LAMOST - Optics


The optics of LAMOST comprise two roughly rectangular mirrors, each made up of a number of 1.1-metre hexagonal deformable segments, providing a focal plane 1.75 metres in diameter corresponding to a five-degree field of view. This focal plane is tiled with fibre-positioning units, attached to 4000 fibres which transfer light to sixteen spectrographs below. The larger spherical mirror MB (37 segments, fitting in a 6.67m x 6.09m rectangle) is located at an angle in a large slanted tunnel attached to a tower; the smaller corrector mirror MA (24 segments, fitting in a 5.72m x 4.4m rectangle) is in a dome at ground level. Looking at the image opposite, MB is at the top of the left-hand supporting column of the tower, MA is in the left of the two domes at the right of the image (the rightmost, grey dome is an unrelated telescope), and the spectrographs are inside the right-hand column of the tower.

Each spectrograph has two 4k x 4k CCD cameras, using e2v CCD chips, with 'blue' (370-590 nm) and 'red' (570-900 nm) sides; the telescope can also be used in a higher spectral resolution mode where the range is 510-540 and 830-890 nm.

Using active optics technique to control its reflecting corrector makes it a unique astronomical instrument in combining large aperture with wide field of view. The available large focal plane may accommodate up to thousands of fibers, by which the collected light of distant and faint celestial objects down to 20.5 magnitude is fed into the spectrographs, promising a very high spectrum acquiring rate of ten-thousands of spectra per night.

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