🔭 astrophysics

THÉMIS: a calibration-free solar telescope

This paper demonstrates that the TH'EMIS solar telescope retains its calibration-free status following the installation of an Adaptive Optics system, as evidenced by raw images and vector magnetic field maps showing negligible instrumental polarization and consistent physical results.

Original authors: V. Bommier, B. Gelly, R. Douet

Published 2026-07-10

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CC BY 4.0

Original authors: V. Bommier, B. Gelly, R. Douet

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the Sun as a giant, glowing stage where magnetic fields dance in invisible patterns. To see these dances, astronomers use special telescopes that act like super-sensitive polarized sunglasses, capable of detecting the direction and spin of light waves. But here's the tricky part: the telescope itself is made of glass and mirrors, and sometimes, just by looking at the light, the telescope accidentally adds its own "spin" to the story. It's like trying to listen to a quiet song while wearing headphones that hum with their own noise. If that happens, you can't tell if the music is real or just the headphones making a racket.

Enter THÉMIS, a solar telescope perched high on a 25-meter tower in Tenerife, Spain. Since 1998, this telescope has been famous for being "calibration-free." Think of it as a perfectly silent pair of headphones that never adds its own hum. It was designed to analyze light before it hits any slanted mirrors, ensuring the story it tells is purely from the Sun, not the machine.

The Great Upgrade: Adding a "Smart Lens"
In the mid-2010s, the team decided to give THÉMIS a major upgrade: Adaptive Optics. You can think of this as adding a "smart lens" that instantly fixes the blurry wobbles caused by Earth's atmosphere, making the Sun look as sharp as a diamond. However, fitting this new lens required a complete overhaul of the light's path inside the telescope. They had to squeeze the light into a single beam to hit the new mirror, which meant changing how the light bounced around.

The big question was: Did this makeover ruin the telescope's "silence"? Did the new path introduce that unwanted "hum" (instrumental polarization) that would mess up the data?

The Ultimate Test: The "Silent" Line
To answer this, the team played a clever trick in September 2024. They looked at a specific color of light from the Sun, a spectral line called Fe I 5576.1 Å. This particular color is special because, according to the laws of physics, it cannot show any magnetic spin from the Sun itself. It's like a mute button on a guitar; no matter how hard you strum, it makes no sound.

If the telescope was still "noisy" (introducing its own fake polarization), this silent line would start "singing" with fake signals. But when they looked at the data, it was perfectly silent. The line showed zero polarization. Meanwhile, the other lines they looked at (which should show magnetic spin) were singing loudly with real solar data.

This proved that even with the new Adaptive Optics and the complex path of light bouncing off mirrors at angles like 22.5° (instead of the old 45°), the telescope remained "calibration-free." The light path was tweaked so that every reflection canceled out any potential noise, keeping the signal pure right up until it hit the cameras.

What They Found in the Sunspots
With the telescope confirmed to be working perfectly, the team turned their gaze to sunspots in July 2025, specifically a region called NOAA 14142. They mapped the magnetic fields, electric currents, and forces in these spots with incredible detail.

Here is what they saw:

The Magnetic Flow: Around a sunspot with a "positive" magnetic charge, the magnetic field lines spread out like a diverging fountain. Around a "negative" spot, they converge inward like water going down a drain.
The Electric Currents: The electric currents flowing through the Sun wrapped around these spots like a hula hoop. They spun clockwise around the positive spots and counter-clockwise around the negative ones.
The Invisible Grip: Most importantly, they calculated the Lorentz force (the magnetic push and pull). They found this force was always pointing centripetally—pulling inward toward the center of the spot. It's as if an invisible hand is constantly squeezing the sunspot, holding it together against the chaos of the solar surface.

The Bottom Line
The paper confirms that THÉMIS is still the "gold standard" for clean, calibration-free solar observation. Even after the massive 2016-2021 upgrade to include Adaptive Optics, the telescope's design ensures that the data it collects is a true reflection of the Sun, not a trick of the instrument. The team measured the telescope's "Mueller matrix" (a mathematical description of how it handles light) and found it to be incredibly stable, changing very little even as the telescope moved across the sky.

