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Pollux test bench: from NUV to FUV polarimetric measurements

This paper details the design and methodology of a vacuum-based test bench developed to validate the performance and increase the technology readiness level of the Pollux spectropolarimeter's NUV, MUV, and innovative FUV polarimetric channels for the HWO mission.

Original authors: Adrien Girardot, Coralie Neiner, Jean-Michel Reess, Olivier Dupuis, Margarita Carret

Published 2026-02-12

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CC BY 4.0

Original authors: Adrien Girardot, Coralie Neiner, Jean-Michel Reess, Olivier Dupuis, Margarita Carret

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

The Cosmic Light-Filter: Building a High-Tech "Sunglasses" Tester

Imagine you are trying to study the most distant, mysterious stars in the universe. To see them clearly, you can’t just use a regular camera; you need a specialized pair of "cosmic sunglasses" (a spectropolarimeter) that can tell you not just how bright a star is, but also the direction the light is vibrating. This information tells scientists about the magnetic fields and environments of those stars.

A group of European scientists is designing a futuristic instrument called Pollux for a massive upcoming space telescope (the Habitable Worlds Observatory). But before they launch this expensive equipment into space, they need a way to test it on Earth.

They have built a "Test Bench"—essentially a high-tech laboratory workbench—to make sure these "cosmic sunglasses" work perfectly.

The Challenge: The "Invisible" Light Problem

Light comes in different "flavors" (wavelengths).

NUV/MUV (Near and Mid-Ultraviolet): This is like bright, visible light. For this, scientists can use crystals (like magnesium fluoride) that act like prisms to bend and manipulate light.
FUV (Far-Ultraviolet): This is much more extreme. It’s so energetic that those crystals simply won't work—they become useless. To handle this "extreme" light, the scientists have to use a completely different design made entirely of mirrors.

It’s like trying to build a lens for a camera: for normal photos, you use glass; but if you were trying to take a photo through a blast furnace, glass would melt, and you’d have to use mirrors instead.

The Solution: The Two-in-One Laboratory

Because the "glass" method and the "mirror" method are so different, the scientists built a test bench that can "shape-shift" between two modes:

The MUV-NUV Mode (The Crystal Setup): This uses a special lamp and crystals to create a controlled beam of light. It’s like having a master light-painter who can create any specific pattern of light to see if the "sunglasses" can detect it accurately.
The FUV Mode (The Mirror Setup): Since crystals won't work here, they use a special plasma light source (a tiny, controlled lightning bolt) and a setup made of mirrors to test the most difficult part of the instrument.

The "Vacuum" Secret Sauce

You can't test this kind of light in a normal room. Air is like a thick fog to ultraviolet light; it absorbs the signal before it even reaches the sensor.

To fix this, the entire experiment is housed inside a giant vacuum chamber. They suck all the air out until it’s emptier than deep space. This ensures that the light travels in a "clean" environment, just like it would in the void of space.

Precision: The Needle in the Haystack

The scientists aren't just looking for "good enough." They need a precision of $10^{-3}$ .

To put that in perspective: imagine you are trying to measure the thickness of a single human hair, but you have to do it while standing on a moving train, and you have to be right every single time.

To achieve this, they use:

Tiny Motors: "Piezoelectric" motors that move parts in increments so small they are almost invisible.
Extreme Alignment: They use sensors to make sure their mirrors are aligned within "arcseconds"—which is like trying to hit a moving target on the moon with a laser pointer from Earth.

Why does this matter?

By building this test bench, the team is moving the technology from "cool idea on paper" to "ready for space" (what scientists call increasing the TRL or Technology Readiness Level). Once Pollux is flying, it will act as a high-definition eye, helping us understand the magnetic secrets of the universe.

Technical Summary: Pollux Test Bench for NUV to FUV Spectropolarimetric Measurements

Problem Statement

The Pollux instrument is a proposed high-resolution spectropolarimeter for the future Habitable Worlds Observatory (HWO). A major technical challenge lies in its wide spectral coverage (100–1,800 nm). While the Near-UV (NUV) and Mid-UV (MUV) channels can utilize traditional birefringent materials (like $\text{MgF}_2$ ) for polarization, no such materials exist for the Far-UV (FUV) range (100–123 nm). Consequently, the FUV polarimeter must rely on an innovative, entirely mirror-based architecture. There is a critical need for a specialized experimental facility capable of validating these different polarimetric technologies to ensure they meet the required precision of $10^{-3}$ .

Methodology

The researchers developed an adaptable vacuum ultraviolet (VUV) test bench designed to operate in a clean-room vacuum environment ( $2 \times 10^{-6}$ mbar) to prevent atmospheric absorption and contamination. The bench is divided into two distinct configurations:

MUV-NUV Configuration: Designed to characterize the CASSTOR nano-satellite polarimeter. It uses a deuterium lamp and an integrating sphere to create a stable, unpolarized collimated beam. Polarization states are generated using a Wollaston prism and a Babinet–Soleil compensator. The light is then passed through the polarimeter under test (using $\text{MgF}_2$ waveplates) and analyzed by an echelle spectrograph.
FUV Configuration: Specifically designed for the mirror-based FUV polarimeter. Because standard lamps are insufficient for the extreme UV, this setup utilizes a deuterium plasma light source generated directly within the vacuum chamber to produce emissions below 118 nm.

Key Technical Features:

Alignment & Control: The bench uses nine vacuum-compatible piezoelectric motors for fine alignment. The most complex component, the objective mirror, is mounted on a five-axis stage.
Detection: A Teledyne CIS120 sensor is used, with its PCB coated in Mapsil for vacuum compatibility.
Subsystem Calibration: The Babinet–Soleil retarder is calibrated for precise zero-retardance positioning, and the collimator is aligned using a wavefront sensor to minimize aberrations.

Key Contributions

Dual-Mode Architecture: The creation of a single, reconfigurable test bench that can handle both traditional birefringent optics (NUV/MUV) and novel reflective optics (FUV).
FUV Validation Framework: The development of a methodology to test FUV spectropolarimetry, a field previously lacking experimental validation tools due to the absence of UV-transparent birefringent materials.
High-Precision Alignment Protocols: Implementation of sub-micron positional accuracy and arcsecond-level angular alignment within a vacuum environment.

Results

Calibration Precision: The Babinet–Soleil retarder achieved a positioning accuracy of 0.01 mm, resulting in a thickness uncertainty of only $4.36 \times 10^{-5}$ mm.
Collimator Performance: Through iterative adjustments with a wavefront sensor, the team achieved an angular alignment precision of 30 arcseconds and a positional accuracy better than 1.5 µm.
Feasibility: Simulations and early calibration efforts confirm that the bench can provide the controlled, repeatable conditions necessary to reach the target polarimetric precision of $10^{-3}$ .

Significance

This work represents a major step in increasing the Technology Readiness Level (TRL) of UV spectropolarimetry. By providing a means to validate both existing and next-generation (mirror-based) polarimeters, the test bench directly supports the development of the Pollux instrument. Ultimately, this infrastructure is vital for the success of the Habitable Worlds Observatory (HWO), enabling the high-precision astrophysical measurements required to study the universe in the challenging far-ultraviolet spectrum.