The Jiao Tong University Spectroscopic Telescope Project

: The Jiao Tong University Spectroscopic Telescope (JUST) is a 4.4-meter f/ 6 . 0 segmented-mirror telescope dedicated to spectroscopic observations. The JUST primary mirror is composed of 18 hexagonal segments, each with a diameter of 1.1 m. JUST provides two Nasmyth platforms for placing science instruments. One Nasmyth focus fits a field of view of 10 ′ and the other has an extended field of view of 1 . 2 ◦ with correction optics. A tertiary mirror is used to switch between the two Nasmyth foci. JUST will be installed at a site at Lenghu in Qinghai Province, China, and will conduct spectroscopic observations with three types of instruments to explore the dark universe, trace the dynamic universe, and search for exoplanets: (1) a multi-fiber (2000 fibers) medium-resolution spectrometer (R=4000-5000) to spectroscopically map galaxies and large-scale structure; (2) an integral field unit (IFU) array of 500 optical fibers and/or a long-slit spectrograph dedicated to fast follow-ups of transient sources for multi-messenger astronomy; (3) a high-resolution spectrometer (R ∼ 100000) designed to identify Jupiter analogs and Earth-like planets, with the capability to characterize the atmospheres of hot exoplanets.


Introduction
Observing facilities play a fundamental role in advancing our understanding of the universe.These facilities, including ground-based telescopes, space observatories, and specialized instruments, provide astronomers the necessary tools to gather data from distant celestial objects and phenomena, and explore the properties, compositions, and behaviors of objects such as stars and galaxies, leading to remarkable discoveries and profound insights into the nature of the universe.Moreover, long-term observations with these facilities enable monitoring of transient events, and probe the cosmos across various wavelengths, which is essential for unveiling cosmic mysteries.Observing facilities are indispensable for pushing the boundaries of astronomical knowledge and fostering scientific breakthroughs.The development of powerful observing facilities, whether for general-purpose use or dedicated surveys, has become a critical requirement for astronomers to achieve groundbreaking advancements.The progress of astronomy hinges on the construction of large telescopes which are currently evolving to possess large apertures, wide fields of view, and high spatial and spectral resolution.Given the disparity in associated cost between acquiring images and spectra, there is a notable shortfall in high-quality spectroscopic observational facilities compared with the plentiful availability of image-based observational facilities.This gap highlights the need for further attention and investment in advancing spectroscopic capabilities to complement the existing observational landscape.
Astronomical spectroscopy enables the precise measurement of redshift, identification of specific chemical elements, and the determination of kinematics of celestial objects.It leads to a deeper understanding of the nature and characteristics of the observed objects.Spectroscopic observations offer a wealth of information that complements and enhances the insights gained from imaging observations.To fulfill the scientific needs for spectroscopic observations, as well as owing to the great success of spectroscopic projects such as the Sloan Digital Sky Survey (SDSS 1 ; York et al., 2000), multiple new projects are thriving.The Dark Energy Spectroscopic Instrument (DESI2 ; DESI Collaboration et al., 2016) is the first stage-IV dark energy survey project, comprising a 4-meter telescope with 5 000 robotic fiber positioners to feed a collection of spectrographs covering the 360-980 nm wavelength range.It has reportedly finished over 50before the planned 5 years of run time, demonstrating its high efficiency in observation.
They will provide the spectroscopic follow-up required for full scientific exploitation of other projects, such as the Gaia, LOFAR and Apertif surveys.The MegaMapper (Schlegel et al., 2019) will be a dedicated cosmology facility with highly efficient redshift measurements on a 6.5 m telescope.8-meter-class projects include the Subaru Prime Focus Spectrograph (PSF4 ; Tamura et al., 2022) project, and the Multi-Object houses the WFST, which is dedicated to imaging surveys.JUST will be placed at Position B (at 4322 m).
Photo credit: Bin Chen.
Optical and Near-IR Spectrograph (MOONS 5 ; Cirasuolo et al., 2020).With increasing telescope size, the Maunakea Spectroscopic Explorer (MSE 6 ; Hill et al., 2018), SpecTel (Ellis & Dawson, 2019) and the Fiber-Optic Broadband Optical Spectrograph (FOBOS 7 ; Bundy et al., 2019) are 10-meter-class projects.All of these large telescopes will equip instruments with thousands to tens of thousands of optical fibers, aiming at simultaneous spectroscopic observations of multiple objects at once.In China, optical telescopes currently fall behind world-class standards.However, with improved funding availability and technological capabilities, observatories and universities have initiated the construction of optical telescopes with diameters exceeding 2 meters.This initiative is driven by diverse scientific objectives, and aims to facilitate distinctive observational research, enabling universities within China to make substantial progress with medium-sized observing facilities, such as the Wide Field Survey Telescope (WFST or "Mocius" 8 ) is one of them and is on commissioning phase (Wang et al., 2023).
For spectroscopic observations, there are several telescopes, either proposed or under construction, such as the 4.4-meter Jiao Tong University Spectroscopic Telescope (JUST 9 ), and 6.5-meter MUltiplexed Survey Telescope (MUST 10 ), as well as the stage II of Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST-II11 ).Construction has already begun on JUST, and the first light observations are expected within three years.This telescope will be equipped with dedicated spectroscopic instruments to explore the dark universe, trace the dynamic universe, and search for exoplanets.In this paper, we provide basic information about details such as the site, structure, optical system, and science motivations of JUST.The structure of this paper is outlined as follows.We first introduce the site condition in Section 2., followed by the conceptual design of the JUST telescope in Section 3..An overview of the planned instruments and science motivations is presented in Section 4..We summarize the JUST project in Section 5..