So, the next time you hear about the Sun's magnetic secrets, you can trust that THÉMIS is listening with ears that hear nothing but the Sun's own voice.

Technical Summary: THÉMIS: a calibration-free solar telescope

Problem and Context
The Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires (THÉMIS), operational since 1998 at the Izaña site in Tenerife, was originally designed as a "calibration-free" solar telescope. Its core innovation lay in performing polarimetric analysis at the primary focus (F1), before any oblique reflections occurred, thereby eliminating instrumental polarization. However, between 2015 and 2021, the telescope underwent modernization to include an Adaptive Optics (AO) system. This upgrade necessitated a fundamental restructuring of the optical path, specifically converting the previous dual-beam polarimetric setup into a single beam to accommodate the adaptive mirror. The central problem addressed by this paper is whether the telescope retains its calibration-free status after these modifications, particularly since the two beams required for polarimetry are now separated only immediately before the cameras, rather than at the F1 focus.

Methodology
To verify the preservation of the calibration-free character, the authors adapted the optical design to manage oblique reflections along the entire ray path. The methodology involved two primary optical modifications:

Beam Splitting and Propagation: The polarisation analyser at F1 was modified to function as a dual superimposed beam splitter. Instead of separating beams early, it creates two superimposed beams distinguished by orthogonal linear polarization states. These beams propagate together through the transfer optics and are separated only just before the spectral camera.
Minimizing Retardance: To ensure the transfer optics preserve these linear polarization states, the authors replaced all 45° reflections with pairs of 22.5° reflections. This non-linear adaptation significantly reduces retardance (variation range of ±2.8° compared to ±15° at 45°). Additionally, the field derotator was redesigned using two Crova prisms and a flat mirror, where the Mueller matrices of the components compensate for one another, ensuring the beam's linear polarization remains practically unmodified.

To empirically test for instrumental polarization, the authors observed the unpolarisable spectral line Fe I 5576.1 Å. This line has a Landé factor of zero due to its specific quantum levels ( $J=0$ to $J=1$ ), meaning it cannot exhibit Zeeman or Hanle effects from solar magnetic fields. Any polarization detected in this line would be strictly of instrumental origin.

Key Results

Instrumental Polarization Test: Raw images taken on September 12, 2024, across the sunspot in active region NOAA 13814 show clear polarization signals in the polarizable lines (Ni I 5578.7 Å and Fe I 5577.0 Å) but a complete absence of polarization in the unpolarizable Fe I 5576.1 Å line. This confirms that instrumental polarization remains negligible up to the point of camera separation.
Mueller Matrix Characterization: The telescope's Mueller matrix at approximately 6000 Å was determined to be nearly diagonal, with off-diagonal elements representing minimal cross-talk (e.g., $M_{21} = -0.009$ ). The inverse matrix was calculated, though the authors note that applying it to data treatment chains resulted in no significant changes to derived magnetic field maps in test cases.
Vector Field Observations: Using data from July 2025 (Active Region NOAA 14142), the team derived vector magnetic fields, vector current densities, and vector Lorentz forces. The results align with previous findings:
- Magnetic fields diverge from positive polarity spots and converge toward the center of negative polarity spots.
- Vector current density is primarily horizontal, wrapping clockwise around positive polarity spots and counter-clockwise around negative polarity spots.
- A strong horizontal current component crosses the active region neutral line.
- The Lorentz force vector is systematically centripetal relative to the spot center, consistent with the physical mechanism maintaining sunspots.

Significance and Claims
The paper concludes that the THÉMIS telescope remains a calibration-free instrument despite the integration of Adaptive Optics and the subsequent reconfiguration of its optical path. The authors assert that the specific adaptations to the oblique reflections and the field derotator successfully preserved the integrity of the polarization states. Consequently, the telescope can continue to provide high-fidelity vector magnetic field, current density, and Lorentz force maps without the need for complex polarization calibration, relying instead on the inherent design that minimizes instrumental artifacts. The work serves as a validation of the telescope's modernized architecture and its continued utility for precise solar magnetic field studies.

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