Spectro-polarimetric Observations at the NVST: I. Instrumental Polarization Calibration and Primary Measurements

TL;DR

This paper evaluates the polarimetric accuracy and sensitivity of the NVST instrument using spectro-polarimetric observations of the Fe I 532.4 nm line, demonstrating calibration stability and identifying current limitations.

Contribution

It provides a detailed calibration and performance assessment of the NVST polarimeter, including instrumental polarization characterization and measurement accuracy analysis.

Findings

Calibration accuracy up to 3×10^{-3} achieved.

Residual cross-talk around 4×10^{-3} after calibration.

Polarimetric sensitivity of about 10^{-3} with 20s integration.

Abstract

This paper is devoted to the primary spectro-polarimetric observation performed at the New Vacuum Solar Telescope 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 technology and then characterize its instrumental polarization based on the operation in 2017 and 2019. It is verified that the calibration method making use of the instrumental polarization calibration unit (ICU) is stable and credible. The calibration accuracy can reach up to 3$\times 10^{-3}$ . Based on the scientific observation of the NOAA 12645 on April 5th, 2017, we estimate that the residual cross-talk from Stokes $I$ to Stokes $Q$, $U$ and $V$, after the instrumental polarization calibration, is about 4$\times10^{-3}$ 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 seconds. Slow modulation rate is indeed an issue for the present system. The present NVST polarimeter is expected to be integrated with an high-order adaptive optics system and a field scanner to realize 2D magnetic field vector measurements in the following instrumentation update.