01��. A sensor composed of a commercial plug-and-play webcam and a polarized filter is presented in [15]. It proved that the image-based Sun position sensor has high immunity to different weather conditions and achieved a tracking accuracy of 0.1��.The stability of the solar tracking system is a key factor to obtain the maximum electric power from a PV system. We have developed an image-based Sun position sensor to increase the stability and accuracy of Sun-tracking. The image-based Sun position sensor consists of a self-design reflecting Cassegrain telescope and webcam. The reflecting telescope can enlarge and adjust Sun pictures to an adequate size to achieve optimum tracking accuracy and view angle. This article describes the development of an image-based Sun position sensor and the algorithm for how to point at the Sun precisely by using image processing.

The paper is organized as follows: In Section 2, the design and analysis of a reflecting Cassegrain telescope are described. In Section 3, the control strategies of Sun-tracking are examined. In Section 4, the experimental results that prove the good performance of our image-based tracking system are presented. Finally, some conclusions are offered in Section 5.2.?Design and Analysis of a Reflecting Cassegrain TelescopeObtaining a clear photo with a large enough Sun image is the first and determining step for accurately estimating the solar center. If the Sun image taken in the photo is too small, due to the digitizing effects, it may introduce uncharacteristic noise and errors to the image that become difficult to filter out to estimate the solar center.

A reflecting type Cassegrain telescope [16,17] can reflect and enlarge Sun images. To weaken the Sun luminosity and shorten the telescope length, we put in a right angle prism to change the light direction and Brefeldin_A allocate a suitable eyepiece to get an enlarged Sun image and also a clear outline. We thus developed a Cassegrain type telescope with a right angle and eyepiece as shown in Figure 1.Figure 1.A reflecting type Cassegrain telescope�Coptical structure.In the plot, there are two concave mirrors and a right prism at the right side, and a convex mirror at the left side, as well as an eyepiece above the right prism. The two concave mirrors reflect light to convex mirror, and then the light reflected by the convex mirror goes to the right prism.

Subsequently, the light is refracted to the eyepiece. Finally, the light goes through the eyepiece and focuses on somewhere above the eyepiece.The OSLO? tool [18] is used to analyze the characteristics of the self-designed telescope. In assessing performance of the telescope, as shown in Figure 2, modulation transfer functions (MTF) give an idea about how much contrast a telescope design will be able to show [19].Figure 2.MTF analysis of a reflecting type Cassegrain telescope.