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The VLTI's Astronomical Multi-Beam Combiner Telescopes and Instrumentation DOI: 10.18727/0722-6691/5106 The Life and Times of AMBER: The VLTI’s Astronomical Multi-BEam combineR Willem-Jan de Wit1 instruments are scientifically inaugurated called the closure phase. The absolute Markus Wittkowski1 via their “first fringes”, which is the inter- phase of incoming light waves is scram- Frederik Rantakyrö 2 ferometric equivalent of “first light”. By bled by atmospheric turbulence, resulting Markus Schöller 1 the early 2000s, the integration of ESO’s in distortion over a pupil and global Antoine Mérand 1 interferometer into the VLT architecture phase shifts between the apertures in the Romain G. Petrov 3 was on track. array (called the piston). The degree and Gerd Weigelt 4 frequency of the scrambling increases Fabien Malbet 5 The first Paranal interference fringes were towards shorter wavelengths. As a result, Fabrizio Massi 6 produced by the VLT INterferometer the coherence time of the incoming Stefan Kraus7 Commissioning Instrument (VINCI) and wave ranges from a few milliseconds to Keiichi Ohnaka 8 MID-infrared Interferometric instrument (at best) some tens of milliseconds in the Florentin Millour 3 (MIDI), instruments that combined the optical regime. There is no way to beat Stéphane Lagarde 3 light from two telescopes. VINCI’s pur- the turbulence and recover the phase Xavier Haubois1 pose was to commission the interferome- without additional aids. When combining Pierre Bourget 1 ter’s infrastructure. MIDI, on the other three telescopes arranged in a closed Isabelle Percheron1 hand, was the first scientific instrument in triangle one can retrieve a new observa- Jean-Philippe Berger 5 operation using the VLTI in conjunction ble by adding the phases. This resulting Andrea Richichi 6 with the 8.2-metre Unit Telescopes (UTs). closure phase is invariant to atmospheric The second scientific VLTI instrument to perturbations, as the atmospheric phase arrive on Paranal was AMBER. It had noise terms from each individual tele- 1 ESO been conceived as a potential sea scope cancel out. The technique was first 2 Gemini Observatory change in optical interferometry, exploit- applied in radio interferometry. Physically, 3 Université Côte d’Azur, France ing the idea of spectro-interferometry — the closure phase quantity is a proxy 4 Max-Planck-Institut für Radioastrono- obtaining spatial information on milliarc- for the degree of asymmetry in the sci- mie, Bonn, Germany second scales at high spectral resolution. ence target. Closure phase information 5 Institut de Planétologie et d’Astrophy- It comprised three spectral settings, is a pre-requisite to reconstructing sique de Grenoble, France including a high spectral resolution of R = images from interferometric observables 6 INAF–Osservatorio Astrofisico di Arcetri, 12 000, and was foreseen to work at a (for example, Jennison, 1958; Baldwin Italy high sensitivity and with high visibility et al., 1996) and AMBER was the first 7 University of Exeter, UK accuracy in three infrared atmospheric instrument at the VLTI to deliver it. 8 Universidad Católica del Norte, Chile windows (J-, H-, and K-bands). Yet, argu- ably its most important asset was the AMBER produced clear first fringes of capacity to combine the light beams from the star θ Centauri on the night of 20 The sharpest images on Paranal are three separate telescopes at long base- March 2004 using two telescopes at a produced by the beam-combining lines, a novelty in long-baseline optical baseline of 64 metres, marking a mile- instruments of the Very Large Tele- interferometry which allowed millarcsec- stone after seven years of work. The scope Interferometer (VLTI). Currently, ond-resolution images to be synthesised instrument was offered to the community the VLTI is close to completing a tran- at high spectral resolution. for the first time in observing period 76 sitional period, moving away from the (starting October 2005), fed by the large first generation of instruments (AMBER, The consortium of four institutes driving apertures of the UTs. MIDI) and offering new instruments the AMBER project consisted of the and subsystems to the community. In Observatoire de la Côte d’Azur (OCA: the this article, we report on the life and Principal Investigator institute) in Nice, the Optical principle and early years achievements of the recently decom- Laboratoire d’Astrophysique de l’Observ- missioned, near-infrared beam com- atoire de Grenoble (LAOG at the time, AMBER’s design corresponds broadly to biner instrument AMBER, the most pro- now called IPAG), the Max-Planck-Institut an optical configuration similar to the one lific optical interferometric instrument für Radioastronomie (MPIfR) in Bonn, and that creates fringe patterns in a Young’s to date. the Osservatorio Astrofisico di Arcetri in interference experiment, i.e., overlapping Florence. It built on the European exper- images coming from multiple telescopes tise of designing two-telescope combin- (or beams). Most importantly, before AMBER, a three-telescope combiner ers capable of exploiting spectro-inter- the light is