Commissioning and on-sky performance of FiberPol: a fiber-fed spectropolarimetric system for the SAAO 1.9 m telescope

Résumé

At the South African Astronomical Observatory (SAAO), we have developed FiberPol, a fiber-fed spectropolarimetric front-end for the SpUpNIC spectrograph on the 1.9 m telescope. In conjunction with SpUpNIC, it combines two niche techniques: fiber-based spectroscopy and polarimetry to enable a wide range of science cases, particularly for studies of the interstellar medium, and serves as a low-cost technology pathfinder for larger facilities such as the 10 m Southern African Large Telescope. Commissioned in early 2025, FiberPol has already demonstrated key performance benchmarks. Based on an initial data reduction pipeline, we find that the uncertainties in the measured q and u Stokes parameters—and hence the degree of polarization p—are typically in the range of 0.2%–0.3% in 5 nm spectral bins across the 400–700 nm range. With improved calibration and refined analysis, we expect to reach our design goal of 0.1% accuracy per bin. Several science programs are already underway following its successful commissioning. FiberPol employs a Wollaston prism and half-wave plate system to analyze linear polarization and extract spectropolarimetric data comprising wavelength (λ) and the Stokes parameters (I, q, u). Its compact and modular design uses primarily small, commercially available off-the-shelf optical and optomechanical components, making it straightforward to adapt to other spectrographs, including those on larger and next-generation telescopes. As high-accuracy and broadband fiber-fed polarimetry remains relatively unexplored in astronomy, we conducted extensive laboratory tests prior to telescope deployment. These tests confirmed the feasibility of achieving a polarimetric accuracy of 0.1% for point sources across the 400–700 nm range. Together, the lab validation and on-sky results demonstrate the viability of high-accuracy fiber-fed spectropolarimetry, opening the path for integrating polarimetric capabilities into conventional spectrographs, and making spectropolarimetry accessible to a broader astronomical community. This paper presents the design, laboratory validation, and commissioning results of FiberPol, and outlines planned upgrades to further enhance its scientific impact.

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