Research On Measurement And Evaluation Method Of Integral Field Unit's Focal Ratio

With the continuous progress of astronomical technology,the three-dimensional imaging technology based on integral field unit(IFU)has gradually become the mainstream technology,which is used by astronomical researchers in the production of astronomical telescopes.This technique can improve the observation efficiency and spatial resolution of the telescope.As the core component,the performance of IFU directly affects the subsequent spectral analysis and then affects the quality of the whole telescope.In this paper,using the IFU prepared before,we studied the test method of optical fiber focal ratio degradation in IFU by simulation,modeling and experiment.In this paper,the design principle of the measured IFU is introduced.What is more,the transmission theory of step index and multimode fiber used in IFU is described from the view of geometric optics and physical optics respectively.On this basis,we further explored the phenomenon of fiber focal ratio degradation and the factors causing focal ratio degradation.Apart from that,the paper illustrates the CCD method of focal ratio test,which provides the theoretical basis for the subsequent focus ratio test system.Considering the difficulty and heavy workload of measuring the focal ratio of all optical fibers in IFU,we adopted optic transformation to obtain near-field spot image,which can test focal ratio.Firstly,the error of the transformation coefficient in the optical transformation method was analyzed.The Zemax software simulation was used to solve the transformation coefficient.The simulation results show that the change of the transformation coefficient of the optical system to the output of focus ratio can be considered as a constant,when the optical fiber is within the radial deviation range of –5?5 mm from the optical axis.According to the mismatch between the microlens array and the optical fiber array,the Zemax simulation results are obtained: the radial deviation has no effect on the output of focus ratio in the range of 9 ?m;the axial deviation does not affect the defocus ratio degradation under the actual fabrication process;the angle deviation synthesizes the three type mismatches,whose effect on the focal ratio degradation performance is most obvious.In view of the problem that the single fiber spot can not be analyzed because of the aliasing,the single fiber spot is successfully dissociated from the composite strip spot by using the image processing method,so that we can process the data of the single fiber spot and obtain the focal ratio of the corresponding fiber.In the absence of optical transformation,the far-field spot was analyzed by the method of spot energy field modeling to test focal ratio of IFU.Two-dimensional fitting of the energy distribution of the line in which the spot diameter is located was carried out by using the super gaussian model,the flattened gaussian model and the double-gaussian model.Moreover,the three-dimensional expansion of the gaussian model with order N=4 and the flattened gaussian model with order N=2 model were used to fit spot energy field distribution.The energy field superimposed by N fiber was used to simulate the strip spot of IFU by using the super gaussian model with relatively simple function form.On this basis,a curve with a fixed function was obtained by solving the energy ratio of the longitudinal center line in the EE90 diameter of each fiber spot.By using the standard obtained by simulation,the EE90 diameter of the strip spots of IFU was solved.What is more,the focal ratio degradation of each fiber was obtained by using the linear fitting method.