## 1 Introduction  
A sparse-aperture imaging system collects beams through multiple small-diameter subapertures, and the beams interfere with each other to achieve high-resolution imaging, thus obtaining a spatial resolution equivalent to that of a large-aperture optical system [1]. However, due to the discrete subapertures and the complex light path structure, the alignment of a sparse-aperture system needs to be conducted at a nanoscale, which brings great challenges to the beam adjustment structure design and the specific alignment [2].  
The Golay3 array structure was first proposed by [PERSON] in 1970 [3]. In the actual system development, the detection and correction of the phase error between subapertures are important for optical synthetic-aperture telescope systems that have separate mirror structures. One of the important reasons why the six-subaperture synthetic telescope--MMT (Multiple Mirror Telescope) established by the University of Alexandria in 1978--was forced to be replaced with a single-aperture telescope was that the cophasing problem was not being well addressed [4]. A three-aperture test platform Phasar was developed by the Air Force Research Laboratory in 1988. Each sub-telescope in the system has an optical path difference adjuster (OPDA) consisting of a fast steering mirror, and each OPDA has four actuators for the correction of the two-dimensional tilt error and piston error. The four actuators are driven by the signals of the wavefront piston and the tilt sensor [5], enabling the system to realize the cophasing control between the subapertures. However, the system structure is too complex. In 2004, Lockheed Martin developed a novel and adaptable optical telescope comprising an array of nine identical afocal sub-telescopes, which was named the multiple instrument distributed aperture sensor (MIDAS). The light path of each sub-telescope, which consists of five plane mirrors, is an integrated component. A retro-reflecting roof mirror is used to precisely control the optical path length without introducing beam tilt, and can be used to correct small image rotation errors [6; 7]. This scheme contains many sub-mirrors, as the processes of beam alignment and cophasing error adjustment are complicated. [PERSON] et al., from the Rochester Institute of Technology, improved the sparse-aperture design framework by the genetic aperture method in 2015 [8; 9]. They experimented with a 4f optical system with a sparse-aperture mask plate and introduced the wavefront error into the system, conducting a lot of research on imaging quality [10]. They did not, however, make independent cophasing error adjustments for each aperture. It can be seen that a stable and reliable phase adjustment device is critical to the coefficient aperture system, and the extremely complex design will make it difficult to adjust and control [11]. According to the existing report, it is found that the structure design of the beam control scheme is either too complicated or does not consider the adjustment function. Therefore, it is necessary to design a concise, stable, and practical cophasing error control structure.  
Considering that the sparse-aperture imaging system realizes simultaneous imaging by steering in multiple light paths, this paper systematically analyzes the core component--beam steering structure. An integrated design has been achieved for steering and phase control. The mechanical and optical structure and properties are optimized and realize an efficient and stable beam steering and adjustment structure.