Site and dome
Mauna Kea in Hawaii and certain summits and plateaus in northern Chile are among the best observing sites on the Earth.Over the past two decades, much effort has been dedicated to the search for excellent astronomical sites in China.Recently, the summit of Saishiteng Mountain in Lenghu, located on the Tibetan Plateau, was identified as possessing favorable observing conditions.Site monitoring has shown that the summit of Saishiteng Mountain, situated at an altitude of 4 200 to 4 500 meters, experiences clear nights for approximately 70% of the year and boasts good median seeing of 0.75 arcsec (Deng et al., 2021).
The climate in the surrounding area of the site is extremely arid, and the sky background is exceptionally dark due to minimal light pollution.Furthermore, the time zone of the Lenghu site is distinct from that of nearly all observatories worldwide, facilitating complementary timedomain astronomy.
The planned installation of the JUST telescope is shown in Fig. 1, at position B, situated at an altitude of 4322 m on the summit of Saishiteng Mountain, close to position C where WFST is located.
Spectroscopic observations with JUST will complement imaging observations made with WFST, providing essential photometric and spectroscopic data for advancing researches across various domains of astronomy.Fig. 2 illustrates the design concept of the telescope dome for JUST.It incorporates a classical semispherical dome, featuring a shutter that can be opened to allow the telescope to observe.The dome will also integrate ventilation systems to regulate temperature and air circulation, improving dome seeing.
Additionally, it will include lighting and other equipments to support telescope operation and maintenance.
The construction of the site infrastructure and telescope dome is scheduled to commence in 2024.

General parameters
The telescope project encompasses three main functional subsystems: telescope optics, support and structure, and telescope control.The telescope optics subsystem comprises optical mirrors and mirror support with active optics.The support and structure subsystem includes a tracking mount and telescope tube.The control subsystem incorporates the telescope control subsystem (TCS), observation control subsystem, and active optics control subsystem.A lightweight telescope design is achieved through the selection of a horizontal tracking mount and a truss-type telescope tube structure.The overall conceptual view of the telescope is illustrated in Fig. 3. Nasmyth focus 1 is a purely reflecting system, in which high-resolution wide-wavelength instruments or infrared instruments can be mounted.The image quality is defined as the full width at half maximum (FWHM) of the image profile.The intended image quality of Nasmyth focus 1 is 0.09 ′′ .With manufacturing, alignment, and control error, this value can be reduced to 0.35 ′′ .
Nasmyth focus 2 is used for the Multi-Object Fiber Spectroscopic Survey.The diameter of the focal plane is 570 mm, and about 2 000 optical fibers can be accommodated.At a zenith distance of 60 • , observation site altitude of 4 200 m, and the wavelength range of 0.35-1.30.35 − 1.3 µm, the atmospheric dispersion is 3.1 ′′ .However, astigmatism rapidly increases with the square of the FoV, so it is essential to include a corrector for widening FoV and compensating for atmospheric dispersion.The corrector consists of four silica lenses, two of which are the atmospheric dispersion correctors (ADCs).The target image quality of Nasmyth focus 2 is 0.51 ′′ .With errors, the delivered image quality will be 0.7 ′′ , which is close to the value of median seeing at the Lenghu site.As a reference, we list in Table 1 the main optical parameters of JUST.

The mirrors
The primary mirror (M1) is composed of 18 hexagonal segments, with an effective aperture of 4.4 m.
The detailed configuration is depicted in Fig. 5.Each segment is equipped with its own support system to maintain the correct optical surface.The axial support system utilizes an 18-point whiffletree support for the segments, while the radial support employs a central flexible support.This support system serves the following functions: -Accurate installation of the segments onto the main truss; -Support for the segments to meet the requirements for the mirror surface shape; -Active optical technology to control the closed-loop segmented system of the primary mirror, mitigating the effects of temperature and gravity, and achieving co-focusing/co-phasing of the segments.
The secondary mirror (M2) support module is designed to preserve its original machining accuracy and stabilize its spatial position.The support system for the secondary mirror includes a bottom support, lateral support, and centering mechanism.The bottom support features a suspended whiffletree support structure, while the lateral support uses a lever-balanced weight support structure.The centering mechanism employs a bi-directional membrane structure in both radial and axial directions.
The tertiary mirror (M3) uses a whiffletree floating support structure, with the centering mechanism also adopting a bi-directional membrane structure to serve the radial and axial directions, in addition to function as lateral support.

Active optics
The M1 is designed to use active optics technology for real-time closed-loop control splicing to join the segments into a single mirror surface.The active optical system primarily comprises core devices such as segment surface support, displacement actuator, and an active optical wavefront sensor.-Segment surface support system: Ensures that the support surface meets and exceeds the technical requirements for the segment surface shape.
-Displacement actuator: Utilizes nano-electromechanical displacement actuators controlled in parallel by multiple active optical intelligent controllers to achieve nanometer-level displacement resolution and millimeter-level displacement range output accuracy and stroke under the full load of the segments.
-Active optical wavefront sensor: Uses a Shack-Hartmann wavefront sensor based on physical optics.
It measures the surface shape, imaging quality of individual segments, and the segmented primary mirror.It uses the central star as a target source, providing precise feedback to continuously drive the displacement actuators for segmented mirror calibration and maintenance.
Upon implementation of active optics technology, the telescope will achieve co-focusing of the primary mirror and obtain imaging quality close to the visibility limit of the telescope's location, in conjunction with the optical system design.

