Advances in Optical/Infrared Interferometry

Abstract

After decades of fast-paced technical advances, optical/infrared (O/IR) interferometry has seen a revolution in recent years. ▪ The GRAVITY instrument at the Very Large Telescope Interferometer (VLTI) with four 8-m telescopes reaches thousand-times-fainter objects than possible with earlier interferometers, and the Center for High Angular Resolution Astronomy (CHARA) array routinely offers up to 330-m baselines and aperture-synthesis with six 1-m telescopes. ▪ The observed objects are fainter than 19 mag, the images have submilliarcsecond resolution, and the astrometry reaches microarcsecond precision. ▪ This led to breakthrough results on the Galactic Center, exoplanets, active galactic nuclei, young stellar objects, and stellar physics. Following a primer in interferometry, we summarize the advances that led to the performance boost of modern interferometers: ▪ Single-mode beam combiners now combine up to six telescopes, and image reconstruction software has advanced over earlier developments for radio interferometry. ▪ With a combination of large telescopes, adaptive optics (AO), fringe tracking, and especially dual-beam interferometry, GRAVITY has boosted the sensitivity by many orders of magnitudes. Another order-of-magnitude improvement will come from laser guide star AO. In combination with large separation fringe tracking, O/IR interferometry will then provide complete sky coverage for observations in the Galactic plane and substantial coverage for extragalactic targets. Expected final online publication date for the Annual Review of Astronomy and Astrophysics, Volume 61 is August 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Coherent Imaging with Photonic Lanterns

“…Spatial filtering also increases coherence timescales, by averaging out temporal fluctuations within the subaperture. The atmospheric decorrelation time increases with increasing the subaperture size when the pupil phases are averaged by spatial filtering (Conan et al 1995;Kellerer & Tokovinin 2007;Eisenhauer et al 2023). This enables longer exposure times and thus observation of fainter targets.…”

Artificial Intelligence in Astronomical Optical Telescopes: Present Status and Future Perspectives

“…Concurrently, the Nanjing Institute of Astronomical Optics & Technology of the Chinese Academy of Sciences is developing an optical interference project that incorporates three 600 mm aperture telescopes and a baseline length of 100 m. Optical interference, due to its advantages of lengthy baselines and a wide spectrum of observable wavebands, is attracting widespread interest. However, current optical interference experiments have resolutions far below the theoretical limit owing to factors such as detector noise and telescope vibrations [177]. As a result, the development of AI techniques to enhance the wavelength and baseline dynamic range of optical interference will be a critical area for future exploration.…”

Scientific detector workshop 2022 on‐sky performance verification of near‐infrared e−APD technology for wavefront sensing and demonstration of e−APD pixel performance to improve the sensitivity of large science focal planes

“…Both the fringe tracker and the wavefront sensors outperform the specifications of GRAVITY (Deen et al 2016; Eisenhauer, 2017). The CIAO systems are located in the Coude rooms of the VLT telescopes and have bimorph deformable mirrors.…”

“…The electron avalanche photodiodes improved the sensitivity for fringe tracking by more than 2 magnitudes. The limiting magnitude for coherent exposures with GRAVITY was m k ∼ 17 at first light in 2017 (Eisenhauer, 2017) and has been increased to m k ∼ 19 by several improvements of various GRAVITY instrument components and by long detector exposure times for deep imaging of the galactic center (Eisenhauer & GRAVITY Collaboration, 2022). This sets an impressive new sensitivity standard in infrared interferometry, made possible by the deployment of eAPD technology, which is now widely being used in near‐infrared interferometry and for wavefront sensing on a worldwide scale (Goebel et al 2018).…”