Fringe test is the method which can detect the relative optical path difference in optical synthetic aperture telescope array.
To get to the interference fringes, the two beams of light in the meeting point must be within the coherence length. Step
scanning method is within its coherence length, selecting a specific step, changing one-way’s optical path of both by
changing position of micro displacement actuator. At the same time, every fringe pattern can be recorded. The process of
fringe patterns is from appearing to clear to disappearing. Firstly, a particular pixel is selected. Then, we keep tract of the
intensity of every picture in the same position. From the intensity change, the best position of relative optical path
difference can be made sure. The best position of relative optical path difference is also the position of the clearest fringe.
The wavelength of the infrared source is 1290nm and the bandwidth is 63.6nm. In this experiment, the coherence length
of infrared source is detected by cube reflection experiment. The coherence length is 30μm by data collection and data
processing, and that result of 30μm is less different from the 26μm of theoretical calculated. In order to further test the
relative optical path of optical synthetic aperture using step scanning method, the infrared source is placed into optical
route of optical synthesis aperture telescope double aperture. The precision position of actuator can be obtained when the
fringe is the clearest. By the experiment, we found that the actuating step affects the degree of precision of equivalent
optical path. The smaller step size, the more accurate position. But the smaller the step length, means that more steps
within the coherence length measurement and the longer time.