Science cases and scientific instruments
JUST has two Nasmyth platforms and will be installed with three types of spectrographs on both.The basic parameters of these spectrographs are listed in Table 2.
-Galaxies and large-scale structures: JUST will be equipped with multiple fiber positioners and mediumresolution spectrometers to conduct spectral surveys of a large number of galaxies.
-Multi-messenger astronomy: JUST will be equipped with hundreds of optical fibers to form an Integrated Field Unit (IFU) array and/or a long-slit spectrograph for follow-up observation of a large number of transient sources.
-Exoplanet detection and characterization: JUST will use advanced high-resolution spectrometers to detect cold giant planets and earth-like terrestrial planets, and to provide detailed atmospheric characterization for hot exoplanets.

Exploring the dark Universe
More than 95% of the Universe remains dark to humanity, whether in the form of dark matter or dark energy (Planck Collaboration et al., 2020).The first step toward understanding the dark Universe requires the accurate measurement of the growth of cosmic structures on scales ranging from a few kiloparsecs to hundreds of megaparsecs, with highly-multiplexed spectroscopic surveys of galaxies (Weinberg et al., 2013).
To complement the current stage-IV surveys that focus on the galaxy distribution at linear scales, JUST will dedicate its multi-object spectroscopic (MOS) capability to the mapping of structures from quasilinear to highly non-linear scales, centered on massive galaxy clusters at z<0.6.JUST aims for complete spectroscopic coverage of the galaxies at r < 20 mag in the cosmic web surrounding clusters below z < 0.6, complementing DESI spectra.
The JUST spectroscopic cluster survey(SCS) will improve cluster cosmology as one of the most sensitive probes of cosmic growth, through the mitigation of systematic uncertainties in the cluster redshifts, satellite membership assignment, and various projection effects associated with photometric cluster finders (Erickson et al., 2011;Noh & Cohn, 2012;Zu et al., 2017;Costanzi et al., 2019).The spectroscopic cluster catalogue will provide stringent constraints of key cosmological parameters, including the matter density, the amplitude of matter clustering, the equation-of-state of dark energy, and the sum of neutrino masses (Sartoris et al., 2016).The combination of the redshift-space distortion(RSD) of infalling galaxies (Lam et al., 2013;Zu & Weinberg, 2013;Hamabata et al., 2019;Shirasaki et al., 2021) and the weak lensing of background sources (Johnston et al., 2007;Simet et al., 2017;Wang et al., 2022) by galaxy clusters will enable stringent tests of theories of cosmic acceleration and distinguish between dark energy and modified gravity on inter-cluster scales (Zu et al., 2014;Koyama, 2016;Joyce et al., 2016;Baker et al., 2021).Meanwhile, JUST-SCS will fully sample cluster galaxies in both the velocity phase space(cluster-centric radius vs. line-of-sight velocity) and the color-magnitude diagram, from infall to the splashback regions, and into the virialized cores of clusters (Fillmore & Goldreich, 1984;Bertschinger, 1985;Kravtsov & Borgani, 2012;Diemer & Kravtsov, 2014;More et al., 2016;Walker et al., 2019).Such spectroscopic coverage of the cosmic web will provide a comprehensive picture of galaxy formation in different environments surrounding galaxy clusters (Kauffmann et al., 2004).
In recent decades, Chinese astronomers have made significant contributions to revealing the nature of the dark universe with contributions such as measuring and quantifying large scale structure, elucidating the galaxy-halo connection, constraining the cosmological parameters.Among these efforts, representative work includes establishing the halo occupation distribution model (Jing et al., 1998) based on the Las Campanas Redshift Survey (Shectman et al., 1996), establishing the conditional luminosity function model (Yang et al., 2003) based on the 2-degree Field Galaxy Redshift Survey(2dFGRS) (Colless et al., 2001), establishing the halo-based group finder (Yang et al., 2005(Yang et al., , 2007) ) based on 2dFGRS and the Sloan Digital Sky Survey (York et al., 2000), and making dark energy model constraints (Zhao et al., 2017) based on the Baryon Oscillation Spectroscopic Survey (Alam et al., 2015).Most of these achievements were made based on either public data releases or through international collaborations of large galaxy redshift surveys.
With JUST-SCS, we will have greater opportunity to explore the dark universe with our own observational data set.To maximize the science return of the MOS survey on cluster cosmology and galaxy evolution, JUST-SCS will include three layers as summarized by Figure 6.We will discuss each of the three in the subsections below.

