The Large Sky Area Multi-Object Fiber Spectroscopic Telescope

    As a result of many years' efforts, Chinese astronomical community has laid down a solid astronomical observational foundation, which is figured by the 2.16-m and 1.56-m optical telescopes, the 1.2-m infrared telescope, the solar magnetic field and multi-channel telescope, the 13.7-m millimeter wave radio telescope, the meter wave aperture synthesis array and the VLBI stations. It widened and deepened astronomical researches in this country, and raised the standing of Chinese astronomy in the world. For further progress, Chinese astronomical community, analyzing the developing tendency of contemporary astronomy and astrophysics, and the status quo of Chinese astronomy, in consideration of the needs and possibility of the present social development in China, proposed the "Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST)" as one big-science project in the Ninth Five-Year Plan period (1996-2000). This project aims at the wide field astronomy and astrophysics, and seizes the valuable opportunity to open up the optical spectroscopic observation in large scale, it realizes a breakthrough in combining large aperture with wide field of view in optical telescope by using original concepts and ingenious design. This new telescope will bring Chinese astronomy into 21st century with a leading role in wide field spectroscopy, and in the field of large scale and large sample astronomy and astrophysics, whether extra-galactic or galactic. This project was proposed by a research group headed by Wang Shouguan and Su Dingqiang, members of the Chinese Academy of Sciences (CAS), and was supported widely by astronomical community. A formal proposal for the project submitted by CAS has been listed, after repeated reviews, into "The National Mega-Science Facilities Program" in 1996. The proposal was approved by State Planning Commission (SPC) in April, 1997, and the feasibility study was approved by SPC in August, 1997. The preliminary design is under preparation at present time. LAMOST is a meridian reflecting Schmidt telescope laid down on the ground with its optical axis fixed in the meridian plane, as shown in the figure. It consists of a reflecting Schmidt corrector MA at the northern end, a spherical primary mirror MB at the southern end and a focal plane in between. Both the primary mirror and the focal plane are fixed on their ground bases, and the reflecting corrector, as a coelostat, tracks the motion of celestial objects. Celestial objects are observed around their meridian passages. The light collected is reflected from MA to MB, then reflected by MB and forms image of the observed sky on the focal plane. The light of individual objects is fed into the front ends of optical fibers accurately positioned on the focal plane, and then transferred into the spectrographs fixed in the room underneath, to be dispersed into spectra and recorded on the CCD detectors, respectively and simultaneously. The quasi-meridian layout with both the primary mirror and the focal plane fixed, while only the reflecting corrector moving, makes the long focal length possible, which is needed by increasing the aperture of a Schmidt telescope. Its long tube will not be needed to rotate for tracking, simplifying the design and construction of the spherical primary mirror and its support structure, which is the biggest part of the telescope. Besides, the large focal plane connected with many optical fibers will not be needed to move with the tube also, as in the case of conventional telescopes. The alt-azimuth mounting for the reflecting corrector reduces the structure and the enclosure as well. The southern part of the telescope is higher than the other end, with its optical axis tilted by an angle of 25o to the horizon for the sky coverage. The declination of observable sky area ranges from -10o to +90o. The primary mirror has a size of 6.67m x 6.05m with a radius of curvature of 40m, which consists of 37 hexagonal spherical submirrors, each of them having a diagonal of 1.1m and a thickness of 75mm. The reflecting corrector is located at the center of curvature of the primary mirror, and its size is 5.72m x 4.40m, which consists of 24 hexagonal plane submirrors, each of them having a diagonal of 1.1m and a thickness of 25mm. The clear aperture of the telescope is around 4m that would become a little larger or smaller depending on the declination of the sky to be observed. Its focal length is 20m, with the focal ratio of 5 correspondingly. The active optics technique for segmented thin mirrors will be applied to the reflecting Schmidt corrector. Combining both the thin mirror and segmented mirror active optics, it not only controls the aspherical shape of the corrector to correct the spherical aberration of the primary mirror, but also controls the co-focus of all 24 submirrors. This system gives excellent image quality within the 5o field of view, with the largest image of 1.77 arcseconds only on the edge of the field of view. The large focal plane of 1.75m in diameter, corresponding to the 5o field of view, may easily accommodate up to 4000 optical fibers. The fiber positioning mechanism is to place these optical fibers precisely to their pre-determined positions on the focal plane in a relatively short time. The parallel controllable fiber positioning promises simultaneously moving the fibers in their own small regions and fine adjusting. This technique will break the present world record of positioning simultaneously 640 optical fibers. For a conventional Schmidt telescope with a wide field of view, it is very difficult to make its transmission corrector large, meanwhile for a reflecting telescope with a large aperture, its field of view is small. With the new concepts and design, LAMOST is expected to be a unique astronomical instrument in combining a large clear aperture and a wide field of view. Since its 4m aperture enables it to obtain the spectra of faint celestial objects down to 20m.5 with 1nm spectral resolution in 1.5 hours exposure, and its 1.75m focal plane corresponding to the 5o field of view can accommodate several thousands of optical fibers, it could obtain 4,000 spectra of celestial objects simultaneously, becoming the telescope of the highest spectrum acquiring rate of several ten-thousands of spectra per night. The optical spectrum contains abundant physical information of distant celestial objects, and acquiring spectra of a large number of celestial objects is desperately needed in astronomy, which touches various cutting-edge researches of contemporary astronomy and astrophysics. However, among tens of billions various celestial objects recorded by imaging survey, only a very small part (about one of ten thousands) has been observed spectroscopically. As the telescope of the highest spectrum acquiring rate, LAMOST will break through the "bottleneck" of spectroscopic observation in astronomy, becoming the most powerful spectroscopic survey telescope for researches of wide field and large sample astronomy. The spectroscopic survey carried out by LAMOST of tens of millions of galaxies and others will make substantial contribution to the study of extra-galactic astrophysics and cosmology, such as galaxies, quasars and the large-scale structure of the universe. Its spectroscopic survey of large number of stars will make substantial contribution to the study of stellar astrophysics and the Galaxy. Its spectroscopic survey combining with the surveys in other wavebands, such as radio, infrared, X-ray and (-ray will make substantial contribution to the cross-identification of multi-waveband of celestial objects. The engineering of LAMOST consists of seven subsystems, that is, optical system, active optics and support system, mounting and tracking system, telescope control system, focal plane instruments, data processing and computer integration, and telescope enclosure. The telescope will be located at the Xinglong station of Beijing Astronomical Observatory, CAS. As the national facilities, LAMOST will be opened to the whole Chinese astronomical community. The investment for the project is RMB 235 million yuan. Its construction period is 7 years, coming into operation at the end of 2004 .