Pollux UV & FUV polarimeters: first lab results

Imagine you are a cosmic detective trying to solve the mystery of how stars are born and how planets form. To do this, you don't just want to take a picture of a star; you want to understand the invisible magnetic forces that shape its light. This is where a branch of science called "polarimetry" comes in. Think of light not just as a beam of energy, but as a swarm of tiny, vibrating ropes. Usually, these ropes vibrate in every direction at once, like a chaotic crowd. But when light bounces off a planet or travels through a magnetic field, the ropes start vibrating in a specific, organized pattern. This "polarization" is a secret code that tells astronomers about the magnetic fields and the materials the light has touched.

The challenge is that some of the most interesting secrets are hidden in a part of the light spectrum called the "ultraviolet" (UV), specifically the "far-ultraviolet" (FUV). This is a high-energy, invisible zone where the air around us acts like a thick fog, blocking the light from reaching our eyes or standard telescopes. To catch these clues, scientists need special tools that can work in a vacuum and handle light that is so energetic it can't be bent by normal glass lenses. The paper you are about to read is about building and testing a brand-new, high-tech "light decoder" designed to crack these UV codes, paving the way for a future space telescope that will help us find out if other worlds could support life.

---

The Pollux Project: Building a Super-Decoder for Cosmic Light

The paper introduces "Pollux," a proposed high-tech instrument designed to ride on a future giant space telescope called the Habitable Worlds Observatory (HWO). Think of the HWO as a massive, 6-to-8-meter eye in space, planned to launch after 2040, intended to look for Earth-like planets. Pollux is the special camera attached to this eye, designed to not only see the light from stars but to decode the polarization of that light with incredible precision. The goal is to measure polarization with an accuracy of 1 part in 1,000 (or $10^{-3}$), which is like trying to spot a single grain of sand on a beach from a satellite.

The tricky part is that Pollux needs to work across a huge range of colors, from the far-ultraviolet (FUV) to the near-infrared. In the middle and near-ultraviolet ranges, the team can use special crystals (made of magnesium fluoride, or MgF2) that act like prisms to sort the light. However, in the far-ultraviolet (below about 118 nm), these crystals simply don't exist; the light is too energetic for them to work. To solve this, the team invented a clever "all-reflective" design for the FUV part, using mirrors instead of crystals to sort the light.

Before building the final space instrument, the team had to prove their ideas work on the ground. They built a dedicated "test bench" inside a vacuum chamber at the LIRA lab in France. This bench is a miniature version of the instrument, designed to test two different setups: one for the middle/near-ultraviolet (MUV-NUV) and one for the far-ultraviolet (FUV). The paper reports the very first successful tests of this new lab setup.

Testing the Middle and Near-Ultraviolet (MUV-NUV)

First, the team tested the setup for the 120–290 nm range. They assembled a chain of five blocks, starting with a deuterium lamp that acts as a bright, unpolarized light source. To make sure the light was truly "clean," they passed it through an integrating sphere (a hollow ball with a rough white interior) that scrambled the light's direction, making it completely random.

They then shone this light through a series of mirrors and a special prism (a Rochon prism) that splits the beam into two separate paths based on how the light is vibrating. When they took the first pictures, they saw a beautiful rainbow of light (a spectrum) stretching from 138 to 290 nm. A key discovery here was that the two paths of light had almost exactly the same brightness. This was a lucky break because the manufacturers of the mirrors didn't promise this; usually, one color of vibration gets lost more than the other. Having them balanced makes the measurements much easier and more accurate.

Next, they tested their ability to create specific patterns of polarized light. They used a device called a Babinet-Soleil compensator, which acts like a tunable filter to twist the light's vibration into any shape they wanted. They compared what they measured against computer predictions (using something called "Mueller matrix" calculations) and found they matched perfectly. Finally, they ran a full test where they rotated a modulator (a spinning part that changes the light's pattern) and measured the light at six different angles. By doing this, they successfully reconstructed the original "Stokes vector" (the mathematical description of the light's polarization) and compared it to what they expected. The results showed that their entire chain, from creating the light to decoding it, works as planned.

Tackling the Far-Ultraviolet (FUV) Challenge

The second part of the paper deals with the much harder FUV range (98–120 nm). Here, the rules change completely. Because the air absorbs this light and even the glass windows of standard lamps block it, they couldn't use a normal lamp. Instead, they built a "windowless" deuterium plasma source. Imagine a tiny, glowing fire of gas floating directly inside the vacuum chamber, with no glass barrier between it and the rest of the machine. To keep the chamber from filling up with gas, they used a clever trick: the gas flows through a tiny 200 µm pinhole, and powerful pumps suck it away so fast that the pressure stays low enough for the machine to work (around $1.6 \times 10^{-5}$ mbar).

They also had to solve a tricky alignment problem for a "K-mirror," a special mirror setup that rotates to modulate the light. They adapted a method used for another project (MICADO) to align this mirror with extreme precision. They used two light sources—one to check the position and one to check the angle—and moved the mirror until the light hit the exact center of rotation, ensuring the mirror spins perfectly around its own axis.

Finally, they built and tested the "analyser" for this range. Since they can't use crystals, they coated a mirror with two special layers: a layer of boron carbide (B4C) and a layer of magnesium fluoride (MgF2). They tested this mirror at 120 nm and found it worked as a polarizer, blocking one type of vibration while letting the other through. Specifically, it blocked the "P-polarized" light so effectively that the remaining light was only 10% as bright as the "S-polarized" light. This gives them a "polarization extinction ratio" of 10, which is a solid first step. They plan to test this again at a massive particle accelerator (a synchrotron) to confirm the results across the whole range.

What's Next?

These first results are a major milestone. The team has proven that their vacuum test bench works, that they can generate and measure polarized light in both the MUV-NUV and FUV ranges, and that their new mirror-based designs are viable. While the FUV analyser still needs more testing to confirm its full performance, the successful integration of the MUV-NUV chain and the creation of the windowless FUV source mean the Technology Readiness Level (how ready the technology is for space) is rising. These tests are the essential groundwork for the future Habitable Worlds Observatory, bringing us one step closer to decoding the magnetic secrets of distant worlds.