JUST Cluster Cosmology Survey
The upcoming China Space Survey Telescope (CSST; Miao et al., 2023)) will detect approximately 300,000 photometric halo-based cluster candidates with halo mass above 10 14 M ⊙ /h up to z < 1.5 (Yang et al., 2021), serving as the basis of target selection for JUST-SCS.In particular, the JUST cluster cosmology survey will target ≃50, 000 clusters over 10, 000 deg 2 at z<0.6, producing an unprecedented spectroscopic cluster sample for cosmological analysis.For each cluster, JUST will be used to obtain spectra for the brightest cluster galaxy (BCG) and the bright member galaxy candidates down to r=20, including but without re-observing the spectra from the full DESI survey.This program will not only provide secure spectroscopic redshifts for a cosmologically significant volume of individual clusters, but also improve the centering of clusters both perpendicular and along the line-of-sight (Sohn et al., 2021).
Such an accurate localization of individual clusters in three dimensions enables cosmological analyses using massive dark matter haloes, instead of galaxies, as spectroscopic tracers of the large-scale structure.
With spectroscopic redshifts for up to 20 member galaxy candidates, JUST will be able to disentangle the chance alignment of structures along the line of sight, and mitigate interlopers from any correlated structures on the velocity phase diagram.The spectroscopically confirmed satellite galaxies will enable mass estimates of individual haloes through the velocity dispersion (Evrard et al., 2008;Wu et al., 2013;Ntampaka et al., 2015) and caustic boundary (Diaferio, 1999;Gifford et al., 2013;Rines et al., 2013), improving the calibration of the cluster mass-observable relation beyond the optical richness (Rozo et al., 2009).Meanwhile, JUST will probe the properties of the intracluster medium, particularly the circumgalactic medium of cluster galaxies, by measuring metal absorption lines recorded in the DESI spectra of background quasars in the cluster fields (Zhu & Ménard, 2013;Lee et al., 2021;Zu, 2021;Anand et al., 2022;Napolitano et al., 2023).

JUST Cluster Infall Survey
In the intermediate redshift range between 0.1<z<0.4,JUST-SCS aims to achieve a complete spectroscopic coverage of galaxies within an approximately 20 Mpc/h radius surrounding each cluster down to r = 20, on top of the existing spectra from the DESI Bright Galaxy Survey (BGS; Hahn et al., 2023).In addition, JUST will spectroscopically cover a large number of non-cluster fields to the same depth, as the control sample of field galaxies for the cluster-galaxy cross-correlation measurements and the galaxy evolution study.The target selection of the non-cluster fields will be optimized based on the signal-to-noise forecast of the cluster RSD analysis.
The JUST cluster infall survey will push the(E G ) method (Zhang et al., 2007) from the linear regime to the infall region around clusters, where the potential imprint of modified gravity remains unscreened and the signal-to-noise of the RSD and weak lensing measurements is high.In particular, JUST will accurately measure the cluster-galaxy cross-correlation function in the redshift-space on projected scales below 20 Mpc/h, allowing high-fidelity reconstruction of the galaxy infall kinematics (GIK) as a function of distance to the cluster center.The GIK reconstruction provides a unique probe of the average dynamical mass profile of clusters in the infall region, which will enable stringent tests of the theories of cosmic acceleration when compared with the cluster mass profile measured from weak lensing (Zu et al., 2014).
In addition, the spectr dense spectroscopic sampling of the infall region allows individual measurements of the cluster dynamical mass using the caustics technique.
One of the primary systematics in cluster cosmology is the projection effect due to the 2D aperture adopted by photometric cluster catalogues, leading to the correlation between cluster richness and largescale overdensity, hence the bias in the large-scale weak lensing signals of clusters (McClintock et al., 2019;Sunayama, 2023;Salcedo et al., 2023).The JUST cluster infall survey will mitigate this projection effect by adopting a 3D aperture in the velocity phase space for measuring cluster mass observables.Meanwhile, this program will provide a panorama of the star formation, chemical enrichment, and dynamical evolution of galaxies across the cosmic web.Spectral stacking at different cosmic web environments will allow a robust reconstruction of the average histories of star formation and chemical evolution, as galaxies are funneled through the filaments into clusters (Andrews & Martini, 2013;Lin & Zu, 2023).By comparing the galaxy population surrounding clusters with those observed in the non-cluster fields, JUST will provide the key observational evidence on the concept of "nature versus nurture" in galaxy formation.

JUST Cluster Galaxy Evolution Survey
In the nearby universe below z<0.1, JUST will obtain spectra for galaxies within the virial radius of each SDSS galaxy group (Yang et al., 2007) above 10 13 M ⊙ /h down to a stellar mass of ∼10 8 M ⊙ .Focusing on the faint end of the conditional luminosity function of groups (Lan et al., 2016;Golden-Marx et al., 2023), the JUST cluster galaxy evolution survey will explore the star-forming histories of dwarf galaxies inside the group and cluster-size haloes, and ascertain the existence of a characteristic stellar mass of quenching among the satellites (Meng et al., 2023).With the accurate measurement of the group/cluster masses, JUST will provide strong constraints on the stellar-to-halo mass relation of the dwarf satellites via abundance matching and satellite weak lensing (Li et al., 2014;Niemiec et al., 2017;Sifón et al., 2018;Dvornik et al., 2020;Danieli et al., 2023).
The JUST cluster galaxy evolution survey will reveal the co-evolution between cluster galaxies and dark matter haloes, by connecting the spectroscopic observations to the individual halo assembly histories predicted by ELUCID, a state-of-the-art constrained simulation that accurately reconstructed the initial density perturbations within the SDSS volume below z=0.1 (Wang et al., 2014(Wang et al., , 2016)).Another unique aspect of this program is the exciting synergy with the FAST All Sky HI Survey (FASHI) (Zhang et al., 2023), which will provide the largest extragalactic HI catalogue at z < 0.1 using the Five-hundred-meter Aperture Spherical radio Telescope (FAST; Nan et al., 2011).
Meanwhile, JUST will reserve a fixed set of fiber assignment for a sample of low-surface brightness targets(e.g., ultra-compact dwarfs) to allow spectral coverage down to ≃23 magnitudes per arcsec 2 in the r-band (Liu et al., 2020;Wang et al., 2023).For extended sources of interest(e.g., including the outskirts of BCGs and bar galaxies), MOS-mode observations can be supplemented by follow-up observations with the IFU instrument (Gu et al., 2020;Chen et al., 2022).Taking advantage of the synergy with ELUCID and FAST, the versatility of JUST will present an exquisite view of cluster galaxy evolution in the local universe.

