Messenger no143


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Messenger no143

  1. 1. Brasil to join ESOThe 2nd generation VLTI instrument GRAVITYSpectroscopy of planet-forming discsLarge Lyman-break galaxy survey The Messenger No. 143 – March 2011
  2. 2. The OrganisationAdriaan Blaauw, 1914–2010In the last issue of The Messenger There follow three tributes to Adriaan Pottasch; and by Raymond Wilson, who(142, p. 51) only a brief obituary of Adriaan Blaauw: by Tim de Zeeuw, current led the Optics Group during his tenure asBlaauw, the second Director General ESO Director General; by his long-term Director General.of ESO, could be included at the time of colleague at the Kapteyn Institute, Stuartgoing to press.Tim de Zeeuw1 but moved to Yerkes Observatory in ESO [M] 1953, becoming its associate director in 1956, and moved back to Groningen1 ESO in 1957, where he was in a key position to contribute to transforming the idea of Baade and Oort into reality. He was Sec-Professor Adriaan Blaauw, ESO’s sec- retary of the ESO Committee (the proto-ond Director General and one of the Council) from 1959 through 1963, amost influential astronomers of the twen- period which included the signing of thetieth century, passed away on 1 Decem- ESO Convention on 5 October 1962.ber 2010. Blaauw became ESO’s Scientific Director in 1968. In this position he also pro-Adriaan Blaauw was born in Amsterdam, vided the decisive push which led to thethe Netherlands, on 12 April 1914. He creation of Astronomy and Astrophysics,studied astronomy at Leiden University, which successfully combined andunder de Sitter, Hertzsprung and Oort, replaced the various individual nationaland obtained his doctorate (cum laude) journals for astronomy, and today iswith van Rhijn at the Kapteyn Laboratory one of the leading astronomy researchin Groningen in 1946. His PhD thesis publications in the world. The articlewas entitled “A study of the Scorpio– by Pottasch (1994) and the following trib-Centaurus Cluster”. During his career, ute provide further details of Blaauw’sBlaauw became renowned for his ground- creative leadership in the founding of thebreaking studies of the properties of European astronomical journal.OB associations (groups of young, hotstars) which contain the fossil imprint Blaauw was Director General from 1970 Figure 1. Adriaan Blaauw in 1973 while Director Gen- through 1974. During this period several eral of ESO. From a photograph taken during a con-of their star formation history. Perhaps his tract-signing ceremony for building works at La Silla.most famous work explained why some telescopes, including the ESO 0.5-metreOB stars are found in isolation travelling and 1-metre Schmidt telescopes, beganat unusually high velocity: the so-called operating at ESO’s first observatory The Messenger may serve to give the“run-away stars”. Blaauw proposed in site, La Silla, in Chile, and much work world outside some impression of what1961 that these stars had originally been was done on the design and construction happens inside ESO.” The continuingmembers of binary systems, and when of the ESO 3.6-metre telescope, which popularity of The Messenger is a testi-one star in the binary experiences a had its first light in 1976. Blaauw decided mony to Blaauw’s foresight.supernova explosion, its companion sud- that it was crucial for this project to movedenly ceases to feel the gravitational ESO’s Headquarters and the Technical After stepping down as Director Generalpull that keeps it in its orbit and hence it Department from Hamburg to Geneva, to of ESO, Blaauw returned to Leiden,“runs away” at its orbital velocity. benefit from the presence of the experi- where I had the privilege to be amongst enced CERN engineering group. He also his students. He continued to play aIn addition to his distinguished research oversaw the development of the Proto- very important role in international astron-career, Blaauw played a central role in col for Privileges and Immunities that is omy. He was President of the Interna-the creation of ESO. In 1953, Baade and critical for ESO’s functioning. In May tional Astronomical Union from 1976 toOort proposed the idea of combining 1974 he launched The Messenger with 1979, during which period he used hisEuropean resources to create an astro- the stated goal: “to promote the partici- considerable diplomatic skills to convincenomical research organisation that pation of ESO staff in what goes on in China to rejoin the IAU. From 1979 tocould compete in the international arena. the organisation, especially at places of 1982 he served on the ESO Council onBlaauw had returned to Leiden in 1948, duty other than our own. Moreover, behalf of the Netherlands. He retired from2 The Messenger 143 – March 2011
  3. 3. his Leiden professorship in 1981 and ESO’s early history with some of us of science, honorary doctorates frommoved back to Groningen, but stayed (see the photograph in The Messenger, the University of Besancon and fromactive in various areas. This included 137, p. 6). During this visit he revealed his l’Observatoire de Paris and, like his pre-organising the historical archives of ESO wish to visit Chile one more time if his decessor as ESO Director General,and of the IAU — work which resulted in health would allow this. It was a pleasure Otto Heckman, the Bruce Medal of thetwo books, ESO’s Early History (Blaauw, to organise this trip in February 2010. Astronomical Society of the Pacific. He1991) and History of the IAU (Blaauw, He met ESO “legends” Albert Bosker, Jan was well known for his warm personality,1994). He also served as Chairman of Doornenbal, Erich Schumann and Daniel wisdom, humour, legendary patience,the Scientific Evaluation Committee for Hofstadt and was driven to La Silla and and the rare gift of being able to slowthe European Space Agency satellite