POPO: Fast-modulating polarimeter with imaging capability

¹ Center for Astronomy, University of Hyogo, 407-2, Nishigaichi, Sayo, Hyogo 679-5313, Japan
² Korea Astronomy and Space Science Institute (KASI), 776 Daedeok-daero, Yuseong-gu, Daejeon 34055, Republic of Korea

^★ Corresponding author: takahashi@nhao.jp

Received: 29 May 2025
Accepted: 15 September 2025

Abstract

Context. Previous fast-modulating polarimeters have successfully achieved precisions on the order of parts per million (ppm). However, they lack imaging capability, which restricts their scientific application.

Aims. We developed POlarimeter for Precision Observations (POPO), an instrument designed to provide either a high precision (≲10 ppm) or a high time resolution (≲1 s), along with imaging capability. In this study, we evaluated the system characteristics and performance of POPO, focusing on the observations of bright point sources.

Methods. Test observations were conducted with POPO mounted on the Cassegrain focus of the 2.0-m Nayuta telescope. Bright (<10 mag) unpolarized and strongly polarized standard stars were observed to evaluate precision, night-to-night stability, and accuracy. In addition, asteroid (4) Vesta was used to demonstrate the potential of POPO in time-series polarimetry.

Results. Under nearly constant conditions, the best precision of Stokes measurement was 5 ppm. We found the precision to be 2.5 times the photon noise limit. In addition, even with a time resolution of 1 s, a precision sufficient to detect 1% polarization was achieved for stars brighter than 10 mag. The night-to-night stability was recorded as ≲10 ppm. The accuracy of the linear polarization degree (P) was evaluated to be ≲0.1% for strongly polarized stars (P ≈ several %) when observed at several position angles of the instrument. POPO successfully confirmed the rotational variation in Vesta’s polarization (ΔP of ~0.06% to ~0.08%).

Conclusions. POPO is suitable for studying small polarimetric variation in bright objects. Furthermore, POPO demonstrated the ability to perform high-time-resolution polarimetry.