Tracing dynamical universe
The Universe is not static.It is in motion and constantly changing.Time domain astronomy, which focuses on dynamic astronomical events, is a promising method to study this in greater detail.In the 2020 NASA decadal survey for astronomy and astrophysics, it is considered an important research frontier in astronomy (National Academies of Sciences, Engineering, and Medicine, 2021).Rapid follow-up observations of unexpected events is crucial in the era of multi-messenger astronomy, allowing astronomers to combine various observation methods such as neutrinos, electromagnetic waves, and gravitational wave signals, which are of great significance for understanding important high-energy astrophysical processes such as black hole and neutron star mergers.The main targets of time-domain astronomy are sporadic events(such as supernova explosions and tidal collapse events.),and the follow-up spectroscopic observation of these events can help to understand the specific physical processes in these transient sources.
There are currently dozens of time-domain astronomical survey projects, such as the Catalina Survey, PanSTARRS, iPTF, ASASSN, ATLAS and ZTF.In the past decade, the number of transient sources discovered has increased tenfold (Gal-Yam et al., 2013).The first gravitational wave electromagnetic counterpart was discovered in 2017 (Abbott et al., 2017) and confirmed to be a millennium nova(kilonova; Coulter et al. 2017).At present, the number of supernovae discovered is increasing year by year, exceeding one thousand per year.Based on large sample studies, new types of supernovae and explosive physical processes have been discovered.At the same time, new processes of active galactic nucleus explosions and tidal disruption events are also being continuously discovered.Time-domain astronomy has evidently become one of the fastest developing frontier astrophysical research fields.The study of time-domain astronomy can answer the following important questions: What is the explosive process of the evolution of massive stars to their final stages?What are the precursor stars of Type Ia supernovae?How did they erupt?Why does the universe accelerate its expansion?What determines the mass, spin, and radius of a dense star?How do supermassive black holes accrete and grow?Although astronomers have made some progress in addressing these issues, they are still far from fully understanding the physical reasons behind these phenomena.
In the future, surveys like LSST, CSST and WFST will obtain larger transient source samples.It is expected that hundreds or thousands of supernovae and other explosive phenomena will be discovered every night.These future surveys will significantly expand the redshift coverage of transient sources and expand the observation wavelength ranges.Space telescopes such as the Einstein Probe(EP), Swift, and Wide-field Infrared Survey Explorer(WISE) will observe the transient sources in X-ray, ultraviolet, and infrared bands, respectively.It can be expected that these larger samples will bring higher statistical significance, to reveal systematic differences among different types of transient sources and to discover extreme cases in each category.For example, in the past decade, the increasing number of transient source events has spawned research on the relationship between Type la supernovae and the star formation rate in their host galaxies (Jones et al., 2018), and the discovery of Type II supernovae that lasted for a year (Arcavi et al., 2017), as well as a new type of thermonuclear explosion supernova(SNe Iax; Foley et al.The transient sources may be induced from many different high-energy phenomena.Among them, events such as gamma-ray bursts, supernovae, and tidal disruption events are generated by cataclysmic processes.AGN flares, X-ray binary bursts, and rapid radio bursts involve periodic and intense physical processes near black holes or compact objects with strong magnetic field.The study of these phenomena not only reveals the specific physical mechanisms, but also helps to test basic theories such as relativity under extreme conditions.JUST will primarily focus on follow-up spectral observations of various transient sources, which is crucial for revealing the driving mechanisms of transient sources.

Supernova identification and classification
Important for cosmological research, Type Ia supernovae can serve as standard candles for cosmological distance determination, ultimately leading to the discovery of accelerated expansion of the Universe.High redshift supernovae are mainly discovered through photometric methods, and subsequent spectral analysis helps to distinguish different types of supernovae.On one hand, distinguishing the different types of supernovae can reduce the impact of other types of supernovae on the distance measurement of high redshift galaxies, improving the accuracy of galaxy distance measurement, to better constrain on the accelerated expansion of the universe.On the other hand, analyzing supernova subclasses can help in understanding the basic parameters of precursor stars, the physical processes of explosions, and the interaction between the outflow material and the interstellar medium.
JUST is capable of rapid response and subsequent spectral observations of supernovae at moderate redshifts(z ∼ 0.1 − 0.3).Within this redshift range, the magnitude of Type Ia supernovae ranges from 18 to 22 magnitudes.The aperture of this telescope is large enough to accomplish this, and the observation conditions at its location(Lenghu) are excellent, which can allow the recording of high signal-to-noise ratio spectra of these sources (Deng et al., 2021).This will significantly increase the number of supernova observations in the medium redshift range and may potentially discover new supernova types.

