Design Of Automatic Focusing System For Fast And High Precision Telescope

Focusing is the essential and fundamental part of the imaging optical system. With the technology improvement, focusing of the optical system is determined by digital imaging analysis system instead of visual comparison. Nowadays, there are many kinds of approaches to focus, such as contrast, phase method, ranging method and so on. But no matter what kind of focusing approach, optics researchers has been pursuing the goal which is the fast, accurate and stable focusing.Currently, the common focusing approaches are image focusing method and ranging focusing method. Image focusing method can improve the position accuracy of the object by the feedback information of its image quality. However, this approach has two defects, that is, the slow focusing speed and the defocus caused by the low contrast between the object and its background. And for ranging focus system, it has a quick focusing speed by the direct feedback ofthe amount of displacement of the focusing system, but because of the lack of the feedback of the image quality, it can not be self-correctted according to the image quality, so it has the relatively low focusing precision.Based on the merits and demerits of image focusing and ranging focusing and in light of the structural characteristics and optical characteristics of Earth Observation reflector telescope, this paper proposes an improved ranging focusing. Through the accurate simulation, the paper concludes the relations between focus position and distance to the target. At the basis of the conclusion,this paper comes up with the speed or displacement compensation approach in the closed-loop platform, aiming at the delay of the real-time focusing. This approach can not only improve the focusing accuracy, but also solve the problem of defocus that is due to lag in the dynamic focusing.First of all, this paper makes a brief introduction about different kinds of telescope optical systems and introduces the internal and external factors influencing the focusing error of the telescope focusing system in detail. Furthermore, through the analysis of finite object point confidence aberration, the paper points out the reasons of the existence of telescope focal depth, and focuses on the research of Cassegrain telescope’s structure, light path and focusing error.Secondly, this paper achieves the ZEMAX simulation analysis of the reflector telescope. Finding the correct search direction of positive focus is the indispensable step to realize highly precision ranging focusing, which requires us to know how to design a telescope and how to distinguish whether the imaging quality is good or not. With the help of ZEMAX software, we can find the positive focus position quickly, which allows us to implement the correction for the formula of positive focus. With the help of MATLAB simulation, we find the optimal off-axis angle and off-axis quantity of the off-axis telescope system in 2 D plane by using the analysis method of geometrical optics.Finally, we design the hardware platform of the automatic focusing system. Besides, we analyze the reason why the automatic focusing system lags behind and put forward the compensation method for the delay problem of ranging focusing. Also, we have measured the static error of the motor and verified the compensation effect by the closed-loop test.