Solar-C EUVST (EUV High-Throughput Spectroscopic Telescope) is a solar physics mission concept that was selected as a candidate for JAXA competitive M-class missions in July 2018. The onboard science instrument, EUVST, is an EUV spectrometer with slit-jaw imaging system that will simultaneously observe the solar atmosphere from the photosphere/chromosphere up to the corona with seamless temperature coverage, high spatial resolution, and high throughput for the first time. The mission is designed to provide a conclusive answer to the most fundamental questions in solar physics: how fundamental processes lead to the formation of the solar atmosphere and the solar wind, and how the solar atmosphere becomes unstable, releasing the energy that drives solar flares and eruptions. The entire instrument structure and the primary mirror assembly with scanning and tip-tilt fine pointing capability for the EUVST are being developed in Japan, with spectrograph and slit-jaw imaging hardware and science contributions from US and European countries. The mission will be launched and installed in a sun-synchronous polar orbit by a JAXA Epsilon vehicle in 2025. ISAS/JAXA coordinates the conceptual study activities during the current mission definition phase in collaboration with NAOJ and other universities. The team is currently working towards the JAXA final down-selection expected at the end of 2019, with strong support from US and European colleagues. The paper provides an overall description of the mission concept, key technologies, and the latest status.
The main characteristics of Solar-C_EUVST are the high temporal and high spatial resolutions over a wide temperature coverage. In order to realize the instrument for meeting these scientific requirements under size constraints given by the JAXA Epsilon vehicle, we examined four-dimensional optical parameter space of possible solutions of geometrical optical parameters such as mirror diameter, focal length, grating magnification, and so on. As a result, we have identified the solution space that meets the EUVST science objectives and rocket envelope requirements. A single solution was selected and used to define the initial optical parameters for the concept study of the baseline architecture for defining the mission concept. For this solution, we optimized the grating and geometrical parameters by ray tracing of the Zemax software. Consequently, we found an optics system that fulfills the requirement for a 0.4” angular resolution over a field of view of 100" (including margins) covering spectral ranges of 170-215, 463-542, 557-637, 690-850, 925-1085, and 1115-1275 A. This design achieves an effective area 10 times larger than the Extreme-ultraviolet Imaging Spectrometer onboard the Hinode satellite, and will provide seamless observations of 4.2-7.2 log(K) plasmas for the first time. Tolerance analyses were performed based on the optical design, and the moving range and step resolution of focus mechanisms were identified. In the presentation, we describe the derivation of the solution space, optimization of the optical parameters, and show the results of ray tracing and tolerance analyses.
The Focusing Optics X-ray Solar Imager (FOXSI) sounding rocket experiment demonstrates the technique of focusing hard X-ray (HXR) optics for the study of fundamental questions about the high-energy Sun. Solar HXRs provide one of the most direct diagnostics of accelerated electrons and the impulsive heating of the solar corona. Previous solar missions have been limited in sensitivity and dynamic range by the use of indirect imaging, but technological advances now make direct focusing accessible in the HXR regime, and the FOXSI rocket experiment optimizes HXR focusing telescopes for the unique scientific requirements of the Sun. FOXSI has completed three successful flights between 2012 and 2018. This paper gives a brief overview of the experiment, focusing on the third flight of the instrument on 2018 Sept. 7. We present the telescope upgrades highlighting our work to understand and reduce the effects of singly reflected X-rays and show early science results obtained during FOXSI's third flight.
The Solar-C_EUVST is a mission designed to provide high-quality solar spectroscopic data covering a wide temperature range of the chromosphere to flaring corona. To fulfill a high throughput requirement, the instrument consists of only two optical components; a 28-cm primary mirror and a segmented toroidal grating which have high reflective coatings in EUV-UV range. We present a mission payload structural design which accommodates long focal length optical components and a launcher condition/launch environment (JAXA Epsilon). We also present a mechanical design of primary mirror assembly which enables slit-scan observations, an image stabilizing tip-tilt control, and a focus adjustment on orbit, together with an optomechanical design of the primary mirror and its supporting system which gives optically tolerant wavefront error against a large temperature increase due to an absorption of visible and IR lights.
The imaging spectroscopic observations for solar soft X-rays are expected to provide us novel and valuable information about the plasma activity in the solar corona, e.g., particle acceleration, heating, shock, etc. However, this type of observations has not been performed yet with enough energy, spatial, and temporal resolutions. In this situation, we plan to realize the imaging spectroscopic observations for solar soft X-rays with a high speed soft X-ray camera and grazing incidence mirrors. Our developing camera consists of a back-illuminated CMOS sensor. This censor has a sensitivity to soft X-rays (0.5 keV - 10 keV), and can perform continuous exposures of 1,000 frame per second for the imaging area of 1k x 100 pixels. We will mount this camera on the FOXSI-3 sounding rocket that is planned to be launched in the summer of 2018. By the combination of our camera and the X-ray mirror on the FOXSI, we can achieve an energy resolution of 0.2 keV, a spatial resolution of ~5 arcsec (1 arcsec sampling), and the temporal resolution of ~10 seconds in an energy range of 0.5 keV - 10 keV. In this presentation, we will explain the science goal, the instrumental design, and the developments of the solar soft X-ray imaging spectrometer.