Paranal by car to enjoy Chile’s beautiful down when the pressure mounted. TheHIPPARCOS, advising on many aspects landscapes. He characteristically engaged personal account of his life, entitledof its scientific programme. When the young people at the telescopes and “My Cruise Through the World of Astron-data became available in 1996, he was in Vitacura in interesting discussions and omy”, published in the 2004 Annualactively involved in the re-analysis of the throughout the visit displayed a crystal- Reviews of Astronomy and Astrophysicsyoung stellar groups that he had studied clear perspective on the development of (Blaauw, 2004), provides an accuratefirst during his PhD research. ESO and on the exciting opportunities and inspiring picture of a truly remarkable for the future programme (a photograph person, who positively influenced theBlaauw remained keenly interested in of this visit is shown in The Messenger, lives of many.developments at ESO. After a discussion 139, p. 61). The characteristic twinkle inwith him in late 2008, he drove himself his eye was as bright as Garching and back in July 2009 in Referencesorder to take another look at the historical Blaauw won many academic distinctions, Blaauw, A. 2004, ARAA, 42, 1documents in the library and to discuss including membership of many academies Pottasch, S. R. 1994, The Messenger, 76, 62Stuart Pottasch1 tronomers of PhD level or higher, with the This is where Adriaan, who was at that result that 75 % of those present agreed time Scientific Director of ESO, came that a new journal was desirable. Simi- in. He suggested, organised and imple-1 Kapteyn Laboratorium, Groningen, lar meetings took place at a somewhat mented a legal status for the new jour- the Netherlands higher level in other countries. At this nal. The basic idea was that ESO would point there was much enthusiasm to begin make use of the fact that it was an offi- a new journal. This led to a meeting of cial European organisation. Its adminis-Adriaan has contributed to many fields of European astronomers on 8 April 1968. trative and legal services were madeastronomy. In the long years we have available to the journal through a formalknown and worked with each other there In spite of the enthusiasm for the Euro- agreement between ESO and the Boardare two aspects that may be less well pean astronomical journal, there were of Directors of the journal. This agree-known and that I would like to highlight. rather difficult problems ahead. These ment was confirmed at the December problems were of a practical nature and 1968 ESO Council meeting, just beforeFirst of all is the deep interest he took in arose because the new journal was to the first issue of the new journal As­the formation of the European journal be a combination of journals published in tronomy and Astrophysics appeared inAstronomy and Astrophysics. Adriaan various European countries. The indi- January 1969. Individual countriestook part in the initial discussions, which vidual journals all had a rather different could now contribute financially to thefirst began to take real shape in 1967 status. Some were owned by private journal, but ESO itself would carry noand especially in 1968. The discussions publishers, some by astronomical organi- financial responsibility for the journal. Atin 1967 took place in several European sations. The French journals were owned the same time the Board would be en-countries. At first they were independent by the ministry in France, which could tirely independent of any influence fromof each other and took place because not contribute financially to a European the ESO side on its scientific policy.of a general feeling in Europe that existing journal without an official treaty betweenEuropean astronomical journals were various countries. The timescale for But this did not end Adriaan’s connec-not being read to the same extent as the such a treaty, essentially the creation of tion with the new journal. He acceptedAmerican journals. In December 1967 an international organisation, was ex- an invitation to become a member ofa meeting took place in France which pected to be long, and the discussions the Board of Directors and was in factwas attended by almost all French as- complicated. elected chairman of that body. The The Messenger 143 – March 2011 3
  4. 4. The Organisation de Zeeuw T., Pottasch S., Wilson R., Adriaan Blaauw, 1914–2010importance of this can be seen in the fact where Adriaan was able to reconcile the to combine his scientific curiosity withthat the journal at the time was more differences. He was chairman of the A&A various administrative responsibilitiesturbulent than it is at present. Not only Board for about ten years. without letting the one cloud out thewere there more disputes between indi- other. I think that he was able to do thisvidual scientists, there were also dis- A second aspect of Adriaan’s career because he approached science inputes between different countries, espe- that is worth highlighting can be stated an unhurried and patient way. Astron-cially about the refereeing. Some of more simply. He remained an active omy interested him; there was alwaysthese disputes were brought to the Board scientist for his whole life, and was able time for it.Raymond Wilson1 building on the CERN campus. The total would have been no active optics at ESO staff in this fledgling technical division and, consequently, no NTT, VLT or E-ELT of ESO cannot have numbered more than project. The readers of this tribute will1 Rohrbach/Ilm, Germany ten or twelve. understand, I am sure, why I hold Adriaan Blaauw in such high esteem. A major contractual problem now emerged.It is an honour and a pleasure to write a I had clearly understood, from Blaauw’s Finally, there was another aspect oftribute to Adriaan Blaauw, whom I con- interview with me, that