🔭 astrophysics

Upgrading LBTI/NOMIC with a quadruple annular groove phase mask and GeoSnap detector for imaging nearby, habitable-zone exoplanets

This paper describes two major upgrades to the Large Binocular Telescope Interferometer's NOMIC instrument—the installation of a quadruple annular groove phase mask coronagraph and a high-performance GeoSnap detector—along with validation of their enhanced contrast and sensitivity for imaging habitable-zone exoplanets as part of the Breakthrough Watch program.

Original authors: Kevin Wagner, Manny Montoya, Steve Ertel, Jarron Leisenring, Pontus Forsberg, Samuel Ronayette, Andre Wong, Mikael Karlsson, Olivier Absil, Denis Defrère, Markus Kasper, Jordan Stone, Dániel Apai, Lai

Published 2026-07-31

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CC BY 4.0

Original authors: Kevin Wagner, Manny Montoya, Steve Ertel, Jarron Leisenring, Pontus Forsberg, Samuel Ronayette, Andre Wong, Mikael Karlsson, Olivier Absil, Denis Defrère, Markus Kasper, Jordan Stone, Dániel Apai, Laird Close, Jamie Dietrich, Ewan Douglas, Jamie Drew, Olivier Durney, Marina Fetisova, Kyran Grattan, Olivier Guyon, Jacob Isbell, Sebastián Jorquera, Petri Karvinen, Markku Kuittinen, Hervé Le Coroller, Jared Males, Brittany Miles, Dillon O'Reilly, Eric Pantin, Sascha P. Quanz, Eckhart Spalding, Vivek Vijayakumar, Zach Werber, S. Pete Worden

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

Imagine the night sky as a giant, glittering stage where stars are the blindingly bright spotlights and planets are tiny, shy actors trying to perform in the shadows. For decades, astronomers have struggled to see these actors because the spotlights are so intense they wash out everything else, like trying to spot a firefly next to a searchlight. This is the challenge of "direct imaging": trying to take a clear photo of a planet orbiting another star. To make this possible, scientists use a special trick called a "coronagraph," which acts like a cosmic pair of sunglasses or a thumb held up against a lightbulb to block the glare, allowing the faint planet to peek through. They also need to look in the "thermal infrared," a type of invisible heat-light, because warm planets glow brightly in this color while the star's glare is less overwhelming than it is in visible light. The ultimate goal? To find "habitable zones"—the cozy "Goldilocks" regions around stars where a planet could have liquid water and perhaps even life.

This paper tells the story of a massive upgrade to a high-tech telescope camera called NOMIC, sitting on the Large Binocular Telescope (LBT) in Arizona. Think of the LBT as a giant pair of eyes working together. The team is giving this camera two superpowers to finally catch a glimpse of Earth-like worlds around our nearest stellar neighbors. First, they are installing a new "mask" called the Q-AGPM, which is like a four-way traffic director made of diamond. Instead of just blocking light in one spot, this mask splits the view into four sections, allowing the telescope to chop between them rapidly to cancel out noise and keep the planet's signal clear. Second, they are swapping out the camera's old sensor for a brand-new "GeoSnap" detector. This new sensor is like upgrading from a grainy, slow film camera to a super-fast, high-definition digital one that doesn't get confused by the telescope's own heat, allowing for much longer, steadier looks at the sky.

The paper describes the design, testing, and installation of these two upgrades. The team has already built the diamond mask and tested it in a cold lab, proving it can block the star's light by a factor of 1,000 to 10,000 times (a "rejection ratio" of $10^3$ to $10^4$ ) across the specific heat-light band they need. They have also ordered the new detector, which is free from a type of electronic noise that previously forced the telescope to shake the image back and forth too quickly to use its full power. By combining these tools with a special way of using the telescope's two mirrors together (called "Fizeau imaging"), the authors show that the system can achieve a contrast gain of about 2 to 4 times better than using just one mirror alone. While the full system isn't on the sky yet, the team plans to install it in the summer or fall of 2026. Once it's running, they expect to be able to detect planets as faint as 30 to 50 micro-Janskys (a measure of brightness) in just 100 hours of observation, which is the predicted glow of a "super-Earth" planet in the habitable zone of stars like Epsilon Eridani. This isn't a guaranteed discovery of life, but it is a major step toward finally taking a picture of a world that could be like our own.

Technical Summary: Upgrading LBTI/NOMIC for Habitable-Zone Exoplanet Imaging

Problem Statement
Directly imaging temperate, terrestrial-mass planets around nearby Sun-like stars remains a primary goal of exoplanet science. While the contrast between an Earth-sized planet and its host star is significantly more favorable in the thermal infrared (order $10^{-7}$ at 10 µm) than in reflected visible light (order $10^{-10}$ ), ground-based detection faces two critical challenges: the bright, fluctuating thermal background and the need for high angular resolution to resolve the habitable zone (HZ) at small separations (0.1–1 arcsec).

