Pollux: high-resolution precision spectroscopy and polarimetry for the Habitable Worlds Observatory

Imagine the universe as a giant, cosmic library where every star, planet, and cloud of gas has a story to tell. For decades, astronomers have been trying to read these stories by catching the light that travels across space. But light isn't just a simple beam; it's a complex message carrying hidden codes about temperature, speed, and even invisible magnetic forces. To crack these codes, scientists use tools called spectrographs, which act like super-prisms, splitting light into a rainbow of colors so fine that they can see tiny fingerprints of chemical elements. However, some of the most exciting stories—like how planets form or how stars are born—are written in ultraviolet light, a color our eyes can't see and that gets blocked by Earth's atmosphere. To read these secret chapters, we need a telescope in space equipped with a "super-spectroscope" that can see the full rainbow from deep ultraviolet to near-infrared, and even detect the subtle "twist" in light waves known as polarization, which reveals the shape of magnetic fields. This is the stage where a new instrument called Pollux is stepping onto the scene.

This paper introduces Pollux, a high-tech instrument proposed by a European team to be the "eyes" of NASA's future Habitable Worlds Observatory (HWO), a massive space telescope planned for the early 2040s. Think of Pollux as a multi-tool Swiss Army knife for light, designed to be attached to the HWO telescope. Its main job is to take the light from distant stars and planets and break it down with extreme precision, measuring colors from 100 nanometers to 1,750 nanometers. The paper outlines how Pollux will use five different "channels" (or light paths) to handle different parts of the spectrum simultaneously. It aims to achieve a resolution of about 100,000 in the ultraviolet and 65,000 in the visible and near-infrared, meaning it can separate light into incredibly thin slices to spot the tiniest details. Uniquely, Pollux can also act as a polarimeter, a device that measures how light waves are oriented, which is like checking the direction of a spinning top to understand the magnetic fields around it. The paper details the instrument's design, its ability to study everything from our own solar system's icy moons to the birth of galaxies, and the current progress in building its high-tech parts like mirrors, detectors, and special coatings.

The core of the paper is a proposal and a design update. The authors are not claiming to have built the final machine yet; rather, they are presenting a mature concept that is ready to be developed further. They describe how Pollux splits light into five channels: Far-Ultraviolet (FUV), Mid-Ultraviolet (MUV), Near-Ultraviolet (NUV), Optical (OPT), and Near-Infrared (NIR). The FUV channel is a special, separate arm that focuses on the shortest wavelengths (100–123 nm) and includes a dedicated polarimeter that cannot be removed, though a new design allows it to work in pure spectroscopy mode to boost signal strength. The other four channels (MUV to NIR) can work together at the same time. The paper explains that the team has updated the design to start the MUV channel at 120 nm instead of 101 nm, which simplifies the mirrors and gratings but means the 100–120 nm range cannot be observed simultaneously with the rest of the spectrum.

The paper also dives into the "how" of building this machine. It discusses the different types of detectors being considered, such as CMOS sensors for visible light and specialized Mercury Cadmium Telluride sensors for the infrared. For the tricky ultraviolet parts, the team is testing polarimeters that use mirrors made of silicon carbide and special coatings like magnesium fluoride to reflect light without losing it. They have run simulations showing that with the right coatings, their gratings (the parts that split the light) could achieve 70% efficiency, though they note a drop in performance below 120 nm, suggesting they might need to switch to a material called Lithium Fluoride for the very shortest wavelengths. The authors emphasize that while the design is solid, they are still in a "concept maturation" phase, aiming to reach a technology readiness level of 5 by the end of 2028. This means they are proving the parts work in the lab before building the final flight model.

Pollux is designed to answer big questions. It wants to find the first stars that had no heavy elements, figure out how planets get their atmospheres, and even look for magnetic fields around exoplanets by detecting auroras. It can also study objects in our own backyard, like asteroids and icy moons, by analyzing how their surfaces scatter light. The paper suggests that by combining high-resolution spectroscopy with polarimetry, Pollux could map the magnetic fields of galaxies and understand how dust grains align in space. The team is currently building a consortium of scientists and engineers from Europe and beyond to secure funding and refine the design. They have even created a software tool to help astronomers calculate how well Pollux will perform for different observations. While the instrument isn't built yet, the paper presents a clear, detailed roadmap for how Pollux could revolutionize our understanding of the universe, provided the necessary funding and international partnerships come together to turn this ambitious blueprint into a reality.