Studying stellar radii and multiplicity was the bread and butter science for the first generations of optical interferometers. New technological possibilities allow now to contribute to astronomical research at the smallest scales of solar system research, and gravitational microlensing for planet hunting over mapping stellar surface proto-planetary disc and planet evolution in detail, as well as of investigating the cosmic distance scale with detailed study of AGN at significant redshifts. This situation motivates scientists throughout the community to develop ideas, concepts and technology to further improve our interferometric observing capabilities, and the access to them.
In the following we cast such developments into a roadmap to the future of interferometry in Europe, reflecting the work present in this collection. To achieve the proposed interferometric roadmap, a strong collaboration between ESO, ESA, EU and other international organizations is essential.
To structure the roadmap, we separate it in time in two parts, the nearer future, parallel to the construction of the first 30 + m class telescopes, and the following decade. Despite of this separation, both phases should be tightly linked for optimal use of the resources. A construction of a new interferometric facility as currently discussed for the second phase, is only feasible at reasonable time and cost, if it is prepared by developing and testing new technologies with the existing facilities in the upcoming years. This happens already now with the use of photonics and newest detector technologies in the latest generation of instruments, and should be emphasized for the next generation of instruments, as discussed below.
Before 2025: Parallel to the construction VLT 3rd generation and E-ELT 1st generation instruments
While the E-ELT and its first suite of scientific instruments are being constructed, a 3rd generation of instruments for the 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 opticsFootnote 3 and piston-stabilized beam trains.Footnote 4 In Defrère et al. [7] on Hi-5, such a focused instrument concept is presented, which could be realized as a visitor-instrument, similar to the successful PIONIER.Footnote 5 This high-dynamic range imager for the thermal infrared relies on a combination of new integrated optics technology which is currently developed for thermal infrared wavelengths [8], and statistically robust data processing techniques, emphasizing the importance of technological progress.
This collection lists several topics of technological developments which should be emphasized at this epoch. The list is not complete, but addresses key elements of interferometric beam combination to extend the sensitivity, sky coverage, operational robustness, imaging capability and wavelength coverage of interferometry in the optical-infrared domain. While 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, the latter being discussed in Labadie et al. [8]. Wavelength up-conversion, as discussed in Lehmann et al. [9], represents an alternative approach towards interferometric science at thermal wavelengths.
Furthermore, we would like to see research ideas become reality 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 [6]. 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 augure a new era of interferometric imaging with reliable image quality benchmarking (Sanchez-Bermudez et al. [10]).
While emphasis shall be put on these technological advancements to fully exploit the scientific potential of the current suite of instruments and infrastructure, and prepare for future instrumentation, we discuss in this collection as well, how the near-term future can benefit from improved community building, teaching and interaction. An overview of various topics to maximize the scientific community exploitation of the VLTI, and the idea of building a network of expertise centres in Europe are discussed in Kraus et al. [11]. Such centres, not unlike the ALMA regional centres, shall lead the process of training users, and bringing established expert knowledge to the broader astronomical community. This effort is supported by EII and the Horizon 2020 programme via the new OPTICON network grant, and is seen as a key element not only to fully exploit the current investments in interferometry, but also to prepare for a future facility beyond the VLTI, either ground and/or space-based [12].
2025–2035: Towards a new facility
After exploiting Gaia, bringing JWST and Euclid in orbit, and having the ELTs 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 (PFIFootnote 6) project [13], 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. Nevertheless, 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. Millour et al. [5] discuss 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 [14], also [15]. A future facility like PFI [13] 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).