recombined, each light beam ferometry and the usage of single-mode is guided through a single-mode fibre. AMBER was one of three ambitious, fibres. Conceptually, to advance from A single-mode fibre acts as a spatial general-user, interferometric instruments two-telescope to three-telescope com- filter and rejects the distorted part of the proposed in 1997 for implementation on biners may seem a small step, but scien- wavefront, leading to a flattened exit the VLTI at Paranal (Paresce et al., 1996), tifically it constituted a leap forward. wavefront. The phase fluctuations are following the recommendations of the traded against fast intensity fluctuations Interferometry Science Advisory Commit- The crucial consideration is to provide (which are recorded) and a global piston tee to ESO. In optical interferometry, new access to the observational quantity (which is measured from the slope of 8 The Messenger 174 – December 2018 the dispersed fringes). Hence, AMBER AMBER consortium in 2007–2008, and (April 2009) standard AMBER operations implements three photometric channels through the continuous improvement of for medium and high spectral resolution for the simultaneous monitoring of the the VLTI infrastructure. were done in conjunction and simultane- beam intensity for each telescope beam. ously with FINITO. Since 2011, the FINITO Recombination, and with it the produc- A report analysing the accuracy in data have been delivered alongside the tion of fringe patterns, is done after absolute visibility, closure phase and dif- AMBER ones for optimised data reduc- forming three exit pupils. The exit pupils ferential phase identified critical software tion and post-processing purposes. With are physically placed in a non-redundant and hardware improvements required the advent of GRAVITY fringe tracking manner such that the set of three con- by AMBER (Malbet et al., 2008). The main has become an integral part of the sci- tained spatial frequencies in the final modifications in AMBER were the ence observations and the data from the interferometric image are fixed (i.e., non- replacement of its polarisation filters fringe tracker are used in the data reduc- homothetic) but different and identifiable. which were responsible for parasitic tion process. The four beams — three intensity moni- Fabry-Perot fringes in all the spectro-in- toring beams and the one interferometric terferometric measurements, and Continual enhancements of AMBER and beam containing all the information for improvements in its operation and main- the VLTI resulted in steady improvements the three baselines — are then spectrally tainability. On the VLTI, after a significant of the limiting magnitude and operational dispersed before detection (Petrov et al., improvement in the delay line models, a efficiency. AMBER’s self- coherencing 2007). continuous effort resulted in the progres- was introduced in April 2012 for the low- sive reduction of the vibrations in the resolution setup. This mode allowed auto- The integration of AMBER operations into coudé trains of the UTs. A decisive factor matic real-time fringe centring at a rela- the complex VLTI and telescope architec- was the implementation of a faster loop tively low cadence when fringe tracking ture was an iterative process (see Mérand to counter the flux dropouts seen in the with FINITO was not possible (for exam- et al., 2014). For example, operations instrument – a higher correction rate was ple, because of seeing conditions or low began with the UTs equipped with the made possible by offloading the meas- source flux). The instrument intervention Multiple Application Curvature Adaptive ured IRIS tip-tilt to the feeding optics of performed at the end of 2012 changed Optics (MACAO) guiding systems, before AMBER. AMBER’s performance could the spectrograph beamsplitter that the arrival of the versatile Auxiliary be further improved thanks to the arrival caused internal reflections; this resulted Telescopes (ATs). The ATs were commis- of the Fringe-tracking Instrument of NIce in almost a 30% improvement in through- sioned in the summer of 2006 and first and TOrino (FINITO) (Haguenauer et al., put in the interferometric channel. Better offered in April 2007 with a limited set 2008). polarisation control, by means of birefrin- of baselines. On the VLTI side, the injec- gent lithium niobate (LiNbO3) plates, was tion of the light into the instrument’s introduced in October 2014, following an single-mode fibres was optimised by Arrival of FINITO earlier implementation in the Precision controlling the tunnel tip-tilt inside the Integrated Optics Near-infrared Imaging VLTI laboratory using the InfraRed Image The art of fringe tracking was introduced ExpeRiment (PIONIER). Such plates allow Sensor (IRIS). This sensing sub-system into AMBER operations for the Period 80 the equalisation of the phase difference was operated from 2006 onwards but call for proposals in October 2007. The between the two polarisation stages and using the telescopic XY table as a correc- purpose of fringe tracking is to nullify the add them incoherently, improving the tive system.
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