Jiao Tong University Spectroscopic Telescope (JUST) is a 4.8-m composite spectroscopic telescope which integrates the high-resolution observation in a small field of view (FoV) with the spectral survey in a wide FoV. The primary mirror of JUST adopts the latest segmented mirror active optics technology, which is composed of 18 hexagonal segments with a diameter of 1.1 meters each. There are two Nasmyth focal stations which can be switched by rotating the tertiary mirror. One station with a 10 arcmin FoV diameter and its image quality (full width at half maximum, FWHM) can be below 0.35 arcsec with the error budgets, which is suitable for installing high-resolution spectroscopic and imaging instruments. The other station is equipped with the wide-field correctors, which can achieve a FoV diameter of scientific 1.2 degrees and correct atmospheric dispersion. The wide-field Nasmyth focal station operates at a wavelength band ranging from 0.35 to 1.3μm and for the zenith distance ranging from 0 to 60 degrees, the image quality (FWHM) falls within a 0.75 arcsec diameter with error budgets. It is used for broad observations and mainly used for wide-field spectroscopic survey of stars, galaxies, etc. This article will describe the optical system design, the baffle design and stray light analysis, error budget, and thermal effect of the telescope in detail.
An innovative Chinese space mission, the Earth 2.0 (ET) mission, is being developed to combine the transit and microlensing method together to search for Earth-sized exoplanets in the Galaxy, including the most precious ones—Earth 2.0s, i.e., habitable Earth-sized (0.8-1.25 Earth radii) planets orbiting solar type stars, cold and free-floating low-mass planets. ET’s 6 transit telescopes will monitor a FoV of 500 square degrees (covering the Kepler field) continuously for at least four years and generate a huge database containing high-cadence and ultra-high photometry precision light curves of 1.2 million FGKM dwarfs. With such a high value database in hand, many unsolved issues in the exoplanet field and even stellar sciences will be well addressed. Besides looking for Earth 2.0s and constraining its occurrence rate, ET will be dedicated to map a much wider radius-period diagram of terrestrial-like exoplanets than ever and reveal how it depends on the stellar properties and environments. With the 4-yr legacy data of Kepler, ET will observe some planet systems for up to 8 years and catch additional components in a multi-planet system, e.g. cold Giant, cold sub-Earths, exomoons, exorings and even exocomets. Are exomoons and exocomets common in a planet system? What’s the favorite number of planets in a multi-planet system? What’s the most common orbital configuration of planet systems? With these new data, ET will deepen our understandings on how unique our Solar system is and how do multi-planet systems evolve. In addition to exoplanet sciences, ET’s time series data will also benefit the studies in asteroseismology, archeology in the Galaxy, time-domain astrophysics and black hole science.
A space mission called “Earth 2.0 (ET)” is being developed in China to address a few of fundamental questions in the exoplanet field: How frequently habitable Earth-like planets orbit solar type stars (Earth 2.0s)? How do terrestrial planets form and evolve? Where did floating planets come from? ET consists of six 30 cm diameter transit telescope systems with each field of view of 500 square degrees and one 35 cm diameter microlensing telescope with a field of view of 4 square degrees. The ET transit mode will monitor ~1.2M FGKM dwarfs in the original Kepler field and its neighboring fields continuously for four years while the microlensing mode monitors over 30M I< 20.6 stars in the Galactic bulge direction. ET will merge its photometry data with that from Kepler to increase the time baseline to 8 years. This enhances the transit signal-to-noise ratio, reduce false positives, and greatly increases the chance to discover Earth 2.0s. Simulations show that ET transit telescopes will be able to identify ~17 Earth 2.0s, about 4,900 Earth-sized terrestrial planets and about 29,000 new planets. In addition, ET will detect about 2,000 transit-timingvariation (TTV) planets and 700 of them will have mass and eccentricity measurements. The ET microlensing telescope will be able to identify over 1,000 microlensing planets. With simultaneous observations with the ground-based KMTNet telescopes, ET will be able to measure masses of over 300 microlensing planets and determine the mass distribution functions of free-floating planets and cold planets. ET will be operated at the Earth-Sun L2 orbit with a designed lifetime longer than 4 years.
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