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Proof of concept of the COSMOCal project at the IRAM 30m telescope

This paper reports the successful September 2024 validation campaign at the IRAM 30m telescope of the COSMOCal instrument, a novel space-borne concept designed to provide absolute polarization calibration for current and next-generation Cosmic Microwave Background experiments.

Original authors: S. Savorgnano, L. Bizzarri, A. Ritacco, J. Aumont, F. Boulanger, A. Catalano, F. Cuttaia, A. Denis, F. -X. Désert, D. González Ovejero, S. Leclercq, J. -F. Macías-Pérez, B. Maffei, M. Migliaccio, L. M

Published 2026-08-07

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CC0 1.0

Original authors: S. Savorgnano, L. Bizzarri, A. Ritacco, J. Aumont, F. Boulanger, A. Catalano, F. Cuttaia, A. Denis, F. -X. Désert, D. González Ovejero, S. Leclercq, J. -F. Macías-Pérez, B. Maffei, M. Migliaccio, L. Montier, P. Morfin, L. Mousset, M. Murgia, F. Nati, P. Ortu, M. Pérault, G. Pisano, T. Pisanu, N. Ponthieu, L. Terenzi, J. Treuttel, L. Vacher, M. Zannoni

Original paper dedicated to the public domain under CC0 1.0 (http://creativecommons.org/publicdomain/zero/1.0/). ✨ 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 universe as a giant, ancient radio station broadcasting a faint, static-filled signal from the moment of its birth. This signal, known as the Cosmic Microwave Background (CMB), is the oldest light in existence, carrying secrets about how the universe began and how it has evolved. However, this signal is incredibly weak and comes with a twist: it is "polarized," meaning the light waves vibrate in specific directions, like a rope being shaken up and down versus side to side. To read the universe's hidden messages—such as the ripples caused by the very first split-second of creation—scientists need to measure these directions with extreme precision.

The problem is that our telescopes are like imperfect rulers. Over time, the instruments themselves can get slightly "out of true," causing them to misread the angle of the light waves. If a telescope thinks a wave is vibrating at 10 degrees when it's actually at 10.1 degrees, it could miss the most important clues about the Big Bang. To fix this, astronomers need a "master reference," a perfect, unchanging signal source that they can point their telescopes at to check their accuracy. Usually, they use bright stars or nebulae for this, but even those have their own wobbles and uncertainties. The COSMOCal project proposes a radical solution: instead of looking at a distant star, we build a tiny, artificial satellite that acts as a perfect, man-made lighthouse in the sky, beaming a signal with a known, unshakeable polarization angle.

This paper describes a crucial "proof of concept" test for that idea. Before launching a real satellite into space, the team built a ground-based prototype and tested it against the IRAM 30m telescope, a massive radio dish located on a mountain in Spain. The goal was to see if this artificial "lighthouse" could successfully talk to a real telescope and help it calibrate its measurements.

The team set up their prototype source about 2.3 kilometers away from the telescope, essentially creating a near-field test where the source was close enough to be seen clearly but far enough to mimic a distant object. They used two clever tricks to know exactly where their source was pointing. First, they took photos of the ground with a camera on the source to triangulate its position (photogrammetry). Second, they shone a laser through a polarizer and watched the resulting diffraction pattern (a specific light shape) to determine the angle of the signal.

When they turned on the source and pointed the telescope at it, the results were a resounding success. The telescope's detectors, which are incredibly sensitive superconducting sensors, picked up the signal immediately. The team could see the signal flickering in rhythm with the source's rotating parts, proving that the "lighthouse" was working and the telescope was listening. They then rotated the source's polarizer to different angles and watched the telescope's readings change accordingly. The data showed a clear, linear relationship: when the source changed its angle, the telescope's measurement changed in perfect step.

However, the test also revealed some bumps in the road. While the system worked, the precision wasn't quite as sharp as the team hopes for the final space mission. The measurements had an uncertainty of about 3 degrees, which is much larger than the target of 0.1 degrees needed for the most sensitive future experiments. The authors explain that this "fuzziness" comes from the fact that they were testing in a "near-field" setup (the source was relatively close) and because of some optical quirks in the telescope's mirrors that slightly scrambled the light. They also noted that the telescope's internal components introduced some small, predictable errors.

Despite these limitations, the experiment proved the core idea works. The team successfully demonstrated that an artificial source can provide a stable, independent reference for calibrating a large telescope. They showed that by combining camera photos and laser patterns, they could reconstruct the source's angle with high confidence. The paper concludes that while this specific ground-based setup had some hurdles, the method is solid. It paves the way for the next step: building a more advanced version that can be launched into space, where it will serve as a permanent, perfect calibration beacon for telescopes around the world, helping them unlock the deepest secrets of the cosmos.

