Parallel Analogy Method for Measuring FRD of the FASOT-IFU

Abstract A fiber integral field unit (IFU) was chosen for the Fiber Array Solar Optical Telescope (FASOT) to obtain high-spectral-resolution polarization solar spectral images. This IFU features a dual-array-end design, complemented by a dozen slit ends. Each consists of 2 sub-slits corresponding to the dual array ends. In order to accurately and quickly measure the focal ratio degradation of 8064 fibers in FASOT-IFU, a parallel analogy method (PAM) for FASOT-IFU was proposed. This method primarily relies on characterizing the focal ratio based on the full-width at half-maximum (FWHM) of the fiber’s output spot, and the linear relationship between the output focal ratio of the spot and the FWHM. By precisely calculating the focal ratio of the positioning fiber in the FASOT-IFU and treating it as a reference, the output focal ratio of the working fiber can be obtained by comparing the FWHM of the working fiber spot and the positioning fiber spot at the same emission distance. The average value of the output focal ratio by PAM corresponding to the 4032 fibers from Array A of the IFU is 6.0 and the standard deviation is 0.6. The values corresponding to the Array B are 6.2 and 0.40, respectively. The mean value exceeds the contractually specified F /5.5, and the proportion of optical fibers with a focal ratio greater than 5.5 accounts for 88% of the total number of optical fibers, meeting the design requirements of the FASOT system.