The fiber positioning system on the focal plane is an important part of the LAMOST. All of 4000 fibers are required to be positioned on 5 degrees focus surface precisely and quickly to right positions. The main technical requirements of LAMOST fiber positioning system are described as follows:

1. The 2-D positioning error of any among the 4000 fibers is less than 40m (respect to 0.4arcsec in focal surface).

2. The time needed for positioning all the 4000 fibers once is shorter than 10 minutes.

3. During observation, errors can be compensated.

The convex focal plate of the telescope (with the diameter of 1.75m) is divided into about 4000 individual domains. Each domain contains a controllable unit which can move a fiber quickly anywhere within its designated circular area on the convex focal surface. A mobile parallel control system is set to control all the units, so the 4000 fibers can move separately. In this system positioning errors can be compensated at real time, and prepare time of each observation is shorten to several minutes.

The fiber positioning system consists of a base plate (the focal plate), 4000 fiber positioning mechanical units, electronic controlling system and a fiber position measuring equipment. 4000 fiber positioning units are accommodated on the focal surface with a 25.6mm distance between each others. Each unit drives a fiber moving in a circle of 33mm in diameter. The unit is driven by two step motors in double rotation movement, thus there are 8000 step motors in LAMOST. With wireless control technology, influence on focal plane tracking system by thousands wires in tons of 8000 step motors was avoided. The positioning error is 40 micron maximum. It is an innovation by LAMOST.

Figure1 shows the focal plane with 4000 fiber positioning units

Figure 2 shows the fiber positioning units