Research About Polarization Measurement Of Primary Window For New Vacuum Solar Telescope

At present, one- meter infrared solar telescope(also called New Vacuum Solar Telescope, NVST for short) is the biggest solar telescope in C hina. Its main scientific goal is to get high-resolution vector magnetic field of the lower atmosphere for solar. In the process of magnetic field measuring, the telescope instrumental polarization will largely affect the accuracy of the measurement. For NVST, the vacuum window is one of the main source of instrumental polarization.To predict the polarization effect of vacuum window, t his paper introduces N VST instrumental polarization model and illustrates the great influence of vacuum window. Then, due to the simulation results of the temperature and strain birefringence effect on the vacuum, it shows that because of the different temperatures of the upper and lower surfaces, the main temperature gradient of window is parallel to the optical axis, and the deformation generally is an order of magnitude smaller than the vacuum stress. So, its contribution to the instrument polarization can be ignored; Vacuum stress and gravity mainly causes the signal crosstalk between Q and U, Q and V, U and V, the magnitude of Q and U crosstalk is 10-5, and the magnitude of Q and V crosstalk is around 10-4; and the primary instrumental polarization crosstalk comes from strain is between U and V, the maximum of the crosstalk is 1.28×10-2, and minimum is 0.43×10-2. And the crosstalk increases along with the decrease of the telescope altitude. By a reasonable approximating, the polarization property of vacuum window is equivalent to a rotated retarder. At 532.24 nm wavelength, the maximum rotation a ngle change of the fast optical axis from its original direction is 1.1225 °, and maximum delay angle is 0.7346 °. Finally after comparing different telescope instrument polarization measurements, a reasonable plan of measurement has been built.