I would be the his leadership which I greatly admired.sider to be an underrated Director Gen- leader of a newly-founded Optics Group, Once settled in with my new Opticseral of ESO, above all through being in dealing with all optical aspects of tele- Group, things were going quite well forthe long shadow thrown by his successor scopes (at that time, mainly the 3.6-metre me and I was elected to be Staff Rep-Lodewijk Woltjer. telescope) and instrumentation. How- resentative. In Blaauw’s weekly one-day ever, in the technical group, led by Svend visits to Geneva, I was always the firstI am unable to make any comments Laustsen, the responsibility for telescope person he visited. But he was not con-regarding his achievements in the astro- optics was in the hands of a German cerned about my technical function,nomical field. I am only going to comment astronomer, Alfred Behr, and for instru- which we had organised: he left that toon my personal experience of his work mentation optics in the hands of Anders Laustsen, who had, of course, acceptedas ESO Director General, above all at the Reiz, a Danish astronomer. My role in the new Optics Group, in which Behr’stime when I was engaged by him per- this existing structure appeared only to work was now integrated under my lead-sonally to create and head a new Optics be that of a senior assistant to them, ership. No, he visited me first as StaffGroup on the technical side of ESO’s above all to Alfred Behr. This situation Representative to ask if the staff wereactivities. At this time, his office was still was unacceptable to me and not as I had content or whether there were any prob-in Hamburg, where ESO was founded, understood the scope of the position I lems where he should intervene. Thisabove all, by Professor Otto Heckman, had accepted. proves again his absolutely fair and hu-for the 3.6-metre telescope project. This mane leadership!project was intended to bring ESO up to Blaauw normally only came to Genevathe level of the American telescopes with, for one day a week. However, when I Adriaan Blaauw was not only a great ESOat that time, one of the larger telescopes rang him up and explained the gravity of Director General, he was also an admi-built in the post-Palomar (5-metre) era. the situation and the inevitability of my rable gentleman of impeccable integrity. leaving ESO immediately if he could notI left the firm of Carl Zeiss to go to ESO rectify it, he came at once and we dis-in 1972, when Zeiss, at the time of a cussed the matter over another goodserious recession in German industry, lunch. I emphasised my clear position onstarted laying off staff, including those of the matter and that I would try to returnmy own Optical Design department, to Zeiss immediately, in spite of the badwhere I had conceived my idea of active situation there. Blaauw recognised thatoptics. Professor Blaauw interviewed I was very serious and stated he wouldme over a good lunch in Geneva. He inform Laustsen at once that a new Opticsimmediately offered me a senior position Group would immediately be foundedat ESO in Geneva, where, through his under my leadership. Without this boldinitiative, ESO had a small barrack-type and clear direction by Blaauw there4 The Messenger 143 – March 2011
  5. 5. The OrganisationBrazil to Join ESOTim de Zeeuw1 now be submitted to the Brazilian Par- nities for Brazilian high-tech industry to liament for ratification. The signing of contribute to the ESO programme, in- the agreement followed its unanimous cluding the European Extremely Large1 ESO approval by the ESO Council during an Telescope project. It will also bring extraordinary meeting, by teleconference, new resources and skills to the organi- on 21 December 2010. sation at the right time for them to makeOn 29 December 2010, at a ceremony a major contribution to this exciting pro-in Brasilia, the Brazilian Minister of Sci- “Joining ESO will give new impetus to the ject,” added Tim de Zeeuw.ence and Technology, Sergio Machado development of science, technology andRezende and the ESO Director General, innovation in Brazil as part of the consid- The president of ESO’s governing body,Tim de Zeeuw signed the formal acces- erable efforts our government is making the Council, Laurent Vigroux, concluded:sion agreement, paving the way for Brazil to keep the country advancing in these “Astronomers in Brazil will benefit fromto become a Member State of the Euro- strategic areas,” said Minister Rezende. collaborating with European colleagues,pean Southern Observatory. Brazil will and naturally from having observing timebecome the fifteenth Member State and “The membership of Brazil will give the at ESO’s world-class observatories atthe first from outside Europe. Since the vibrant Brazilian astronomical community La Silla, Paranal and APEX at Chajnantor,agreement implies accession to an inter- full access to the most productive obser- as well as on ALMA, which ESO is con-national convention, the agreement must vatory in the world and open up opportu- structing with its international partners.”Figure 1. ESO Director General, Tim de Zeeuw,(right) in discussion with the Brazilian Ministerof Science and Technology, Sergio MachadoRezende, during the accession ceremony inBrasilia on 29 December 2010. The Messenger 143 – March 2011 5
