of 16G. The closed-loop control with command and telemetry interface is done by the flight software against the launch vibration of 16G. The closed-loop control with command and telemetry interface is done by the flight software on the DSP processor. The use of the fast processor brings in the significant reduction in the weight and size of the control- electronics, more flexible control system, and shorter design and testing period.
We present the development status of the normal incidence XUV multilayer mirrors for XUV Doppler telescope, which observes coronal velocity fields of the whole sun. The telescope has two narrow band-pass multilayer mirrors tuned to slightly longer and shorter wavelengths around the Fe XIV line at 211.3 Angstrom. From the intensity difference of the images taken with these two bands, we can obtain Dopplergram of 1.8 MK plasma of the whole sun. It is required that the multilayer has high wavelength-resolution ((lambda) /(Delta) (lambda) approximately 30 per mirror), anti-reflection coating for intense He II 304 angstrom emission line and high d-spacing uniformity of approximately 1%.
We present an overview of a sounding-rocket experiment that is scheduled to be launched by the Institute of Space and Astronautical Science (ISAS) in January 1998, the rising phase of the 11-year activity cycle of the sun. The purpose of this experiment is (1) to obtain whole-sun images taken in an XUV emission line, Fe XIV 211 A, using the normal incidence multilayer optics with a high spectral resolution of about 40, and (2) to carry out the velocity-field measurement with a detection limit as high as 100 km/s.
We present an overview of an ongoing Japanese sounding rocket project with the Solar XUV Doppler telescope. The telescope employs a pair of normal incidence multilayer mirrors and a back-thinned CCD, and is designed to observe coronal velocity field of the whole sun by measuring line- of-sight Doppler shifts of the Fe XIV 211 angstroms line. The velocity detection limit is estimated to be better than 100 km/s. The telescope will be launched by the Institute of Space and Astronautical Science in 1998, when the solar activity is going to be increasing towards the cycle 23 activity maximum. Together with the overview of the telescope, the current status of the development of each telescope components including multilayer mirrors, telescope structure, image stabilization mechanism, and focal plane assembly, are reviewed. The observation sequence during the flight is also briefly described.