Technical Summary: Proof of Concept of the COSMOCal Project at the IRAM 30m Telescope

Problem Statement
Current and next-generation Cosmic Microwave Background (CMB) polarization experiments require unprecedented accuracy in the calibration of the absolute polarization angle to detect primordial gravitational waves (via the tensor-to-scalar ratio $r$ ) and search for cosmic birefringence. Existing astronomical calibrators, such as the Crab Nebula, provide only relative references because their measured polarization angles are subject to the systematic uncertainties of the observing instruments. Furthermore, artificial calibration strategies using ground-based or drone-mounted sources cannot serve as common far-field calibrators for large-aperture ground-based observatories, as the required far-field distances exceed the Earth's atmosphere. To address this, the COSMOCal project proposes a space-borne, geostationary polarized microwave source to provide a stable, absolute reference for multiple observatories. Before space deployment, a terrestrial prototype was developed to validate the concept and observational strategy.

Methodology
In September 2024, a proof-of-concept campaign was conducted at the IRAM 30m telescope on Pico Veleta. The campaign utilized a COSMOCal prototype emitting a monochromatic, linearly polarized signal at 265 GHz, matching the polarization-sensitive channel of the NIKA2 camera. The source was installed approximately 2.3 km away at the University of Granada site, creating a near-field observational geometry.

The methodology relied on two independent, complementary approaches to reconstruct the emitted polarization angle:

Photogrammetry: A CCD camera on the calibrator imaged ground-based reference targets (measured via RTK GPS) to reconstruct the instrument's attitude (yaw, pitch, roll) relative to the telescope line of sight using a PnP (Perspective-n-Point) formulation.
Diffraction Pattern Analysis: A reference laser passing through the calibrator's wire-grid polarizer created a diffraction pattern on the CCD. The orientation of this pattern provided a direct proxy for the polarization direction in the camera frame.

These reconstructed angles were combined to define the absolute polarization reference. The NIKA2 detector, equipped with a rotating half-wave plate (HWP) and an external chopper, observed the source through fixed-track scans. The data reduction pipeline involved removing atmospheric fluctuations and instrumental polarization (specifically HWP synchronous signals) using the chopper modulation as a reference, followed by the extraction of Stokes parameters ( $I, Q, U$ ) to determine the polarization angle as measured by the telescope.

Key Contributions and Results

System Validation: The campaign successfully demonstrated the detection of the COSMOCal signal by the NIKA2 detectors and the recovery of both the chopper and HWP modulation signatures, confirming the system's ability to interface with a state-of-the-art millimeter-wave polarimeter.
Alignment Optimization: Through iterative scans, the pointing alignment between the source and the telescope was refined, identifying optimal offsets of $-5'$ in azimuth and $-14'$ in elevation to maximize coupling efficiency.
Polarization Reconstruction: The study verified Malus' law for both total and polarized intensity as a function of the source polarizer angle. A clear linear relationship was observed between the polarization angle reconstructed by the independent COSMOCal system and the angle measured by NIKA2, with a slope consistent with the expected optical configuration ($-1$).
Uncertainty Budget: The total uncertainty on the polarization angle was estimated at approximately $\sigma_\psi \sim 3^\circ$ $σ_{ψ} \sim 3^{\circ}$ . This is significantly larger than the sub-degree target ( $0.1^\circ$ $0. 1^{\circ}$ ) required for the future space mission. The dominant sources of uncertainty were identified as:
- Near-field effects: The extended-source geometry and non-ideal propagation led to partial depolarization and reduced measured polarization degrees.
- Instrumental systematics: A spatial gradient of polarization angle across the NIKA2 focal plane, associated with the dichroic element, and residual calibration uncertainties.
- Photogrammetry limitations: The restricted field of view of the optical system limited the number and geometry of ground targets, increasing the dispersion of the attitude reconstruction.

Significance and Claims
The paper claims that the IRAM 30m campaign successfully validates the core principles of the COSMOCal approach, demonstrating a consistent reconstruction of polarization observables through independent methods. While the current near-field implementation does not yet achieve the $0.1^\circ$ precision required for the final space mission, it establishes a solid foundation for the next phase of development.

The authors emphasize that the campaign serves as a guide for system optimization. They note that the dichroic-related systematics are expected to be mitigated by the upcoming NIKA2+ upgrade (March 2025), and the near-field limitations can be addressed by a future configuration with a significantly larger source distance or a setup more representative of far-field conditions. Ultimately, the results support the feasibility of a space-based COSMOCal calibrator, suggesting that in a space configuration, the need for complex photogrammetric reconstruction may be relaxed if spacecraft pointing and instrument geometry are independently controlled with sufficient precision.

Technical Summary: Proof of Concept of the COSMOCal Project at the IRAM 30m Telescope

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