The Large Binocular Telescope Interferometer (LBTI) Nulling-Optimized Mid-Infrared Camera (NOMIC) is a capable thermal-infrared system, but its original configuration is limited by the legacy Raytheon AQUARIUS detector. The Si:As AQUARIUS array suffers from excess low-frequency noise (ELFN), necessitating rapid chopping to suppress noise. This chopping requirement is fundamentally incompatible with Fizeau interferometry, which requires the stellar image to remain fixed on the detector to maintain fringe lock. Furthermore, the existing coronagraphic capabilities were insufficient for the specific N-band ( $\sim$ 11 µm) requirements of the LBTI dual-aperture mode.

Methodology and Upgrades
The paper details two complementary hardware upgrades designed to transform NOMIC into "GNOMIC," unlocking high-contrast, high-resolution Fizeau imaging:

Quadruple Annular Groove Phase Mask (Q-AGPM) Coronagraph:
- Design: A single 20 mm polycrystalline diamond substrate hosts four annular groove phase masks (AGPMs). Each AGPM is a vector-vortex coronagraph (charge 2) formed by subwavelength gratings.
- Motivation: In the LBTI dual-aperture imaging mode, each of the two telescope beams requires a pair of masks to allow chopping between them without losing integration time for single-star observations. In Fizeau mode, the four quadrants provide redundancy, allowing the selection of the best-performing mask regions or averaging over a four-point nod.
- Fabrication: The masks were fabricated at the Ångström Laboratory using electron-beam lithography and inductively coupled plasma etching. The grating period (4 µm) ensures zero-order operation down to $\approx$ 9.5 µm.
- Performance: Cryogenic testing at CEA Saclay (November 2025) showed on-axis rejection ratios of $10^3$ – $10^4$ across the N' science band (10.2–12.5 µm), with a filter-weighted mean rejection of $\approx$ 2300.
Teledyne GeoSnap Detector:
- Replacement: The legacy AQUARIUS array is replaced by a 13 µm-cutoff HgCdTe GeoSnap-18 focal-plane array ( $2048 \times 2048$ , 18 µm pitch).
- Advantages: Unlike Si:As detectors, HgCdTe does not exhibit ELFN. This eliminates the need for rapid chopping, enabling long, stable integrations and closed-loop phase control required for Fizeau imaging.
- Specs: The detector offers higher quantum efficiency ( $\sim$ 75–85%), larger well depth ( $2.6 \times 10^6$ e $^-$ ), and faster, more linear readout. It operates at a relaxed temperature of $\sim$ 45 K compared to the $\sim$ 7 K required for AQUARIUS.

Key Results and Demonstrations

Fizeau Imaging Gain: The paper presents a high-contrast Fizeau imaging sequence obtained using the new FFTCam fringe tracker (replacing PhaseCam) on the LBTI in M-band (4.77 µm). This demonstration confirmed the interferometric gain over a single aperture.
Contrast Performance: Relative to an equal-time single-aperture exposure, the co-phased Fizeau reduction achieved a contrast gain of a factor of $\sim$ 2–4 across separations of 0.2–1 arcsec. The reduction reached an S/N = 3 contrast of $\sim 1 \times 10^{-5}$ beyond 0.9 arcsec.
Sensitivity Projections: Combining the Q-AGPM, the GeoSnap detector, and Fizeau imaging, the authors project a sensitivity improvement of a factor of $\gtrsim 6$ $≳ 6$ over the NEAR experiment (which used a single VLT aperture). This is derived from:
- Elimination of chopping (factor of 2).
- Increased collecting area (2.1 $\times$ ).
- Lower thermal background at Mount Graham (1.25 $\times$ ).
- Improved quantum efficiency and reduced noise (1.15 $\times$ ).
- Concentration of signal into a smaller PSF core ( $\sqrt{3}$ ).
- Removal of noise terms present in NEAR (e.g., central glow, $\sqrt{2}$ ).
- The conservative adopted gain is $\gtrsim 6\times$ , potentially reaching raw improvements of $\sim 10\times$ .

Significance and Claims
The authors state that these upgrades position the LBTI as a unique pathfinder for the ELT era. By combining the 22.7 m edge-to-edge baseline of the LBT with the Q-AGPM and GeoSnap, the instrument can resolve the habitable zones of the nearest single Sun-like stars (e.g., $\epsilon$ Eridani, $\tau$ Ceti, 61 Cygni) at angular separations ( $\sim$ 0.1 arcsec) that are currently inaccessible to space-based facilities like JWST/MIRI at these wavelengths.

The paper claims that the upgraded GNOMIC instrument, as part of the Breakthrough Watch program, will perform the deepest thermal-infrared observations yet of habitable zones around nearby single stars. The projected sensitivity ( $\sim$ 30–50 µJy in $\sim$ 100 hours) targets temperate super-Earths ( $\sim 2 R_\oplus$ ) and giant planets. The work serves as a critical technology demonstration for the upcoming ELT/METIS instrument, maturing the hardware and techniques required for future extremely large telescope coronagraphy.

Status
Both upgrades are scheduled for installation at the LBT during the Summer/Fall 2026 shutdown, with first light on sky expected by the end of 2026. The instrument is supported by a guaranteed allocation of roughly 45 nights for the Breakthrough Watch campaign.

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