SQUIDPOL: Seoul National University QUadruple Imaging Device for POLarimetry

TL;DR

SQUIDPOL addresses the need for accurate, multi-channel polarimetry with a low-cost, compact design by using a rotating half-wave plate, a non-polarizing beam splitter, and four wire-grid filters coupled to four dedicated CMOS cameras to measure all four linear Stokes components simultaneously. The team carefully characterizes and corrects a polarization-angle offset introduced by the commercial NPBS, demonstrates stable on-sky performance with a polarization precision around $σ_P \,\sim\,0.15\%$ on bright standards, and provides detailed tolerancing, geometric, and stability analyses to ensure reliable operation. Key contributions include a practical calibration workflow for NPBS-induced systematics, an optomechanical design that minimizes boresight errors, and validation of SQUIDPOL on a 0.6 m telescope with common-field polarimetric capability across $B$, $V$, $R_C$, and $I_C$ bands. The instrument’s low cost, compact footprint, and demonstrated precision enable scalable polarimetric surveys of Solar System bodies and comparable astrophysical targets, with potential for educational use and future hardware upgrades to further improve field performance.

Abstract

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.

Figures (12)

• Figure 1: SQUIDPOL mounted on the 60-cm telescope at the Pyeongchang Observatory of SNU. The instrument is enclosed by an aluminum frame that supports the optomechanical structure.
• Figure 2: Optical layout of SQUIDPOL. (a) Top view from the +X direction. (b) Isometric view. B1 is the first branch reflected from the NPBS, and B2 is the second branch, transmitted through the NPBS.
• Figure 3: ZEMAX spot diagrams for (a) Camera 1 and (b) Camera 3. The numbers below each diagram indicate the X and Y positions (in mm) on the imaging plane. The overlaid circles represent the Airy disk size for the 60-cm telescope, with a radius of 4.7 $\mu$m at the V-band (550 nm). The spot diagrams for Camera 2 and Camera 4 are similar to those for Camera 3 and Camera 1, respectively, due to their identical optical layouts.
• Figure 4: Sensitivity analysis results. (a)--(c) Boresight errors caused by $\alpha$-tilts (cyan circles) and $\beta$-tilts (blue crosses) of the (a) non-polarizing beam splitter (NPBS), (b) second wire-grid filter (WGF2), and (c) fourth wire-grid filter (WGF4). (d) Boresight errors resulting from despace between the HWP and NPBS. Other tolerances are not shown, as their effects on the boresight error of Camera 1 are smaller than $1"$.
• Figure 5: Probability distribution of boresight errors from 10,000 Monte Carlo simulations. The vertical black line at $1'$ indicates the tolerance limit. The cumulative probability (red solid line) shows that 98% of the simulations meet the criterion.