Pollux test bench: from NUV to FUV polarimetric measurements

TL;DR

Pollux aims to deliver high-resolution UV-to-FUV spectropolarimetry, confronting the lack of birefringent materials in the FUV by employing a mirror-based polarimeter. The authors present a vacuum-compatible test bench with two configurations (MUV-NUV and FUV) to validate polarimetric performance to $10^{-3}$ and to raise the Technology Readiness Level for UV polarimetry. A dedicated detector model and detector simulations support performance forecasting, while detailed alignment and calibration strategies for critical subsystems (e.g., Babinet–Soleil retarder and collimator) ensure precise polarization control. This infrastructure enables controlled, repeatable testing in vacuum, providing a crucial stepping-stone toward in-orbit Pollux operation on the Habitable Worlds Observatory and broader UV spectropolarimetric capabilities.

Abstract

Pollux is a high-resolution spectropolarimeter proposed by an European consortium for HWO. The current design of Pollux features four spectropolarimetric channels, three of which are in the UV range. For the near-UV (NUV) [236-472 nm] and mid-UV (MUV) [118-236 nm] channels, the polarimeters consist of waveplates and prisms made of MgF2, a birefringent material. However, no such birefringent material can be used for the far-UV (FUV) channel [100-123 nm]. Therefore, the polarimeter for this FUV channel is composed solely of mirrors in an innovative assembly. In this talk, we aim to detail the architecture of the test bench that will allow us to validate the performance of these different polarimeters, as part of the HWO GOMaP. Given that we are working in the vacuum ultraviolet (VUV) range, the test bench operates in a vacuum chamber in a clean room. We will discuss the adaptable architecture of the bench based on wavelength and the measurement methodology that we will implement to test if the polarimeters achieve the precision of $10^{-3}$ required for the Pollux instrument. With this test bench, we will successfully increase the Technology Readiness Level (TRL) of UV spectropolarimeters and, for the first time, develop a means to test FUV spectropolarimetry.

Figures (8)

• Figure 2: Concept of the MUV-NUV test bench
• Figure 3: Concept of the FUV test bench
• Figure 4: Detector simulation. Left: the colour represents the spectral order. Right: the colour represents the intensity.
• Figure 5: Pumpdown inside the vacuum chamber
• Figure 6: Vacuum chamber of the test bench