Gravitational wave electromagnetic counterpart properties
The discovery of gravitational wave GW150914 was a milestone event in gravitational wave astronomy, which confirmed the existence of a black hole merger for the first time (Abbott et al., 2016).However, the electromagnetic wave counterpart of the gravitational event was not discovered until 2017, when global synchronous observations of GW170817 confirmed its electromagnetic counterpart for the first time as a binary neutron star merger event (Abbott et al., 2017a,b).Within minutes to hours, Chile's Swope telescope confirmed an optical flare event in NGC 4993 galaxy.In the following weeks, observatories around the world conducted follow-up observations of the event in different wavelengths, providing a panoramic view of the physical process of the binary neutron star merger event (Cho, 2017).The visual magnitude of the optical counterpart of this binary neutron star merger event varies between 17.5 and 23 magnitudes, and JUST can also perform spectral observations of this source and others like it.In the future, more gravitational wave events will be detected.Timely follow-up spectroscopic observation of the source is very important to provide additional information(such as chemical abundance, redshift, and kinematics) to reveal the physical properties of gravitational wave sources.

The physical process of tidal disruption events
If a star is too close to a supermassive black hole, it will be disrupted by tidal forces, causing about half of the material to be accreted, resulting in flares at optical, infrared, ultraviolet, X-ray, and other wavelengths.This is known as a tidal disruption event(TDE), which was theoretically proposed in 1970s (Hills, 1975;Lidskii & Ozernoi, 1979;Rees, 1988;Phinney, 1989;Evans & Kochanek, 1989;Ulmer, 1999) and observationally confirmed in 1990s (Bade et al., 1996;Grupe et al., 1999;Komossa & Greiner, 1999;Greiner et al., 2000).It has become one of the most important targets in time-domain astronomy.With the advancement of various photometric surveys(such as South Sky LSST and North Sky WFST), a large number of TDEs will be discovered.For example, WFST in China expects to discover tens to hundreds of TDEs annually and to obtain complete light curves, including the early brightening phase.TDEs are one of the main targets of the Einstein Probe X-ray telescope in China.TDE detection is an important method to observe supermassive black holes(including quiescent ones) and provides information on black hole mass and spin, accretion disk physics, strong field gravity, and black hole environment(gas, dust environment, and stellar properties).TDEs are also useful to identify intermediate mass black holes, with the potential to resolve the mass gap between stellar mass black holes and supermassive black holes, completing the evolutionary landscape of black holes.
JUST can efficiently perform follow-up spectroscopic observations of TDEs detected by WFST and EP.Its 4.4-meter aperture, fast pointing adjustment, same observation location, and medium resolution spectrograph make it perfectly compatible with WFST(a 2.5-meter telescope) to carry out joint measurements of TDEs.The typical brightness of TDEs is ∼ 20 − 23 mag(with a redshift range of 0.3 − 1), and the light curve variation period is on the order of months, allowing a considerable success rate in obtaining TDE spectra with redshifts below 1.The acquisition of TDE spectra can provide important information such as accretion disk wind properties, stellar/accretion and disk chemical composition (Dai et al., 2018;Parkinson et al., 2020).Combined with the light profile curves of other photometric surveys, it will significantly improve the understanding of the physical mechanisms of TDEs, as well as the strong gravitational field properties.JUST can also provide key spectroscopic evidence for the tidal disruption of white dwarfs in intermediate mass black holes that have been discovered.The IFU observation mode is expected to obtain spectroscopic information on host galaxies, measuring their redshift, dispersion velocity, and chemical composition, to provide further observational constraints on the coevolution of galaxies and supermassive black holes.

Long term monitoring of active galactic nuclei(reverberation mapping)
JUST can also monitor the long-term spectral variability of active galactic nuclei(AGN).A considerable number of AGN with redshift less than 1 have r-band magnitudes brighter than 22 mag, suitable for future observation with JUST.By analyzing the time delay between the variability of the emission lines and the continuum, the reverberation mapping method can be used to analyze the structural characteristics of the broad line region(BLR) near the black hole, and to estimate the mass of the black hole.In addition, by observing the post spectral variability of some sudden flare phenomena in AGN, we can understand the physical reasons behind the changes in the continuum, broad line structure, and kinematics of AGN with the variation of the accretion rate, to better understand the physical processes of accretion by supermassive black holes.

Detection and characterization of exoplanets
The third category of science motivations for JUST is exoplanet detection and characterization.With its high-resolution spectrometer, JUST will enable the discovery of a substantial number of cold giant planets by employing a combination of radial velocity(RV) and astrometric analyses.In its upgraded phase, JUST will feature an exceptionally high-precision spectrograph designed for detecting Earth-like planets.
Leveraging these advanced capabilities, JUST will further enable the characterization of the atmospheres of hot exoplanets, contributing valuable insights into their formation and evolution.