  6. 6. Telescopes and Instrumentation Fisheye image of the interior of the dome for VLT UT4 Yepun. See potw1049 for details. HH 30 2MASSWJ1207334-393254 778 mas 55 AU at 70pc N 200 AU E Detection of intermediate Ten year large mass BH in GCs/Arches programme Orbit of exo- Stellar motions in nuclei Jupiter/Uranus Detection of SR/GR effects of nearby galaxies in cusp star orbits Three year large Astrometric signal Detection of dark halo around SgrA* programme exo-Jupiter/Uranus 3D dynamics of nuclear star cluster Evolution outflows in Gas flows in AGN YSOs & micro-QSOs SgrA* flare dynamics Single season Proper motions massive star cluster campaign Imaging jets/discs in YSOs & CBs Binary dynamics Lensing 10 0 10 2 10 4 10 6 Maximum distance from Earth (pc) Key experiments with GRAVITY are illustrated (see article by Eisenhauer et al. p. 16). Clockwise from S27 S31 S19 S12 top left are: jet/discs in a nearby star-forming S29 region; planet-brown dwarf binary; dust disc with S5 S14 S17 S4 S2 central gap; Arches star cluster; M31 star discs; S6 NGC 1068 outflow/narrow line region; modelling of S39 a Galactic Centre flare; radial precession of stellar S21 orbits; S-star orbits; nuclear star cluster and radio S1 S13 S18 emission in the Galactic Centre. In the central S8 S33 inset the horizontal axis denotes the maximum S9 S24 distance from Earth, the vertical axis the time span of the measurements. 6 The Messenger 143 – March 2011
  7. 7. Telescopes and InstrumentationHARPSpol — The New Polarimetric Mode for HARPSNikolai Piskunov1 and linear polarisations across their pro- This sets very stringent limits on the di-Frans Snik 2 files. For non-degenerate objects, the mensions of the polarimeter, because itAndrey Dolgopolov 3 continuum is mostly unpolarised, which needs to fit in between various mecha-Oleg Kochukhov1 offers a reliable intrinsic calibration that is nisms (calibration light feeds, calibrationMichiel Rodenhuis 2 necessary for measuring very weak fields, mirror and fibre cover) filling the adapter.Jeff Valenti 4 but such measurements require a very The polarimeter consists of the enclo-Sandra Jeffers 2 stable spectropolarimetric instrument. sure hosting a precision horizontal slider.Vitaly Makaganiuk1 The slider holds two identical opticalChristopher Johns-Krull 5 The HARPS spectrograph at ESO’s tables installed perpendicular to the slid-Eric Stempels1 3.6-metre telescope at La Silla is one of ing direction. Each optical table containsChristoph Keller 2 the most successful spectroscopic as- a full set of polarisation optics (Figure 1), tronomical instruments ever built (Mayor separating the incoming light into two et al., 2003). The exceptional temporal beams. Since the polarising beam-splitter1 Department of Physics and Astronomy, and spatial stability of HARPS makes position is fixed relative to the fibres, the Uppsala University, Sweden it an ideal instrument for spectropolarim- polarisation of the incoming light needs2 Sterrekundig Instituut Utrecht, Utrecht etry. The new polarimeter takes full ad- to be converted to the frame of the beam- University, the Netherlands vantage of the two optical fibres to bring splitter. This is achieved by rotating wave3 Crimean Astrophysical Observatory, the collected light, split into two orthogo- plates in front of the beam-splitters: a Crimea, Ukraine nal polarisations, from the Cassegrain half-wave plate for the linear polarimeter4 STScI, Baltimore, USA focus of the 3.6-metre telescope to the and a quarter-wave plate for the circular5 Rice University, Houston, USA HARPS spectrograph. Analysing polari- one. The relative intensity of the two sations at the Cassegrain focus mini- beams at each wavelength carries the mises the influence of instrumentation on information about the polarisation of theThe HARPS spectrograph can now the measurements. The new module, light.perform a full polarisation analysis of called HARPSpol, allows sensitive andspectra. It has been equipped with accurate measurements of both circular The polarising beam-splitters consista polarimetric unit, HARPSpol, which and linear polarisations of stellar light of a Foster prism (a modified Glan–was jointly designed and produced as a function of wavelength, at high spec- Thompson polariser). The primary beamby Uppsala, Utrecht and Rice Univer- tral resolution. In this article we give a suffers from crystal astigmatism, whichsities and by the STScI. Here we pre- short presentation of the polarimeter and is corrected by a cylindrical lens. Thesent the new instrument, demonstrate show some results from the first year of secondary beam is deviated by 45º.its polarisation capabilities and show operation. Beam-channelling prisms align the opti-the first scientific results. cal axis and the focus of the secondary beam with the second HARPS fibre. HARPSpol — What’s inside the box? The selected optical scheme solves twoIntroduction HARPSpol is installed inside the Casse- Figure 1. Schematic of the HARPSpol optical design.Spectropolarimetry is one of a very few grain adapter, located directly below the Left: the view in the sliding direction. Right: side viewdirect ways of detecting and studying primary mirror of the 3.6-metre telescope. of the two polarimeters.magnetic fields. Magnetic fields are pre-sumed to play crucial roles in all kindsof objects and environments in space,stirring turbulence, transporting angularmomentum, converting kinetic energyto radiation, controlling plasma motion,etc. Magnetic fields create polarisation inspectral lines though the Zeeman effect,and thus polarisation measurementsallow us to measure the strength and theorientation of the field vector, providingimportant clues for understanding starformation, the origin of structures in stel-lar atmospheres and stellar activity. Infact, the origin and the evolution of mag-netic fields remains one of the mostimportant topics in modern astrophysics.Spectral lines formed in the presence ofa magnetic field generally exhibit circular The Messenger 143 – March 2011 7
