Development and testing of glass substrate Wolter-1 X-ray focusing mirror

Abstract

The wolter-1 X-ray focusing mirror can reflect grazing incidence X-ray to the focal plane, which plays an important role in the astronomical detection and other fields due to its good image detecting capability. A geometric model of the optical system is established for theoretically deriving the optical path equations which is useful in this glass based focusing mirror designing, all the design parameters of the focusing mirror can be obtained by solving these equations. In the manufacturing process, the D263T glass is chosen to be the structural material of the focusing mirror due to its light weight and super smooth surface, after a slumping process, the flat glass mirror will have the shape of Wolter-1 X-ray focusing mirror. This slumping process has been used successfully in the manufacturing process of an American mission named The Nuclear Spectroscopic Telescope Array, which was launched in 2012. According to X-ray reflecting theory, the reflectivity of the Wolter-1 mirror can be improved significantly by coating metal film on the surface of the mirror. In this work, an iridium film is coated on the surface of the glass mirror through a vacuum evaporating process. In order to learn the influence of the focal spot caused by the mirror shape tolerance, the morphology of the mirror is tested by using a 3-D laser scan instrument. The results show that 50% of the total test points are located in the tolerance range of-10-10 m, in which the tolerance represents the difference between the actual lens profile and the ideal lens profile. Then the focal spot test is carried out with the help of a visible light test system:a laser collimator is installed in front of focusing mirror as an incidence light source, and a charge coupled device (CCD) is placed in the focal plane to gather the image of the focal spot, by calculating the gray level distribution of the focal spot image taken by the CCD, the energy distribution characteristic of focal spot can be obtained. The experimental results show that the focal length of the focusing mirror is 1.6 m, and the half-power surrounding diameter of the focal spot is 0.33 mm, corresponding to the angular resolution of 0.7 arc min.