Detection of cold giants
The planets in our Solar System and most of the known exoplanets are thought to form in a bottomup fashion through collisions of dust, pebbles, and planetesimals.This so-called "core accretion"(CA) mechanism is able to form Jupiter-like planets through the processes of core formation, envelope formation and contraction.However, this formation channel is probably not efficient to form substellar companions on wider orbits before the dispersion of a protoplanetary disk in ∼10 Myr (Kratter & Lodato, 2016).
These objects are more likely to form like stars in a top-down fashion through the so-called "gravitational instability"(GI) mechanism (Boss, 1997).However, due to the flexibility and ambiguity of the features of substellar companions predicted by CA and GI, it is challenging to determine which formation channel is responsible for specific giant companions such as the four directly imaged giant planets around HR 8799 (Marois et al., 2008).Hence, a statistically significant sample of giant planets on wide orbits(or cold giants) would be essential to statistically distinguish between GI and CA and draw a boundary between these two formation channels.
Thanks to the high precision astrometry catalogs released by Gaia (Gaia Collaboration et al., 2016, 2018, 2021, 2023) and the long baseline formed by Gaia and its precursor, Hipparcos (Perryman et al., 1997;van Leeuwen, 2007), many substellar companions detected by the radial velocity method are confirmed and their absolute masses are determined by combined analyses of radial velocity(RV) and astrometry data (Snellen & Brown, 2018;Brandt et al., 2019;Kervella et al., 2022;Feng et al., 2022).However, these detection are limited to super-Jupiters or more massive companions due to the limited precision and time span of the current Gaia data and the limited number of stars with high precision RV data.
While the precision and time span of Gaia data will be significantly improved in Gaia DR4, it is hard to significantly increase the current sample of stars with high precision RVs because of the limited number of high resolution spectrographs and the low efficiency of the current high precision RV survey.
To facilitate the detection of large number of cold giants with the combined RV and astrometry method, JUST will be equipped with the High Resolution Spectrograph(HRS), which can measure RV with a precision of about 1 ms −1 .HRS will be a fiber-fed, white-pupil spectrograph with a design resolution of R=60,000-80,000 and a wavelength of 380-760nm.The instrument design will be based on the successful high resolution spectrograph on the LAMOST (Zhang, Tianyi and Zhu, Yongtian and Hou, Yonghui and Zhang, Kai and Hu, Zhongwen and Wang, Lei and Chen, Yi and Jiang, Haijiao and Tang, Zhen and XU, Mingming and Jiang, Mingda, 2019) and HARPS-N (Cosentino et al., 2012) on the TNG telescope.In order to obtain a precision radial velocity(PRV), HRS will be enviromentally stabilized in the vacuum enclosure and via two optical fibers will provide simultaneous measurement of the science source and a spectral calibration source.Like other PRV instruments, the HRS will includes three main subsystems, 1) front-end module to correct for atomspheric dispersion, reimage the telescope beam onto the science fiber, stablize the image with fast tip-tilt corrections, 2) calibration unit to enable the injection of different light sources and 3) spectrograph is vibrationally and thermally isolated from the room.To ensure optimal optical performance and superior angular resolution, HRS will be integrated with the first Nasmyth focus of JUST.

Detection of Earth twins
One holy grail of exoplanetology is to find the most Earth-like planets.These so-called Earth twins are Earth-sized planets located in the habitable zones of Sun-like stars (Kasting et al., 1993).These temperate worlds can sustain liquid water on their surface and probably also have other habitable conditions such as plate tectonics, magnetic fields, and stable orbits.The Earth twins are perfect targets for future missions such as LUVOIR, HabEx (The LUVOIR Team, 2019;Gaudi et al., 2020) and Habitable World Observatory(HWO; Mamajek & Stapelfeldt 2023).
However, it is challenging to detect Earth twins due to limited instrumental precision and stellar activity.The measurement error of single RVs is typically >0.3 ms −1 for second-generation spectrometers such as ESPRESSO on VLT (Pepe et al., 2010), Maroon-X on Gemini-North (Seifahrt et al., 2022), NEID on WIYN 3.5m (Schwab et al., 2016), and the Keck Planet Finder (Gibson et al., 2016).With advanced data analysis techniques, we are able to detect RV signals as small as 0.3 ms −1 (Feng et al., 2017;Faria et al., 2022).
While instruments like ESPRESSO and KPF have achieved an RV precision of sub-ms −1 for detecting habitable Earths, stellar activity introduces noise reaching several ms −1 , surpassing the planetary signal.
The challenge in using RV to detect habitable Earths lies in effectively distinguishing this "red noise" from the planetary signal, given its time dependence.Advanced noise modeling techniques such as Gaussian processes have been used to mitigate such red noise (Haywood et al., 2014;Rajpaul et al., 2015).However, these techniques may lead to false negatives due to over-fitting (Feng et al., 2016;Ribas et al., 2018).
To mitigate the impact of wavelength-dependent stellar activity noise on radial velocity, traditional methods involve measuring the intensity of spectral lines characterizing stellar magnetic fields to remove the velocity variations linearly correlated with these so-called "activity indicators" (Dumusque, 2016;Dumusque et al., 2017;Zechmeister et al., 2018).However, different types of stars respond differently to various stellar activity indicators, and the linear removal of velocity correlated with these indicators introduces inherent noise.Therefore, recent research favors directly selecting spectral lines from the spectrum that are less "contaminated" by stellar activity (Dumusque, 2018;Lisogorskyi et al., 2019).
In the upgraded phase of JUST instrumentation, an ESPRESSO-like spectrograph, named Extremely high Resolution Spectrograph(ERS), will be built for the detection of Earth twins.ERS will have a resolution of at least 100,000 and can measure RV with a precision of about 0.1 ms −1 .It will be built following the design of CHORUS on GTC12 .With this spectrometer, JUST will survey a sample of 20-40 Sun-like nearby stars over 5 years to discover Earth twins.Given the uncertainty of the current occurrence rate of Earth twins (Ge et al., 2022), we expect to discover at least 1-3 Earth twins as golden samples for future direct imaging missions such as LUVOIR and Habex (The LUVOIR Team, 2019;Gaudi et al., 2020).

