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Astronomy at High Angular Resolution Astronomical News Report on the ESO Workshop Astronomy at High Angular Resolution held at ESO Headquarters, Garching, Germany, 24–28 November 2014 Henri Boffin1 Linda Schmidtobreick1 Gaitee Hussain1 Jean-Philippe Berger1 1 ESO A workshop took place in Brussels in 2000 on astrotomography, a generic term for indirect mapping techniques that can be applied to a huge variety of astrophysical systems, ranging from planets, single stars and binaries to active galactic nuclei. It appeared to be timely to revisit the topic given the many past, recent and forthcoming improvements in telescopes and instru- mentation. We therefore decided to repeat the astrotomography workshop, Figure 1. Photograph of the participants at the work- but to put it into the much broader con- shop in the entrance hall at ESO Headquarters. text of high angular resolution astron- omy. Many techniques, from lucky and – single-dish tomographic reconstruction telescope in La Silla (Merkle et al., 1989). speckle imaging, adaptive optics to methods (astrotomography), with AO is now part of many systems at the interferometry, are now widely employed microarcsecond resolution, but appli- Paranal Observatory, as well as in all to achieve high angular resolution and cable only to certain classes of objects. major observatories, and has led to many they have led to an amazing number of the most important ESO discoveries: of new discoveries. A summary of the The workshop was structured to present the supermassive black hole at the centre workshop themes is presented. the various techniques, with extensive of the Milky Way, the first image of an reviews, and then to consider the various exoplanet and imaging of the young discs scientific fields that have profited from around stars. AO comes in many differ- The workshop brought together people these techniques: exoplanets and brown ent flavours and can be defined by the from different communities (see the con- dwarfs, stars, binaries, and large-scale total corrected field of view versus the ference photograph, Figure 1), who use phenomena. We give a very short sum- Strehl ratio which the system achieves. various techniques to construct images at mary of the material presented; the pro- These flavours include ground-layer AO very high angular resolution, with the aim gramme of the workshop is available1 and (GLAO), multi-conjugated AO (MCAO) and of reviewing these methods, the progress the presentations can be downloaded2. eXtreme AO (XAO). All major telescopes in the field, the new harvest of results that have second generation AO instruments have been collected, as well as to pre- currently in operation, which can reach pare the next generation of astronomers Beating the seeing: Reaching the typically 15–50 milliarcsecond (mas) res- to use these tools and techniques. The diffraction limit olution with high Strehl ratios and have various techniques used to beat the ten times more actuators than the first seeing and the telescope diffraction limit Atmospheric turbulence creates inhomo- generation AO systems. can be roughly distributed as follows: geneities in the refractive index of air that – single-dish imaging methods have affects the image quality — the infamous At the Very Large Telescope (VLT), the been developed to beat the seeing at “seeing”. To counter this, astronomers second generation AO system is SPHERE the focus of telescopes (e,g., adaptive have devised a series of techniques, such (an XAO system), which is beginning reg- optics, lucky/speckle/holographic as lucky imaging, speckle imaging and ular operations (see article on Science imaging, sparse aperture masking) adaptive optics. Verification by Leibundgut et al., p. 2) and aiming at sub-arcsecond spatial reso- is based on a system with 1600 actuators lution up to the diffraction limit of single Adaptive optics (AO), which has a long (see the result in Figure 2, right). A GLAO telescopes; history at ESO, aims at neutralising the system (part of the VLT Adaptive Optics – techniques that allow the diffraction atmospheric turbulence by a closed-loop Facility [AOF]) is under development at limit to be beaten in order to achieve system. As Julien Milli reminded us in ESO to assist instruments such as MUSE milliarcsecond angular resolution using his presentation, the first astronomical and Hawk-I, combining diffraction-limit direct (optical interferometry) and system ever online was Come-On at the imaging and relatively wide-field imaging indirect (e.g., spectro-astrometry) tech- Observatoire de Haute-Provence, which spectroscopy. Not least, the whole niques; and later became operational at the 3.6-metre concept of the European Extremely Large 52 The Messenger 159 – March 2015 Figure 2. Showing the NaCo L: sented in detail by Mike Ireland. In par- N progress achieved in ticular, he pointed out that there are AO: a comparative study of the disc around HR some moderate to high Strehl regimes, E 4796 A by NACO (left; where working in the Fourier plane using from Milli et al., 2014) a non-redundant mask offers