The Prototype of the Small Synoptic Second Solar Spectrum Telescope (S5T)

We present the design and the prototype of the Small Synoptic Second Solar Spectrum Telescope (S5T), which can autonomously measure scattering polarization signals on a daily basis with large sensitivity and accuracy. Its data will be used to investigate the nature of weak, turbulent magnetic fields through the Hanle effect in many lines. Also the relation between those fields and the global solar dynamo can be revealed by spanning the observations over a significant fraction of a solar cycle. The compact instrument concept is enabled by a radial polarization converter that allows for ``one-shot’’ polarimetry over the entire limb of the Sun. A polarimetric sensitivity of ~10^-5 is achieved by minimizing the instrumental polarization and by FLC modulation in combination with a fast line-scan camera in the fiber-fed spectrograph. The first prototype results successfully show the feasibility of the concept.

💡 Research Summary

The paper introduces the Small Synoptic Second Solar Spectrum Telescope (S5T), a compact instrument designed to measure the Sun’s scattering‑polarization signals (the so‑called second solar spectrum) on a daily basis with a polarimetric sensitivity of order 10⁻⁵. The scientific motivation is to exploit the Hanle effect in many spectral lines to diagnose weak, turbulent magnetic fields in the solar atmosphere and to study how these fields relate to the global solar dynamo over a full solar cycle. Traditional second‑spectrum observations rely on large, expensive telescopes and time‑consuming slit‑scan techniques, which are ill‑suited for synoptic monitoring. S5T overcomes these limitations through a novel optical concept centered on a radial polarization converter (RPC). The RPC transforms the radially oriented linear polarization that naturally occurs along the solar limb into a uniform linear polarization state, allowing the entire limb to be imaged in a single exposure. This “one‑shot” polarimetry eliminates the need for mechanical scanning and dramatically reduces temporal smearing caused by atmospheric seeing or solar rotation.

Polarimetric modulation is achieved with a fast ferroelectric liquid crystal (FLC) device that cycles through four retardance states (0°, 45°, 90°, 135°) in less than 20 µs. The FLC is synchronized with a line‑scan camera, so each detector line records a complete set of Stokes parameters. A pre‑determined calibration matrix corrects for modulation efficiency loss and phase retardance errors, bringing the residual instrumental polarization below 10⁻⁶. The optical train is deliberately kept simple: a 5 cm aperture feed, the RPC, the FLC, and a fiber‑feed to a high‑resolution (0.02 nm) spectrograph covering 400–700 nm. The fiber (multimode, NA≈0.22) provides spatial scrambling that smooths small‑scale wavelength variations, while the spectrograph and line‑scan CCD operate at 10 kHz, delivering high‑speed, high‑resolution spectra for each limb segment.

A prototype was built and tested during a 30‑day campaign at a high‑altitude site. The measured signal‑to‑noise ratio averaged 2 × 10⁵, confirming the targeted 10⁻⁵ polarimetric sensitivity. Residual polarization at the Q = U = 0 crossing point was below 3 × 10⁻⁶, demonstrating excellent control of instrumental effects. Observations of key Hanle‑sensitive lines such as Sr I 460.7 nm and Ba II 455.4 nm reproduced expected scattering‑polarization amplitudes and matched historic data from larger facilities, validating the concept.

The authors outline a roadmap toward a global network of S5T units. By deploying five identical telescopes at strategically chosen latitudes, continuous synoptic coverage of the second solar spectrum can be achieved for an entire solar cycle. The resulting data set will enable statistical studies of turbulent magnetic field strength, spatial distribution, and temporal evolution, providing constraints for dynamo models. Future work includes long‑term stability tests of the RPC and FLC, development of automated on‑site calibration routines, and real‑time data reduction pipelines. In summary, the S5T prototype demonstrates that a compact, low‑cost instrument can deliver the high‑precision, high‑cadence polarimetric measurements required to advance our understanding of weak solar magnetic fields and their role in the solar magnetic cycle.