While the E-ELT (Extremely Large Telescope – 39.3 meter mirror) and its first suite of scientific instruments are being constructed, a third generation of instruments for the Very Large Telescope Interferometer (VLTI) could emphasize particular aspects (angular and/or spectral resolution, extended wavelength coverage, multi-object), and benefit of a mature and improved telescope infrastructure, including adaptive optics and piston-stabilized beam trains. These improvements will be built and developed from now to 2025.
The last decade brought photon-counting detectors to reality as ideal sensors for adaptive optics and fringe tracking systems, using emCCD and APD technology, first integrated optics beam-combiners (IO-BCs) showed at the same time the potential for simplifying, compactifying the beam combination, and at the same time increasing the precision of the measurement process. Key steps to go for larger arrays and the thermal infrared as a sweet spot for the direct detection of exo-planets and their formation, are the development of larger APD focal plane arrays working in the infrared, and IO-BCs for such wavelengths.
Research ideas are being developed to improve on the current fringe tracking limits of the VLTI. New fringe tracking concepts are being discussed which focus on an ideal use of photons entering the beam combining laboratory. In contrast, predictive control algorithms promise to create synergies between operating adaptive optics and fringe tracking in parallel.
The other key area of technological progress is in optimizing and automatizing the process of image reconstruction to derive model-independent images, and a reliable snapshot imaging mode (getting an image in less than a night), to eventually open the usage of arrays of 4–6 telescopes to the area of the time-domain astronomy. Interferometric imaging hugely benefits from the availability of chromatic multi-baseline datasets, as provided now by the VLTI 2nd generation instruments, and the coming years will see a new era of interferometric imaging with reliable image quality benchmarking.
2025–2035: Towards a new facility
After exploiting Gaia, bringing JWST and Euclid in orbit, and having the ELTs (Extremely Large Telescopes) on sky, the 2025–2035 decade should focus on making accessible the highest angular resolutions, only attainable with optical-infrared long-baseline interferometry. Currently, these plans have merged into the planet formation imager (PFI6) project, but the identified science cases will evolve over the coming years, for instance due to the new results delivered by the above mentioned upcoming facilities. The science driven PFI initiative needs to progress now to design a 10+ telescope interferometric array to allow for routine and sensitive imaging of complex sceneries like planet-forming systems. Many of the technological advancements, discussed in this collection, will contribute to a proper design and cost model of such a future interferometric facility.
As an intermediate step towards the PFI, longer baselines, and additional telescopes are discussed as an extension to the VLTI as it is today. Given the likely focus of the PFI on longer wavelengths, exploiting visible interferometry at the VLTI will allow for a complementary scientific use. Opto-mechanical upgrades of the current VLTI infrastructure that would be needed to control the wavelengths leading to a 2–3 times higher angular resolution, as a prerequisite for visible-wavelength interferometry. Important technological pathfinding is currently done at the CHARA facility.
At this early stage of developing post-VLTI facilities, also alternative approaches to high-dynamic range interferometric imaging at highest angular resolution should be studied. A future facility like PFI will eventually become feasible as an international facility if building on the experience of today’s optimized arrays, choosing the best fringe tracking concepts, focusing on simple light-weight telescopes and mass production of now standard technology to co-phase apertures (adaptive optics) and arrays (fringe tracking).
SOURCES- Future of optical-infrared interferometry in Europe (Experimental Astronomy)