The most critical data in astrophysical research is the spectrum, and many key issues in contemporary astrophysics require extensive spectral observations for resolution. Over the years, the international astronomical community has made significant progress in panoramic spectral observations, with notable projects such as Australia's 2dFGRS (Two-Degree Field Galaxy Redshift Survey) and the United States' SDSS (Sloan Digital Sky Survey).

These projects, which simultaneously observe the spectra of hundreds of celestial objects, have produced remarkable scientific achievements in the study of large-scale cosmic structures, galaxy physics, and quasars.

In 2009, China completed its National Major Scientific Infrastructure, the LAMOST (Large Sky Area Multi-Object Fiber Spectroscopic Telescope, also named the "Guo Shoujing Telescope"), which began operational spectral surveys in 2011. LAMOST pioneered large-scale (simultaneous observation of thousands of celestial objects) spectral surveys on an international scale. By March 2024, LAMOST had released a total of 25.12 million spectral datasets, surpassing the cumulative spectra from other international survey telescopes by a factor of 2.2.

This achievement has positioned Chinese astronomy at the forefront of research in the Milky Way and stellar studies. Inspired by the success of LAMOST, international projects such as American Spectral Survey Project DESI (Dark Energy Spectroscopic Instrument), Japanese Spectral Survey Project PFS (Prime Focus Spectrograph), and American SDSS-V (Sloan Digital Sky Survey V) have adopted LAMOST's innovative fiber positioning method, initiating or preparing to launch large-scale (thousands of objects simultaneously) spectral surveys.

Currently, advanced photometric survey telescopes such as the international Large Synoptic Survey Telescope (LSST, 8.4-meter, multi-color photometry), the 1.2-meter space-based Euclid telescope, and the 2.4-meter space-based Roman telescope, are providing extensive new spectral target catalogs with their deep imaging capabilities. This lays the foundation for the next-generation of extremely large-scale optical spectroscopy survey projects. 

Large-scale spectral surveys are increasingly gaining attention on the international stage. In 2014, the European Southern Observatory conducted a survey involving thousands of astronomers, which revealed that large-scale spectral surveys are the most pressing astronomical requirement at present. Currently, countries such as the United States and Europe are proposing the development of 10-meter-class telescopes for extremely large-scale spectral surveys.

The development of extremely large spectral survey telescopes, in conjunction with multi-wavelength, wide-field imaging survey instruments, represents a significant opportunity for China (and the international community) to achieve significant astronomical research accomplishments and advancements in cutting-edge astronomical technologies.

The optical system of conventional spectral survey telescopes consists of two components: reflective and refractive systems working together to correct for image aberrations and atmospheric dispersion. However, the limited aperture of current lenses restricts the aperture and field of view of spectral survey telescopes.

To address this, we propose an innovative concept for a spectral survey telescope: separating the reflective and refractive systems. The proposed design initializes an excellent-quality pure reflective system based on the SYZ relay mirror for telescope with three aspherical miirror independently innovated by Chinese scientists. Additionally, this design incorporates a novel strip-shaped prismatic splice atmospheric dispersion corrector (S-ADC), invented by Su Dingqiang in 2011, which overcomes the limitation of optical glass size on telescope aperture while retaining all the advantages of reflective optical system imaging.

This leads to the design of the next-generation extremely large-scale optical spectroscopy survey telescope, the Extremely Large Survey Telescope (ESST), whose survey capabilities surpass those of its international peers. The ESST also features a superior Cassegrain system, enabling simultaneous large-scale, high-precision Integral Field Unit (IFU) spectroscopic observations.

Upon completion, the ESST will serve as a powerful observational tool for numerous international astronomical frontier fields, ensuring China's continued leadership in extremely large-scale spectral surveys and securing a higher position in the scientific landscape. Relevant studies have been featured as cover articles in SCIENCE CHINA Physics, Mechanics & Astronomy and are included in the Proceedings of SPIE. This research is supported by the National Key R&D Program of China (Grant No. 2022YFA1603001).

Figure 1. Two configurations of the extremely large -scale spectroscopic survey telescope

Figure 2.  Preliminary mount design of  Extremely Large-scale Spectroscopic Telescope

Figure 3.  The ESST Conceptual scheme was selected as the cover paper for SCIENCE CHINA Physics, Mechanics & Astronomy

DOI: https://doi.org/10.1007/s11433-023-2316-3

DOI: https://doi.org/10.1117/12.3021539