LAMOST / Guoshoujing Telescope
Figure 1: Xinglong Map
Map showing the location of Xinglong Observing Station, the LAMOST site, in northern China. Xinglong is about 100 miles northeast of Beijing.
The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) is a National Major Scientific Project undertaken by the Chinese Academy of Science. The telescope, also known as the Guoshoujing Telescope after a 13th-14th century Chinese scientist, is located at Xinglong Observing Station of the National Astronomical Observatories, about 100 miles northeast of Beijing, China (see the map in Figure 1). The large, 4-meter (about 13-foot) mirror of LAMOST enables it to obtain spectra of faint objects. LAMOST is designed with 4000 optical fibers in the optical path, covering a large region of the sky simultaneously. This unique system will be used to conduct a survey of 10 million stars in our Galaxy, as well as millions of distant galaxies, over a 5-year period.
Details About the Telescope:
Figure 2: Lamost Facility
The buildings that house the LAMOST telescope. The entire structure is fixed along the north-south direction (known as the "meridian"). The domed structure on the right contains the movable mirror, which can be pointed to positions on the sky within about 30 degrees of the meridian. Light from this mirror is sent up the long tube to another mirror, then reflected and focused back down the tube to the array of fibers. The light that enters the fibers in the long tube is then sent down long fiber-optic cables to spectrographs in a room inside one of the towers supporting the large tube. A schematic showing this light path is seen in Figure 3. More images of the telescope and Xinglong Station can be found here.
Unlike most telescopes that can be moved to point at nearly any position on the sky, LAMOST is built in a fixed structure aligned along the north-south direction (known as the "meridian"). The telescope can still observe most of the northern sky, but must capture objects as they pass near the meridian.The structure seen in Figure 2 houses the LAMOST telescope. The domed building on the right contains the mirror that can be pointed to capture the light of objects within roughly 30 degrees on the sky of the meridian. In the diagram of the LAMOST optical system shown in Figure 3, this sky-pointing mirror is notated as "Mirror A" (or MA). This mirror, seen in Figure 4, is roughly 4 meters (a little over 13 feet) across, allowing LAMOST to detect and measure rather faint stars and galaxies. Light from Mirror A is directed up the long tube-like structure, seen in Figures 1 and 3, to a second mirror.
Figure 3: LAMOST Schematic
Schematic showing the main optical systems in the LAMOST telescope, with the red arrows showing the path of light through the system. On the bottom right is "Mirror A" (labeled MA in the figure; see an image of MA in Figure 4), a steerable mirror that can be positioned to intercept light from the region of sky to be observed. From this mirror, light is reflected up the long tube toward "Mirror B" (MB in the figure). Mirror B focuses the light back down the tube to the focal plane, where 4000 optical fibers are located (an image of the focal plane and its 4000 fibers is seen in Figure 5). These fibers can be robotically positioned to the known locations of stars and galaxies, and send their light down to the spectrographs in lower floors of the building. The spectrographs are finely-tuned instruments that disperse the incoming light and record the resulting spectrum for the objects whose light passes down each of the 4000 fibers.
The second mirror in the light path (MB in Figure 3) is a spherical mirror that focuses the light back down the tube onto what is known as the "focal plane" (essentially the "plane" where light comes to focus). Both Mirror A and Mirror B are made up of multiple smaller hexagon-shaped mirrors. These smaller mirror segments are adjusted to correct out the distortions of stars' images introduced by Earth's atmosphere. This is done via actuators attached to the back of the mirrors that rapidly change the shapes of the mirrors in response to corrections found by observing a bright star and determining what is needed to re-form the image of that star to its optimal shape. Such a process, known as "active optics", is done in real time as the telescope is observing, and provides dramatic improvements to image quality.
Figure 4: "Mirror A"
"Mirror A", the first mirror in the optical path of LAMOST, is actually made up of smaller hexagonal mirrors butted against each other. This mirror resides in the domed structure seen in Figures 2 and 3. Notice the size of the mirror relative to the person on the catwalk at left -- the large mirror allows LAMOST to gather enough light to detect faint stars and galaxies.
Located in the focal plane (labeled in Figure 3) is an array of 4000 optical fibers, each of which can be moved robotically to the known positions of stars or galaxies in the area where the telescope is pointed. An image of the focal plane and the ends of its 4000 fiber optic cables is seen in Figure 5. This array of fibers is 1.75 meters (about 5 feet 9 inches) across, corresponding to an area on the sky 5 degrees across. This large area on the sky (10 times the diameter of the full moon) populated with 4000 fibers allows LAMOST to survey the sky very efficiently. The starlight is passed down the optical fibers to a carefully-controlled room on a lower floor of the building housing the detectors. The light from the 4000 optical fibers is fed to 16 different spectrographs, which record the information about each of the 4000 stars that were targeted. (More details about spectroscopy can be found under the "LEGUE Science" tab of this website.) By taking 4000 spectra at a time, LAMOST can thus obtain spectra of more than 10000 stars per night, and more than 2 million stars per year.