improved and, more recently, with performance over standard pupil-plane SPHERE (right; from Milli et al. [2015] in prep.). analysis; the example of a putative form- ing planet in a young disc in LkCa 15 was discussed. As a complement, Makoto Uemura showed how sparse aperture N modelling can be applied, in a cross- disciplinary approach, to Very Long 1 arcsec 1 arcsec E Baseline Imaging (VLBI) images, Doppler –16 –11 –5.8–0.7 4.4 9.5 15 20 25 30 35 tomography and gamma-ray Compton imaging. In another vein, Jaeho Choi presented a non-interferometric phase- Telescope (E-ELT) requires the use of that, at the 5-metre Palomar telescope, differential imaging method that uses AO. The future lies also in the develop- AO and lucky imaging are combined, Foucault knife-edge filtering. ment of wide-field AO systems (WFAO), leading to the highest resolution image as shown by Benoît Neichel in his talk. ever taken in the visible on faint targets, By using multiple laser guide stars (LGS), with a resolution of 35 mas in the I-band Beating the diffraction limit: WFAO significantly increases the field and a Strehl ratio of 17 %. Gergely The milliarcsecond horizon of view of AO-corrected images. The first Csépány combined lucky imaging and AO such system in routine operation is GeMS, to study a sample of 38 T Tauri mul tiple In order to reach higher spatial resolu- the Gemini Multi-Conjugate AO System, systems over a 20-year period, and was tions, different techniques, such as long- which uses five LGS and provides 87 by able to cover the orbits in several of them. baseline interferometry where two or 87 arcsecond AO-corrected images in In addition, Mackay presented a proposal more telescopes are combined to obtain the H-band with a resolution of 80 mas. for a mosaic of lucky imagers for the fringes, are required. The technique was Such wide-field images with uniform NTT. This project, called GravityCam, will reviewed by Jean-Baptiste LeBouquin. point-spread functions over the full field consist of no less than 100 CCDs with The contrast (“visibility”) and phase infor- of view are particularly useful for astro- 70 mas pixels and will mainly be used for mation (“closure” or “differential phases”) metric studies, for example in clusters or exoplanet microlensing surveys, down to of the interference fringe packets can at the Centre of the Milky Way. Moreover, below an Earth mass. be used to retrieve direct information on such developments are critical, as all the the observed object brightness distri- ELTs rely on multi-LGS WFAO systems. Speckle imaging was introduced by bution. While the technique has been in Sridharan Rengaswamy, who showed the widespread use in the radio domain for Wolfgang Brandner reviewed lucky advantages of the method and its signifi- several decades, it is now also becoming imaging, which is often presented as a cant contribution to the imaging of binary a routine technique in the optical domain, poor man’s adaptive optics. It exploits stars. More details about this technique in particular at the VLT Interferometer moments of good seeing, that is, it takes is given in Rengaswamy et al. (2014). (VLTI) where the angular resolution in the many short exposures and then retains This technique was further developed, as near-infrared is of the order of a few milli- only those images that have the best speckle holography, by Rainer Schödel, arcseconds. quality (typically 10 % of the whole). The which he calls “lucky imaging on steroids” technique is still quite popular and new to reconstruct images of crowded and Two regimes of data analysis were pre- projects are being devised, especially “wide” fields of view of several tens of sented. In the first, the measurements since the availability of noise-free detec- arcseconds, while keeping the instrument are fitted with a parametric model of the tors such as L3 (Low Light Level, LLL) sensitivity. object. This is particularly well suited and extended multiplication (EM) charge when the number of measurements is coupled devices (CCDs), and the fact In sparse aperture masking (SAM; or limited, the object structure is well under- that these systems are quite affordable. non-redundant masking [NRM] depend- stood or the object is only marginally For example, the AstroLux Sur on the ing on which side of the Atlantic you resolved. This was, and still is, the bread ESO New Technology Telescope (NTT) work), a mask with a few holes is placed and butter of interferometry and is had a total hardware cost of only 50 000 in front of the AO system. This trans- applied to the measurement of precise euros. Lucky imaging is also quite useful forms the primary mirror into a separate- diameters, binary parameters or simple as it generally has fainter limiting magni- element, multiple-aperture interferometer morphological parameters; it is well tudes than most AO systems, and it can and allows diffraction-limited imaging illustrated by the spectacular case of the work in the visible where AO is not so of (bright) astrophysical targets to be expansion of a nova shell (Schaeffer et developed.
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