Objective In addition to intensity and frequency characteristics, celestial radiation also possesses polarization characteristics. Photometry and spectrophotometry can provide information on the intensity and spectral characteristics, while photopolarimetry can provide information on the polarization characteristics. Polarimetric observations of astronomical objects are of great significance for studying gamma-ray bursts, quasars, and spatial targets. To carry out such research, it is necessary for the 2.16-m telescope (Fig.1) to have the capability of polarization photometry, which leads to the development of a polarimeter for the 2.16-m telescope.
Methods The polarimeter adopts a dual-channel time-division linear polarization imaging detection method combining simultaneous and time-resolved imaging (Fig.4), which mainly consists of a polarization calibration unit, a polarization measurement unit, optical lenses, controllers, and cameras (Fig.5-6), enabling polarization calibration, polarization measurement, and multicolor photometry. The polarimeter uses a polarization beam splitter and two cameras for simultaneous imaging. The image acquisition process is completed by two steps. First, images are obtained simultaneously for the 0° and 90° polarization directions; Then, by adjusting the orientation of the half-wave plate, images are obtained simultaneously for the 45° and 135° transmission polarization directions. The working process of the polarimeter (Fig.8) includes the obtaining and processing of bias and flat images, initial angle calibration of the polarizer (Fig.9), polarization calibration (Fig.10), and polarization measurement (Fig.11).
Results and Discussions Following the completion of instrument development, the polarimeter was installed on the 2.16-m telescope for on-sky testing. Following the working process of the polarimeter, a series of unpolarized standard stars were chosen for polarization calibration, and polarimetric measurement observations were conducted (Fig.15). Then, a series of polarization results for polarized standard stars were obtained (Tab.4, Fig.16). The results showed that the polarization measurement accuracy of the polarimeter was better than 0.01. By observing Landolt standard stars (Fig.17), the limiting magnitude result with a polarization measurement accuracy of 0.01 was obtained (Fig.18).
Conclusions The polarimeter adopts a dual-channel time-division linear polarization imaging method and uses two cameras for simultaneous imaging, which allows for the acquisition of the linear polarization degree of an object for two times. The polarimeter has a polarization calibration unit, a polarization measurement unit, enabling polarization calibration, polarization measurement, and multi-color photometry. This paper provides the working process of the polarimeter, including initial angle calibration of the polarizer, polarization calibration, polarization measurement, and multi-color photometry. The paper also gives the processing method for calculating the polarization of targets. After the installation on the 2.16-m telescope, the polarimeter was tested with unpolarized standard stars for polarization calibration, followed by a series of polarized standard stars for testing. The data analysis showed that the polarimeter had a field of view of 4.63′× 4.63′ and a pixel scale of 0.54 (″)/pixel, and the polarization measurement accuracy was better than 0.01. A series of Landolt standard stars were also observed, and the data processing results showed that a 60 s exposure time is capable of capturing a 15.3 magnitude V-band target with a signal-to-noise ratio of 141. The polarimeter enables the 2.16-m telescope to perform V-band linear polarization photometry and multicolor photometry observations.
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