  8. 8. Telescopes and Instrumentation Piskunov N. et al., HARPSpol — The New Polarimetric Mode for HARPSdifficulties: (1) it is highly achromatic, Figure 2. HARPSpol is shown during installa-that is, the image of a star after projection tion. The HARPSpolthrough HARPSpol is essentially the enclosure is on the right.same in the red and in the blue parts of The slider is in the linearthe spectrum; and (2) slight errors in polarisation position. The half-wave plate forpositioning of the slider do not affect the the linear polarimeteroptical/polarisation performance. More is visible in the middle ofinformation about the optical design the picture. The roundof HARPSpol can be found in Snik et al. mirror below the linear polarimeter is one of the(2008, 2010). HARPS fibre heads.The selected wave plates are super-achromatic. They consist of five layers ofbirefringent polymer. This makes thepolarimeters suitable for the entire HARPSwavelength range (380–690 nm) withoutintroducing (polarised) fringes. The simul-taneous measurements in two polari-sation directions, together with the polari-sation modulation by the wave plates,renders the polarimetry with HARPSpolto first order insensitive to seeing andfibre/spectrograph throughput (Semel etal., 1993; Bagnulo et al., 2009).IntegrationOnce installed at the Cassegrain adapter,HARPSpol was integrated with the HARPSinstrument control electronics and soft-ware. When inserted into the optical path,HARPSpol shifts the focus of the tele-scope by approximately 2 mm, which iscompensated for by moving the sec-ondary mirror. Figure 2 shows HARPSpolinstalled inside the Cassegrain adapter.Spectropolarimetry is performed by Figure 3. The total throughput from the telescope to the detector with and without HARPSpol is shown.selecting the corresponding template(s) The sharp drops are not real: they are due to hydro-in the observing software. Calibration and gen lines that are treated differently in spectro-science templates are available for cir- photometry and spectropolarimetry.cular and linear polarimetry. The scienceFigure 4. The combined averageprofile for intensity and polarisation(lower and middle plots) for α Cen A.Left panel shows circular polarisationmeasurements (Stokes parameter V).Middle and right panels are for linearpolarisations. The null profile is shownuppermost. ∆ ∆ ∆8 The Messenger 143 – March 2011
  9. 9. Figure 5. Comparison of the Stokes spectra of a standard magnetic star γ Equ taken at the CFHT with the ESPANDONS spectropolarimeter (red line) and with HARPSpol (black line) is shown. The ESPADONS spectra were taken as part of CFHT’s calibration and engineering plan, and were retrieved from the Canadian Astron- omy Data Centre. The visible differ- ences are mostly due to the higher resolving power of HARPS. Figure 6. One of the HARPS polarisa- tion spectra of a CP star, HD 24712, is shown. Both circular and linear polarisations are detected for practi- cally every spectral line.template allows multiple exposures to to the lower throughput of the “sky fibre” plot in each panel of Figure 4 shows thebe taken in the selected mode (circular (used to carry one of the polarised so-called null spectrum, obtained byor linear) for a sequence of wave-plate beams), but still sufficient to reach rather modifying the analysis in such a way asangles. The full complement of polarisa- faint targets. to destroy the polarisation signal in thetion characteristics can be registered incoming light (Bagnulo et al., 2009).in six or twelve exposures, with the latter Systematic errors limit both the polari- What remains reflects the spurious polari-offering intrinsic control over spurious metric sensitivity and the accuracy. The sation induced inside the instrumentationpolarisation signals. The HARPSpol pipe- sensitivity is the weakest polarisation or by the data reduction.line then processes the data and the detectable with HARPSpol. After accu-final products include the Stokes param- mulating enough photons we expect We do not expect any detectable polari-eters as a function of wavelength. to see spurious polarisation present in sation signal from α Cen A and Figure 4 the light coming to the telescope. We test shows that our new instrument does not this by observing a bright source and detect or induce any polarisation aboveHARPSpol: Performance collecting many photons in a series of the level of 10 –5, which is on a par with many short exposures. Figure 4 shows the best solar polarimeters like ZIMPOLDuring commissioning we have meas- the results of the test for an inactive solar- (Ramelli et al., 2010). The accuracy (theured several characteristics of HARPSpol. type star, α Cen A, where we reach the level at which the HARPSpol measure-The most important ones for the ob- median signal-to-noise ratio of 2 400 per ments match the true polarisation signal)server are the total throughput of the sys- CCD column. Besides combining multi- is assessed by observing objects withtem and the polarimetric sensitivity. The ple exposures we also derive the mean known polarisation spectra. Our observa-throughput (Figure 3) was measured Stokes profiles using the least squares tions of γ Equ demonstrate the highby observing spectrophotometric stand- deconvolution (LSD) technique (Donati et accuracy of HARPSpol. γ Equ is a well-ards, reducing the data, rebinning it al., 1997; Kochukhov et al., 2010), which studied magnetic star showing linear andto match the resolution of the spectro- takes advantage of the fact that most circular polarisations. The lack of notice-photometry and deriving the sensitivity of the spectral lines are affected by mag- able rotation makes γ Equ an excellentcurves for each fibre. The total efficiency netic fields in a similar way. This increases polarisation standard. Figure 5 shows thewith HARPSpol is somewhat lower due the signal-to-noise even further. The top comparison of the HARPSpol polarisation The Messenger 143 – March 2011 9
