## 1 Introduction  
Solar energy is a clean and environmental friendly renewable energy source, which is plentiful and can be widely distributed. Developing and utilizing solar energy is an important way to solve energy shortages and environmental pollution problems [1; 2; 3]. The dish-Stirling concentrated solar power system (DS-CSP system) focuses sunlight onto a metallic coil surface in a heat absorber via a parabolic dish concentrator and heats a working medium (usually hydrogen or helium gas) in a metal coil, which then drives a Stirling engine in order to generate electricity [2; 3]. The DS-CSP system has the advantages of a high solar-to-electricity conversion efficiency (the record is 31.25%), flexible arrangement, and high modularity, and is thus considered a high-grade solar thermal utilization system with wide application prospects in the future [3; 4; 5]. In practice, large dish concentrators are usually assembled with many mirror units forming the parabolic reflecting mirrors and each mirror unit must be accurately fixed in a preset position within the truss structure [6; 7; 8; 9; 10]. The fixation or facet adjustment is realized by three support-adjustment structures located on the posterior side of the mirror unit (Figure 1c). If facet alignment error exists after the mirror unit is installed, the shape accuracy of the parabolic concentrator surface can be affected. This may lead to problems such as a non-uniform flux distribution on the metal coil surface, which can cause unsteady motion of the multi-cylinder engine [11; 12]. This could also lead to the focal spot being moved outside the absorber, which can cause optical intercept losses as well as the formation of \"hot spots\" on the metal coil surface leading to erosion or damage of the metal tube. As such, these problems can directly affect the economics and safety performance of the DS-CSP system during operation. Correctly aligning the mirror units in the dish concentrator while minimizing installment error is therefore of great importance to overall performance [6]. The development of an effective mirror alignment method or alignment tool to guide mirror facet alignment during operation would be extremely valuable. The current methods of the mirror facet alignment and mirror slope error measurement in the solar concentrator field mainly include laser-scanning, reflection imaging, and photogrammetry methods [13; 14; 15]. Laser scanning is derived from a ray tracking method based on the mirror reflection law. A set of reflection mirror detection devices based on Video Scanning Hartmann Optical Test (VSHOT) were previously developed to test the mirror slope error of a dish concentrator [16; 17]. The VSHOT is a laser device used to measure the slope of the reflector directly at many positions on the concentrator mirror surface. [PERSON] and [PERSON] [18] developed a set of optical profilometers based on a He-Ne laser device for measuring the slope error of a trough concentrator. While effective measurements have been made using the scanning process, the method is very time-consuming and difficult to implement in \"real-time\", unable to real-time instruct the mirror facet alignment. Figure 1: (**a**) Photograph of the 38 kW dish-Stirling solar thermal power system (38 kW XEM-Dish System), (**b**) Image of the posterior side of the mirror unit, (**c**) image of the support-adjust structure on the back of the mirror unit, and (**d**) schematic of the assembly model of the support-adjust structure. Using the reflection imaging method, [PERSON] et al. [19] developed a mirror alignment tool with an artificial light source to guide the mirror facet alignment in a multi-dish concentrator (CPG-460). Alignment was accomplished by adjusting the facet aim such that the reflected image falls on a predetermined location. [PERSON] et al. [20] and [PERSON] et al.