The Lyman-alpha Solar Telescope (LST) for the ASO-S mission——Ⅰ. Scientific objectives and overview

As one of the payloads for the Advanced Space-based Solar Observatory(ASO-S)mission,the Lyman-alpha(Lyα)Solar Telescope(LST)is aimed at imaging the Sun and the inner corona up to 2.5 R⊙(mean solar radius)in both the Lyα(121.6 nm)and visible wavebands with high temporo-spatial resolution,mainly targeting solar flares,coronal mass ejections(CMEs)and filaments/prominences.LST observations allow us to trace solar eruptive phenomena from the disk center to the inner corona,to study the relationships between eruptive prominences/filaments,solar flares and CMEs,to explore the dynamical processes and evolution of solar eruptions,to diagnose solar winds,and to derive physical parameters of the solar atmosphere.LST is actually an instrument suite,which consists of a Solar Disk Imager(SDI),a Solar Corona Imager(SCI),a White-light Solar Telescope(WST)and two Guide Telescopes(GTs).This is the first paper in a series of LST-related papers.In this paper,we introduce the scientific objectives,present an overview of the LST payload and describe the planned observations.The detailed design and data along with potential diagnostics are described in the second(Paper II)and third(Paper III)papers,respectively,appearing in this issue.

The Lyman-alpha Solar Telescope(LST) for the ASO-S mission——Ⅱ. design of LST

As one of the three payloads for the Advanced Space-based Solar Observatory(ASO-S)mission,the Lyman-alpha(Lyα)Solar Telescope(LST)is composed of three instruments:a Solar Corona Imager(SCI),a LyαSolar Disk Imager(SDI)and a full-disk White-light Solar Telescope(WST).When working in-orbit,LST will simultaneously perform high-resolution imaging observations of all regions from the solar disk to the inner corona up to 2.5 R⊙(R⊙stands for the mean solar radius)with a spatial resolution of 4.8′′and 1.2′′for coronal and disk observations,respectively,and a temporal resolution of 30–120 s and 1–120 s for coronal and disk observations,respectively.The maximum exposure time can be up to20 s due to precise pointing and image stabilization function.Among the three telescopes of LST,SCI is a dual-waveband coronagraph simultaneously and independently observing the inner corona in the HI Lyα(121.6±10 nm)line and white light(WL)(700±40 nm)wavebands by using a narrowband Lyαbeam splitter and has a field of view(FOV)from 1.1 to 2.5 R⊙.The stray-light suppression level can attain<10^-6 B⊙(B⊙is the mean brightness of the solar disk)at 1.1 R⊙and≤5×10^-8 B⊙at 2.5 R⊙.SDI and WST are solar disk imagers working in the Lyαline and 360.0 nm wavebands,respectively,which adopt an off-axis two-mirror reflective structure with an FOV up to 1.2 R⊙,covering the inner coronal edge area and relating to coronal imaging.We present the up-to-date design for the LST payload.

Development and calibration of the Moon-based EUV camera for Chang'e-3

The process of development and calibration for the first Moon-based ex- treme ultraviolet （EUV） camera to observe Earth＇s plasmasphere is introduced and the design, test and calibration results are presented. The EUV camera is composed of a multilayer film mirror, a thin film filter, a photon-counting imaging detector, a mech- anism that can adjust the direction in two dimensions, a protective cover, an electronic unit and a thermal control unit. The center wavelength of the EUV camera is 30.2 nm with a bandwidth of 4.6nm. The field of view is 14.7° with an angular resolution of 0.08°, and the sensitivity of the camera is 0.11 count s-1 Rayleigh-1. The geomet- ric calibration, the absolute photometric calibration and the relative photometric cal- ibration are carried out under different temperatures before launch to obtain a matrix that can correct geometric distortion and a matrix for relative photometric correction, which are used for in-orbit correction of the images to ensure their accuracy.

Erratum:"The Lyman-alpha Solar Telescope(LST)for the ASO-S mission—I.Scientific objectives and overview"(2019,RAA,19,158)

As a result of an error by the authors,in the paper"The Lyman-alpha Solar Telescope(LST)for the ASO-S Mission.I.Scientific Objectives and Overview"by Hui Li et al.(RAA 2019 Vol.19,No.11,158,doi:10.1088/1674C4527/19/11/158),there is an error occurred in Table 2 about the image size of SCI UV in the'Event'mode:the image size 4608×4608 should be replaced by 2048×2048.This correction is indicated in bold face in the following table.

Wide-field auroral imager onboard the Fengyun satellite

The newly launched Fengyun-3D(FY-3D)satellite carried a wide-field auroral imager(WAI)that was developed by Changchun Institute of Optics,Fine Mechanics and Physics,Chinese Academy of Sciences(CIOMP),which will provide a large field of view(FOV),high spatial resolution,and broadband ultraviolet images of the aurora and the ionosphere by imaging the N2 LBH bands of emissions.The WAI consists of two identical cameras,each with an FOV of 68°in the along-track direction and 10°in the cross-track direction.The two cameras are tilted relative to each other to cover a fan-shaped field of size 130°×10°.Each camera consists of an unobstructed four-mirror anastigmatic optical system,a BaF2 filter,and a photon-counting imaging detector.The spatial resolution of WAI is ~10 km at the nadir point at a reference height of 110 km above the Earth’s surface.The sensitivity is>0.01 counts s^(−1) Rayleigh^(−1) pixel^(−1)(140-180 nm)for both cameras,which is sufficient for mapping the boundaries and the fine structures of the auroral oval during storms/substorms.Based on the tests and calibrations that were conducted prior to launch,the data processing algorithm includes photon signal decoding,geometric distortion correction,photometric correction,flatfield correction,line-of-sight projection and correction,and normalization between the two cameras.Preliminarily processed images are compared with DMSP SSUSI images.The agreement between the images that were captured by two instruments demonstrates that the WAI and the data processing algorithm operate normally and can provide high-quality scientific data for future studies on auroral dynamics.

Solar X-ray and EUV imager on board the FY-3E satellite

The solar X-ray and Extreme Ultraviolet Imager(X-EUVI),developed by the Changchun Institute of Optics,Fine Mechanics and Physics,Chinese Academy of Sciences(CIOMP),is the first space-based solar X-ray and Extreme ultraviolet(EUV)imager of China loaded on the Fengyun-3E(FY-3E)satellite supported by the China Meteorological Administration(CMA)for solar observation.Since started work on July 11,2021,X-EUVI has obtained many solar images.The instrument employs an innovative dual-band design to monitor a much larger temperature range on the Sun,which covers 0.6-8.0 nm in the X-ray region with six channels and 19.5 nm in the EUV region.X-EUVI has a field of view of 42′,an angular resolution of 2.5″per pixel in the EUV band and an angular resolution of 4.1″per pixel in the X-ray band.The instrument also includes an X-ray and EUV irradiance sensor(X-EUVS)with the same bands as its imaging optics,which measures the solar irradiance and regularly calibrates the solar images.The radiometric calibration of X-EUVS on the ground has been completed,with a calibration accuracy of 12%.X-EUVI is loaded on the FY-3E satellite and rotates relative to the Sun at a uniform rate.Flat-field calibration is conducted by utilizing successive rotation solar images.The agreement between preliminarily processed X-EUVI images and SDO/AIA and Hinode/XRT images indicates that X-EUVI and the data processing algorithm operate properly and that the data from X-EUVI can be applied to the space weather forecast system of CMA and scientific investigations on solar activity.