  10. 10. Telescopes and Instrumentation Piskunov N. et al., HARPSpol — The New Polarimetric Mode for HARPS Figure 7. Spectropolarimetry of a K2 dwarf planet- hosting star ε Eri taken with HARPSpol. Circular polarisation profiles (left) are marked with observa- tion times in days. Derived line-of-sight field strength and uncertainty in Gauss are shown against time (in Julian Day) on the right. ∆ ∆spectra of this star with those taken Another example is a chromospherically a pipeline producing science-grade datawith the ESPADONS spectropolarimeter active cool dwarf ε Eri. This nearby star products. The tests and applications(Donati et al., 2006) at the Canada harbours at least two planets and a dust to various types of objects have demon-France Hawaii Telescope (CFHT) with a belt in orbit around it. Polarisation meas- strated high sensitivity and a low levelresolving power of 67 000. urements of stars hosting planets may of systematic effects, making HARPSpol provide an important check for the pres- an ideal tool for detecting and studying ence of starspots that can mimic radial weak magnetic fields, reconstructing fieldHARPSpol: First results velocity variations. Detection of polarisa- topology and many other magnetic phe- tion can reveal signatures of star–planet nomena.One of the obvious applications of magnetic interactions. Our polarisationHARPSpol is in the study of the topology measurements for ε Eri are presentedof magnetic fields on chemically pecu- in Figure 7. Again, we applied the LSD Referencesliar (CP) stars. The goal is to understand technique to enhance the signal-to-noise Bagnulo, S. et al. 2009, PASP, 121, 993the relationship between the field geome- ratio and we see an unambiguous sig- Donati, J.-F. et al. 1997, MNRAS, 291, 658try and the surface/depth distribution nal in circular polarisation. A simplistic Donati, J.-F. et al. 2006, Solar Polarization 4,of chemical elements. This task requires interpretation with a longitudinal field ASP Conf. Series, 358, 362 Kochukhov, O. et al. 2010, A&A, 524, 5a series of observations well spread geometry shows field strength changing Kotov, V. A. et al. 1998, ApJ, 116, 103over the rotation period so as to see all from – 5.8 to +4.7 Gauss with median Mayor, M. et al. 2003, The Messenger, 114, 20visible parts of the stellar surface. Figure uncertainty of 0.1 Gauss! These values Ramelli, R. et al. 2010, SPIE, 7735, 16 shows an example of one measure- are comparable to the disc-averaged Semel, M. et al. 1993, A&A, 278, 231 Snik, F. et al. 2008, SPIE, 7014, 22ment in such a series for a cool magnetic magnetic field of the Sun (Kotov et al., Snik, F. et al. 2010, arXiv: 1010.0397CP star HD 24712. Circular and linear 1998).polarisation were detected in all 13 phasescovering the whole stellar rotation (badweather prevented the collection of one Prospectsset of circular polarisation data) and onecan easily follow the evolution of polari- HARPSpol adds powerful polarimetricsation spectra with stellar rotation. The capabilities to the suite of ESO high-low level of the noise makes the data resolution spectroscopic instruments. Itquite adequate for reconstructing the field is fully integrated into the ESO opera-topology. tional environment and is equipped with10 The Messenger 143 – March 2011
  11. 11. Telescopes and InstrumentationTests of Radiometric Phase Correction with ALMABojan Nikolic1 than temperature fluctuations. ALMA is observed astronomical data can be cor-John Richer1 attempting to correct the effects of these rected for the effect of path fluctuations.Rosie Bolton1 fluctuations through a combination ofRichard Hills 2 two techniques: frequent observations of calibration sources; and direct measure- Water vapour radiometers ment of atmospheric properties along1 Astrophysics Group, Cavendish the line of sight of each of the 54 12-metre The water vapour radiometers (WVRs) Laboratory, University of Cambridge, diameter telescopes using mm-wave are the devices that measure accu- United Kingdom radiometers that measure emission of the rately the absolute brightness of down-2 Joint ALMA Observatory, Santiago, 183 GHz water vapour line. ALMA is welling radiation along the lines of sight of Chile the first telescope to employ phase cor- the antennas. The prototype WVRs for rection based on mm-wave water vapour ALMA were developed by a collaboration radiometers. between the University of CambridgeOf the many challenges facing ALMA, and Onsala Space Observatory. Afterone of the greatest is overcoming Water in the atmosphere is poorly mixed successful laboratory and field testing ofthe natural seeing limit set by the atmos- and the concentration (and phase) of the prototypes, an industrial partnerphere to achieve very high resolution water varies rapidly with position in the (Omnisys Instruments AB, Sweden) wasimages. Its longest antenna separations atmosphere and with time. The underly- contracted for delivery of the produc-(baselines) permit ALMA to synthesise ing reason for this is of course that all tion units. The production stage is nowthe effect of a single antenna with a three phases of water are accessible in already fully complete and ALMA hasdiameter exceeding 15 km, but an ac­ the range of temperatures and pres- taken delivery of radiometers for all of thecurate radio “adaptive optics” system sures typical on the ground and in the planned 54 12-metre required to ensure ALMA’s images atmosphere, leading to various localisedare diffraction limited. With initial test sources and sinks of water vapour. The ALMA radiometers are uniquedata now available from the first ALMA Even at a very high and dry site like among the radiometers used for phaseantennas in Chile, we describe current ALMA, changes of up to 50 % in line-of- correction in that they measure skyprogress towards this goal. sight water vapour can be observed brightness around 183 GHz, as opposed in a matter of minutes. Additionally, water to 22 GHz, which is the spectral region vapour has a high effective refractive where most other WVR systems areAtmospheric limitations to radio