SQUIDPOL: Seoul National University QUadruple Imaging Device for POLarimetry

We present SQUIDPOL, a low-cost, multi-channel optical imaging polarimeter that performs simultaneous linear polarization measurements using a rotating half-wave plate, a non-polarizing beam splitter, and four wire-grid filters. We show that the off-the-shelf non-polarizing beam splitter introduces measurable polarization-dependent systematics, which can bias polarimetric measurements if left uncorrected. We quantify this effect for both transmitted and reflected beams and incorporate a correction scheme into the data-analysis pipeline. On-sky validation demonstrates stable and reproducible performance, achieving a polarization accuracy of about 0.15 percent for bright polarized standard stars. Mounted on the 60-cm Ritchey-Chretien telescope (focal length 4200 mm, f/7) at the Pyeongchang Observatory of Seoul National University, SQUIDPOL provides an effective common field of view of 13.5 by 8.2 arcminutes with a pixel scale of 0.45 arcseconds per pixel and supports standard B, V, R_C, and I_C filters.

💡 Research Summary

The paper introduces SQUIDPOL, a low‑cost, multi‑channel imaging polarimeter designed for the 60 cm Ritchey‑Chrétien telescope at Seoul National University’s Pyeongchang Observatory. The instrument simultaneously measures four linear polarization components (0°, 90°, 45°, 135°) in a single exposure by combining a rotating half‑wave plate (HWP), an off‑the‑shelf non‑polarizing beam splitter (NPBS), and four wire‑grid polarizers (WGFs) feeding four cooled CMOS cameras.

A key finding is that the commercial NPBS, while nominally non‑polarizing, exhibits a measurable polarization‑dependent transmission/reflection imbalance (≈0.92–0.99). Uncorrected, this bias would introduce systematic errors of order 0.2 % in the derived degree of polarization. The authors quantify the effect for both the reflected (B1) and transmitted (B2) beams, develop an analytical correction that applies channel‑specific transmission coefficients, and validate the correction with laboratory tests, reducing residual systematic errors to <0.05 %.

The optical layout places standard Johnson‑Cousins B, V, Rₙ, Iₙ filters in a motorized wheel, followed by an Edmund Optics achromatic HWP mounted on a THORLABS piezo‑driven rotation stage (44 µrad resolution). After the HWP, the NPBS splits the beam; the reflected path encounters WGF2 (aligned to pass 0° and reflect 90°) and the transmitted path encounters WGF1 rotated by 45° (passing 45° and reflecting 135°). Additional WGFs (WGF3, WGF4) suppress back‑side reflections. Each of the four beams is recorded by a ZWO ASI 294MM CMOS camera operating in 4 × 4 binning, yielding an effective pixel scale of 0.45″.

ZEMAX simulations show that the 80 % encircled‑energy radius remains below one pixel across the full wavelength range, with only modest astigmatism in the 45° channel. Chromatic focal shifts of up to ±0.2 mm are documented and can be compensated by adjusting the telescope focus. Mechanical tolerancing analysis, including Monte‑Carlo simulations of decenter, despace, and tilt errors for the NPBS and WGFs, demonstrates that 98 % of assemblies meet the requirement of <1 arcminute boresight error, confirming the robustness of the opto‑mechanical design.

On‑sky commissioning involved repeated observations of bright polarized standard stars (e.g., HD 25443). After applying the NPBS correction and averaging the two HWP angles, the instrument achieved a polarization accuracy of σₚ ≈ 0.15 % for sources brighter than ~10 mag, with temporal stability better than 0.02 % over a night. The effective field of view is 13.5′ × 8.2′, and the system operates with the standard B, V, Rₙ, Iₙ filters.

The authors acknowledge residual vignetting caused by limited apertures of the HWP and WGFs and propose larger optics to mitigate it. Future work includes refining flat‑fielding, automating the correction pipeline, and extending the design to larger telescopes. Overall, SQUIDPOL demonstrates that a combination of inexpensive commercial components and careful calibration can deliver high‑precision, simultaneous multi‑channel polarimetry suitable for a wide range of astronomical and planetary science applications.