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In the present study, we proposed a new type of autocollimator for high-accuracy angular measurement within a large angle range. The new system comprises a traditional autocollimator and Risley prisms, and it employs the normal tracing method to measure the angle. By rotating the Risley prisms, the outgoing beam of the autocollimator can be deflected close to the normal direction of the reflecting mirror and then reflected back to the system by the mirror along the near normal direction to realize normal tracing. Based on the angle measured by the autocollimator and the rotation angles of Risley prisms, we can calculate the tilt angle of the mirror. Since the beam returns to the system close to the original path, the angle error caused by aberration, optical component processing defects, nonuniform refractive index, and so on, can be ignored. Due to the normal tracing measurement method, theoretically, the angle error is not affected by the working distance. ZEMAX non-sequential simulation shows that the angle error caused by aberration in the new system can be significantly reduced.

Abstract-The dust deposition significantly influences the energy gained from the solar power system. The dust deposition of PV modules is decided many factors, such as weather, location, tilt angle, and so on. This paper focuses on the influence factors of the PV modules. In order to realize the relationship between the dust deposition and the sunlight transmittance of the solar PV module, improved models considering the effect of incident angle and tilt angle are developed based on the overlay model. Then simulation with the improved models is conducted to analyze the characteristics of each model. Index Terms-Dust deposition, transmittance, incident angle, tilt angle. I. INTRODUCTIONAs the core component of the solar power system, the solar PV panel is long-running, so the effect of the dust deposition on the system efficiency is significant. The dust deposition of PV module directly leads to the reduction of the solar radiation received and the decrease of the energy gained from the solar power system. The sources of the dust including the tiny particle formed by the action of sand, soil and wind, the blowing dust due to the industry, building, and transportation, and the biomass of animals and plants [1]- [3]. The longer the solar power system is running, the more the dust deposition will be, therefore, the lower transmittance and the less generated energy is caused. With the other conditions being equal, the smaller tilt angle of the PV module leads to the more dust deposition and the less solar radiation received.Although the effect of dust on a single PV cell is noteless, the power loss owning to the dust of the large-scale PV system is great [4]. The research found that in the dry period, efficiency loss resulted from the dust deposition was as much as 15%, and the average annual power generation efficiency reduced 6%.In recent years, more and more researchers pay attention to the influence of the dust deposition on the efficiency of the PV module. Most scholars exposed the PV module in different environments, analyzed the effect of the dust deposition by the comparison of experiment results [5]- [7].

Water entry problems are very common in engineering and sciences. When objects move with relatively high speed, bubble cavities will be generated, and the behavior of moving objects will also be affected conversely. In this paper, the water entry problems are studied using smoothed particle hydrodynamics (SPH) method, which has special advantages in modeling free surfaces, moving interfaces. First, an improved fluid–solid interface treatment algorithm is presented, whose effectiveness is validated by a water entry of a buoyant cylinder. Then the water entry with different velocities and directions are researched. It is found that the velocities and angles of the moving objects will affect the movement of the object greatly, and the SPH model can give optimal predication of these corresponding conditions.

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