Characterization of hot extrasolar giant planets
One of the primordial goals of exoplanet sciences is to characterize exoplanetary atmospheres and inform the formation and evolution history of the diverse planetary systems (Madhusudhan, 2019).Highresolution spectroscopy has offered a unique means to measure chemical species in the atmospheres of close-in hot Jupiters because this type of exoplanets so far offers the best signal-to-noise ratio(see a review by Birkby, 2018).Using the same framework as measuring the extremely precise RV of the planet-hosting stars, this method can be applied to phase-resolved planetary spectral lines which can be identified through the Doppler effects of the orbiting planets.For typical hot Jupiters, the orbital speed is a few orders of magnitude larger than that of the star, and thus the stellar and telluric spectral features are relatively unchanged compared to the planetary spectral lines and can be removed by various detrending methods.The time-varying components of the planetary spectra then reveal compositions in the planetary atmospheres.Typical constituents expected in hot Jupiters' atmospheres include major oxygenand carbon-bearing species such as H 2 O, CH 4 , CO, and CO 2 which are most easily detected in the near-IR and IR wavelengths.In the visible wavelengths, various key heavy elements, including Si, Ti, V, and Fe, have rich spectral features and have been observed in atmospheres of a dozen hot Jupiters(e.g., Yan et al., 2022).These refractory elements offer a valuable window to probe the formation and migration history of hot Jupiters (Lothringer et al., 2021).
The high-resolution spectroscopy has also been applied to the atmospheric characterization of directly imaged exoplanets, i.e., giant planets that are hot, self-luminous, and with large orbital separations.Key molecules including CO and H 2 O have been identified in several directly imaged exoplanets; isotopes are also within reach for some of the best targets (Currie et al., 2023).In addition to composition measurements, the rotationally induced spectral line shapes allow us to determine the rotation periods of directly imaged exoplanets, an important piece of information for tracking how planets accreted their angular momentum when they grew within the disk (Snellen et al., 2014).
The ERS in the upgraded phase of JUST instrumentation should be able to carry out spectroscopic surveys of dozens of hot Jupiters, yielding statistical trends of metallicity and carbon-to-oxygen ratios for the hot Jupiter population.Equipped with extreme adaptive optics, we expect to characterize several directly imaged exoplanets and measure their atmospheric chemical inventories and spin states.

Summary
JUST is a 4.4-meter telescope equipped with a segmented primary mirror and a lightweight framework, allowing for reduced construction costs and rapid switching between observation targets.It features two Nasmyth foci, each offering a field of view of 10 arcmin and 1.2 degree, with the ability to alternate between them by rotating the tertiary mirror(M3).The telescope also boasts three types of spectrographs: a multiple-fiber medium-resolution spectrometer, an IFU array and/or a long-slit spectrograph, and a multiple-fiber high-resolution spectrometer.
JUST will be installed and operated at a high-quality site with an altitude of 4322 meters on

Fig. 1 :
Fig. 1: Bird's-eye view of Saishiteng Mountain.The largest dome at Position C (at an altitude of 4200 m)

Fig. 2 :
Fig. 2: The conceptual design of the dome for JUST.

Fig. 3 :
Fig. 3: The conceptual design of the structure of JUST.

Fig. 4 :
Fig. 4: Optical design of JUST.Left: Nasmyth focus 1 with field of view of 10 ′ ; Right: Nasmyth focus 2 with an extended field of view of 1.2 • .

Fig. 5 :
Fig. 5: Configuration of the segments of M1, showing 18 hexagonal segments.The central area of the primary mirror is vacant, where M3 will be installed.

Fig. 6 :
Fig. 6: Illustration of the JUST-SCS program.Panel(a): Distribution of the photometric galaxies within a simulated lightcone.Galaxies from the same cluster are dispersed over a large line-of-sight distance due to photo-z uncertainties.Panel(b): Distribution of the spectroscopic clusters (red circles) that will be observed by the JUST cluster cosmology survey within the same lightone.Panel(c): A cluster at z = 0.1 within the JUST field of view (white circle) targeted by the JUST cluster galaxy evolution survey.The Background image is Abell 1689.Panel(d): The cosmic web structure centered on a cluster at z≃0.3 targeted by the JUST cluster infall survey.The background image is from the Millennium Simulation.
photometric observations, the spectroscopic observation of transient sources is still insufficient.With the development of more photometric surveys, this difference will only become more severe in the future.JUST, with hundreds of optical fibers, can effectively carry out spectroscopic observations of a large number of transient sources by assigning each target with a fiber.It will also provide information on the two-dimensional kinematics and chemical properties of the host galaxy of the transient source with the fibers forming an IFU array, providing first-hand data for studying its triggering environment mechanism.

Saishiteng
Mountain in Lenghu town, Qinghai province.Expected to achieve first light in 2026, it is poised to become the most powerful telescope for spectroscopic observations in China for a considerable period.Upon completion, JUST will focus on research in three main directions:(1) Exploring the dark universe through spectroscopic surveys of numerous galaxies in the cosmic web;(2) Tracking the dynamic universe by conducting follow-up spectroscopic observations of various transient sources; (3) Detecting and characterizing exoplanets through the acquisition of high-resolution stellar spectra and the precise

Table 2 :
Key parameters of three types of spectrographs.