index at mm and sub-mm wavelengths: designed to observe. This has a numberastronomy one millimetre of precipitable water of advantages, primarily based on the vapour retards radiation by an equivalent very high strength of the water vapourALMA aims to synthesise an antenna of about seven millimetres of path in line at 183 GHz (see Figure 1 for plots ofwith an effective diameter of over 15 km: vacuum. The combination of poor mixing brightness in typical conditions), whichthis would have a diffraction-limited and high refractive index leads to a is about 150 times stronger than the lineresolution of 15 milliarcseconds at a fre- corruption of the wavefront of incoming at 22 GHz. This means that fluctuationsquency of 300 GHz. (Note, however, astronomical radiation. When observing in water vapour content produce muchthat for most projects with ALMA, we with an aperture synthesis array like higher, more readily observed fluctuationsanticipate that a more modest resolution ALMA, these wavefront errors lead to in the observed brightness at this fre-of 50–100 milliarcseconds will be re- phase errors in the recorded visibilities. quency. Besides this, the high strengthquested by scientists.) In comparison, the of the line means that radiation fromuncorrected radio seeing at this fre- In order to correct for these errors, each sources other than atmospheric waterquency would typically limit the resolution of ALMA’s 12-metre diameter antennas vapour has a smaller influence on theof images to 700 milliarcseconds if no has an accurate millimetre-wave radiome- predicted phase corrections. For exam-adaptive optics corrections were applied ter that measures the radiation pas- ple, clouds, spill-over past the primary(see Evans et al., 2003). sively emitted by water molecules in the reflector of the antenna and man-made atmosphere along the line of sight of radio frequency interference (RFI) all haveThe seeing at sub-millimetre and milli- the antenna. The radiometers cover fre- a smaller effect relative to the strength ofmetre wavelengths arises due to atmos- quencies around the 313 -> 220 rotation the line.pheric (specifically, tropospheric) insta- line of the para water molecule, which isbilities that lead to fluctuations of the centred at 183.3 GHz. This line lies about Measurements at these higher frequen-refractive index and consequent path 200 K above the ground state and so is cies do, however, also present a numbererrors in the propagating wavefront. As ideal for tracing atmospheric properties. of challenges:explained in a previous Messenger arti- The principle of radiometric phase cor- 1. Design and production of the hardwarecle (Nikolic et al., 2008), the process rection is that these measurements can is more complex and expensive,is analogous to that affecting the optical be used to compute the quantity of water requiring custom components and highseeing, but the dominant contribution vapour along the line of sight of each precision the refractive index fluctuations is from antenna and, consequently, the equiva- 2. Calibration is more difficult as it needsinhomogeneities in water vapour, rather lent path error. Using these estimates the to be based on very frequent (10 Hz The Messenger 143 – March 2011 11
  12. 12. Telescopes and Instrumentation Nikolic B. et al., Tests of Radiometric Phase Correction with ALMA Figure 1. The water vapour line at 183 GHz. The upper left, upper right and lower left pan- els show how the simu- lated brightness of the atmospheric 183 GHz water vapour line varies with changes in total contents of the water vapour, the atmospheric temperature and atmos- pheric pressure. The lower right panel shows the nominal filter pass- ν ν bands for the ALMA 183 GHz water vapour radiometers. The detec- tion system is double- sideband and so only the average signal of the two filters symmetric around the line centre is measured. ν ν in the case of ALMA) observation of we read out at 1 Hz, which is fast enough and we do not need to try to retrieve the physical internal calibration loads. to capture essentially all the path varia- total extra path due to the water vapour3. The water vapour line is close to satu- tions. in the atmosphere. Additionally, frequent ration and thus subject to non-linear observation of point-like sources will effects, leading to significantly more The task of the phase correction soft- allow ALMA to calibrate the expected complex software requirements. ware is to turn these 1 Hz measurements “zero” phase and it is only the departures in four filters into phase rotations to be from this that are important.Over the past twelve months, extensive applied to the observed astronomical sig-testing of the first WVR systems has nal. The first step in the analysis is to use The close relationship between fluctua-been carried out at the ALMA site. The the four observed sky brightness tem- tions in sky brightness and the pathpreliminary results of these tests suggest peratures, together with ancillary weather errors is illustrated in Figure 2, which isthat the development and production information, to make an inference about based on recent observations by ALMA.stage has successfully met these chal- the total quantity, temperature and pres- During this observation, the telescopelenges. So far, the units installed on the sure of the water vapour. This is impor- was tracking a quasar (i.e., a point-likeALMA antennas appear to be perform- tant because the profile of the water va- source) at a known location on the sky,ing well in terms of noise, stability and pour line is a strong function of these so for a perfect interferometer we wouldreliability. parameters, as shown in Figure 1, and expect to measure visibilities with con- because the near-saturation of the line stant phase and amplitude. The phase means that the observed sky brightness