Relative position measuring for segmented mirrors of large telescope

In order to enhance photon collection and resolution of the telescope, the development of astrophysics needs to increase the size of the aperture. Comparing with the traditional systems, segmented mirrors structure used to develop the large telescope have many advantages. In order to make a surface constructed from a joint of segments emulate the optical performance of a single piece of glass, the segments must be individually oriented and positioned with a high degree of accuracy. In this paper, the relative position measuring methods of segmented mirrors are present. The traditional measuring ways are summarized, thinking of their limitation, a new measuring method which be called the edge sensors way is put forward. Some edge sensors which be used to measure the relative position data between segmented mirrors are placed on the segments, The relative position data of segments (piston P, tilt theta and phi) recorded by edge sensors are sent to the control algorithm which uses them to compute the piston and tilt errors. And the relationship equation between sensor data (delta) and freedom of segmented mirrors can be built according to the coordinates of mounting spot be transformed to the coordinates of fixed mirror.