It is a key problem to accurately calculate beam spots' center of measuring the warp by using a collimated laser. A new method, named double geometrical center method (DGCM), is put forward for the first time. In this method, a plane wave perpendicularly irradiates an aperture stop, and a charge couple device (CCD) is employed to receive the diffraction-beam spots, then the geometrical centers of the first and the second diffraction-beam spots are calculated respectively, and their mean value is regarded as the center of datum beam. In face of such adverse instances as laser intension distributing defectively, part of the image being saturated, this method can still work well. What's more, this method can detect whether an unacceptable error exits in the courses of image receiving, processing and calculating. The experimental results indicate the precision of this method is high.

A method for measuring focal length based on Talbot effect is put forward, which suits determining focal length of long-focus lens. The variation of Moiré fringes occurs after typical phase object of measuring lens is laid between two gratings. The images of fringes taken by CCD camera are processed with MATLAB. The focal length of measuring lens is figured out accurately after calculating the slope of fringes, the separation angle between two grating lines and measuring the distance between lens and the second grating. Experimental data indicate that, for a system with laser wavelength of 0.65μm, measuring lens wth focal length of 420mm, the measurement accuracy is 0.10%.So this method has advantages of simple device, fast processing speed and high precision. It satisfies focal length determination of long focal-length lens used in SG-III Facility.

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