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Demonstration of stellar intensity interferometry with the four VERITAS telescopes


High angular resolution observations at optical wavelengths provide valuable insights into stellar astrophysics1,2, and enable direct measurements of fundamental stellar parameters3,4 and the probing of stellar atmospheres, circumstellar disks5, the elongation of rapidly rotating stars6 and the pulsations of Cepheid variable stars7. The angular size of most stars is of the order of one milliarcsecond or less, and to spatially resolve stellar disks and features at this scale requires an optical interferometer using an array of telescopes with baselines on the order of hundreds of metres. We report on the implementation of a stellar intensity interferometry system developed for the four VERITAS imaging atmospheric Cherenkov telescopes. The system was used to measure the angular diameter of the two sub-milliarcsecond stars β Canis Majoris and ϵ Orionis with a precision of greater than 5%. The system uses an offline approach in which starlight intensity fluctuations that are recorded at each telescope are correlated post observation. The technique can be readily scaled onto tens to hundreds of telescopes, providing a capability that has proven technically challenging to the current generation of optical amplitude interferometry observatories. This work demonstrates the feasibility of performing astrophysical measurements using imaging atmospheric Cherenkov telescope arrays as intensity interferometers and shows the promise for integrating an intensity interferometry system within future observatories such as the Cherenkov Telescope Array.

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Fig. 1: The VERITAS array.
Fig. 2: Temporal and spatial coherence measurements.

Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding authors upon reasonable request. Given the extremely large datasets obtained during the observations, in excess of 40 TB, only the post-correlation data can be reasonably made available.

Code availability

The software used to analyse the data in this study is available upon request to the corresponding authors.


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This research is supported by grants from the US Department of Energy Office of Science, the US National Science Foundation and the Smithsonian Institution, by NSERC in Canada and by the Helmholtz Association in Germany. We acknowledge the excellent work of the technical support staff at the Fred Lawrence Whipple Observatory and at the collaborating institutions for their maintenance of VERITAS and assistance with integrating the SII system. The authors gratefully acknowledge support from the US National Science Foundation grant nos. AST 1806262 and PHY 0960242, and from the University of Utah for the fabrication and commissioning of the VERITAS-SII instrumentation. This work is dedicated to the memory of Paul Nuñez.

Author information

Authors and Affiliations



All authors contributed to the operation of the VERITAS telescopes and the internal review of the manuscript. N.M. and D.K. developed the SII hardware used in these observations with assistance from M.K.D. and G.H. in the integration with VERITAS; N.M. and D.K. wrote the data acquisition software; N.M. developed the FPGA correlator software; N.M. proposed the observations; N.M. and T.H. reduced and analysed the data; N.M., D.K., T.L., T.T.Y.L., D.R., Q.F., J.D., A.U.A. and A.F. took the observations; N.M. wrote the paper with guidance from T.L., M.K.D., D.K., T.H., M.P. and J.L.R.; and T.L. and D.K. provided key insights throughout this work.

Corresponding authors

Correspondence to D. Kieda or N. Matthews.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks William Guerin and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Coverage of the sources in the (u, v)-plane.

Each of the coloured points represents different runs for a given telescope pair.

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Abeysekara, A.U., Benbow, W., Brill, A. et al. Demonstration of stellar intensity interferometry with the four VERITAS telescopes. Nat Astron 4, 1164–1169 (2020).

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