Spectro-polarimetric observations at the NVST:I.instrumental polarization calibration and primary measurements

This paper is devoted to the primary spectro-polarimetric observation performed at the New Vacuum Solar Telescope(NVST)of China since 2017,and our aim is to precisely evaluate the real polarimetric accuracy and sensitivity of this polarimetry by using full Stokes spectro-polarimetric observations of the photospheric line Fe I 532.4 nm.In the work,we briefly describe the salient characteristic of the NVST as a polarimeter in terms of technology and then characterize its instrumental polarization based on the operation in 2017 and 2019.It is verified that the calibration method utilizing the instrumental polarization calibration unit(ICU)is stable and credible.The calibration accuracy can reach up to 3×10^-3.Based on the scientific observation of NOAA Active Region 12645 on 2017 April 5,we estimate that the residual cross-talk from Stokes I to Stokes Q,U and V,after the instrumental polarization calibration,is about 4×10^-3 on average,which is consistent with the calibration accuracy and close to the photon noise.The polarimetric sensitivity(i.e.,the detection limit)for polarized light is of the order of 10-3 with an integration time over 20 s.Slow modulation rate is indeed an issue for the present system.The present NVST polarimeter is expected to be integrated with a high-order adaptive optics system and a field scanner to realize 2 D vector magnetic field measurements in the following instrumentation update.

A novel method for telescope polarization modeling based on an artificial neural network

The polarization characteristics of an astronomical telescope is an important factor that affects polarimetry accuracy. Polarization modeling is an essential means to achieve high precision and efficient polarization measurement of the telescope, especially for the alt-azimuth mount telescope. At present, the polarization model for the telescope(i.e., the physical parametric model) is mainly constructed using the polarization parameters of each optical element. In this paper, an artificial neural network(ANN) is used to model the polarization characteristics of the telescope. The ANN model between the physical parametric model residual and the pointing direction of the telescope is obtained, which reduces the model deviation caused by the incompleteness of the physical parametric model. Compared with the physical parametric model, the model fitting and predictive accuracy of the New Vacuum Solar Telescope(NVST) is improved after adopting the ANN model. After using the ANN model, the polarization cross-talk from I to Q, U, and V can be reduced from 0.011 to 0.007, and the crosstalk among Q, U, and V can be reduced from 0.047 to 0.020, which effectively improves the polarization measurement accuracy of the telescope.