The Telescopio Nazionale Galileo has been invited to collaborate on the La Palma Interferometry project, which will involve several telescopes from the Roque de los Muchachos Observatory

The La Palma Interferometry project (LPI) is an innovative initiative in intensity interferometry within the visible light range, based on a system of Single Photon Avalanche Diode (SPAD) sensors. In its first phase, the project plans to conduct observations with the 3.6-meter Telescopio Nazionale Galileo (TNG). The TNG will contribute to the development of a cryogenic dewar and the characterization of the SPAD-based sensor. This instrument will be temporarily installed as a guest instrument at the Nasmyth B focus of the telescope . In the second phase, a clone of the dewar will be added to the 2.5-meter Nordic Optical Telescope (NOT) to begin intensity interferometry tests.Additionaly, to further expand the network of telescopes in a later phase, contacts have been made both with the GTC and the ING groups at the Roque de Los Muchachos Observatory (ORM).

On November 7 and 8, the Real Instituto y Observatorio de la Armada (ROA), which will be responsible for the complex task of synchronizing the sensors across the different telescopes, hosted a workshop for the initial definition of the project. During this meeting, TNG researchers shared their experience with colleagues from ROA, IAA-CSIC, IAC, IMSE CSIC-US, EPS-UAM, the Nordic Optical Telescope (NOT), the Gran Telescopio Canarias (GTC), the Faculty of Physics of the University of Salamanca, and the Department of Electronics and Computer Technology at the University of Granada.

The presentations covered scientific aspects, temporal synchronization, sensor design, microchip-SPAD control electronics, SPAD camera development for the TNG, NOT, and GTC telescopes, as well as statistical techniques and artificial intelligence for analysing photons observed simultaneously by the telescope network integrated into LPI.

Innovation in Visible Interferometry

For decades, interferometry has revolutionized astronomy, both in radio waves with facilities like the VLA, ALMA, and VLBA, and in the infrared with ESO's VLTI. More recently, the EHT captured the shadow of a black hole in our galaxy and in M87 in the microwave range. However, these techniques rely on amplitude and phase correlations of light. In contrast, intensity interferometry allows the study of photon-photon correlations in the visible spectrum.

Thanks to the ultra-fast reading SPAD sensors and the optical telescope network at ORM , LPI will be able to correlate signals within times close to the coherence of visible radiation, increasing sensitivity and achieving resolutions up to 50 microarcseconds. This is comparable to the EHT, but in the visible spectrum. To achieve this, temporal synchronization at the picosecond level is required, a technological challenge that will benefit from ROA's extensive experience.

Currently, there is only one operational visible interferometer: CHARA, at Mount Wilson, which combines light from six 1-meter telescopes distributed with baseline lengths of up to 330 meters. LPI, by using larger telescopes at the Roque de Los Muchachos and a baseline of 1.5 kilometers, will be able to observe fainter objects and achieve four times the resolution, equivalent to 50 microarcseconds. This corresponds to the angular size of a one-euro coin seen from 65,000 kilometers away.

Acknowledgments

We would like to express our gratitude to the Real Instituto y Observatorio de la Armada for their hospitality and support of the project. Founded in 1753 by Jorge Juan, the ROA has been instrumental in the creation of navigation charts and the dissemination of legal time in Spain with exceptional precision. It is also known for its research in time measurement, geodesy, and astronomy—key disciplines for navigation and scientific exploration.

Visiting the ROA is an essential experience: a true gem of scientific knowledge whose legacy transcends borders and eras. Its library, which holds over 30,000 volumes dedicated to scientific research from the 15th to the 19th centuries, is a testament to its rich history and contribution to the advancement of knowledge.