we actually measure is therefore an esti-Technique is not in general linearly related to the mate of the differential path along the total path error. two lines of sight due to atmosphericThe WVRs provide measurements of fluctuations. This is plotted on the hori-sky brightness in the four filters illustrated The second stage of analysis is to turn zontal axis of the diagrams, and on thein the lower right plot in Figure 1. As the fluctuations in the observed sky vertical axis we plot the difference inALMA WVRs employ a double-sideband brightness into estimates of fluctuation of observations by the WVRs on these twomixing system, only the average bright- effective path to each of the antennas antennas. What can be seen in Figure 2ness of the sky at frequencies symmetric in the array. We only consider the fluctua- is that there is a high degree of correla-around the centre of the line is meas- tions because, as an interferometer, tion between the two quantities, meaningured. The maximum readout frequency ALMA is sensitive to only the difference in that as long as we can forecast the slopefrom the WVRs is 5 Hz, although normally path errors to each of the antennas of this correlation then we can convert12 The Messenger 143 – March 2011
  13. 13. Figure 2. Correlation between the atmos- pheric path error esti- mated from observa- tions of bright point-like objects (horizontal axis) and the differenced WVR signal (vertical axis). Each plot is a two- ∆∆ dimensional histogram where the colour scale shows how many points fall in each bin. The four panels correspond to the four channels of the radiometers (1–4 desig- δ δ nated by the axis label).∆ ∆ δ δfluctuations of WVR outputs to path fluc- (This programme also supports develop- – The software is easy to distribute as atuations for a general observation. ment of Band 5 receivers for ALMA [see binary package that works in conjunc- Laing et al., 2010], and on-the-fly interfer- tion with CASA.The final stage of the analysis is to turn ometry techniques).estimates of path fluctuations into a The software (wvrgcal) for phase cor-phase correction that needs to be applied The software we have been developing is rection is available freely under the Gnuto the astronomical data. This is gener- designed primarily for off-line phase cor- Public License in both source code andally straightforward, although it is impor- rection, i.e., it operates on the observed binary formats1. We also operate atant to take into account the dispersive data after these have been stored on mailing list2 for discussion, improvementeffects of the atmosphere and also to disk. Some of the principal features of our suggestions and community support ofensure that rotations applied using esti- software are: the software.mates derived from WVR data interact – It is closely integrated with the officialcorrectly with other calibrations applied ALMA off-line data reduction suiteto the astronomical data. (CASA), which allows it to be used in a Tests of phase correction straightforward manner by scientists. – The software has a rapid development Since about January 2010, ALMA hasDevelopment of phase correction soft- cycle, with new features and improved been collecting significant amountsware for ALMA under FP6 algorithms appearing regularly. of test observations designed to measure – It uses a robust Bayesian statistical the effectiveness of WVR phase correc-Our recent involvement in WVR phase inference framework to derive optimal tion and to guide the further develop-correction for ALMA has been primar- corrections. ment of algorithms used to translate skyily through development of software and – When certain WVRs are missing from brightness measurements to the phasealgorithms that process the raw data an observation, the software has rotations. In order to fit with the numerousobserved by the WVRs and use these to the ability to interpolate available data other ALMA commissioning activities,calibrate and correct the astronomical to provide phase correction estimates most of these observations were takendata. This work is separate from the base- at those antennas lacking accurate with the antennas in relatively compactline ALMA software and has been funded WVR measurements. configurations, i.e., most data are withas an ALMA enhancement by the Euro- baselines in range 30–100 m, with somepean Union Framework Programme 6. data on baselines of up to 600 m. These The Messenger 143 – March 2011 13
  14. 14. Telescopes and Instrumentation Nikolic B. et al., Tests of Radiometric Phase Correction with ALMAdata have already provided a good dem- Figure 3. Path fluctua- tion estimated fromonstration of effectiveness of phase cor- WVR data for two ob -rection on these relatively modest base- serving sessions. Timeslines. However, we know that the phase are expressed in UT,correction will be most challenging on so the upper panel cor- responds to night-time,long baselines (up to 15 km in length for while the lower panelALMA); this is because the root structure to a time around mid-function of the atmosphere increases as day. Note that the verti- δroughly the 0.6 power of baseline on typi- cal scale is different between the two ALMA baselines. Long baseline testdata are awaited to investigate the effec-tiveness of the technique when ALMA ismaking its highest resolution images.Two typical examples of path fluctuationscomputed from WVR observations areshown in Figure 3. For these plots wehave used data from three antennas,shown by different colours in these plots.Since these are absolute path estimatesfrom the WVRs, it is the differencesbetween the three traces that correspond δto the phase rotations to be applied.For these observations, the antennaswere relatively close to each other andtherefore these differences are quitesmall. These plots illustrate very well thewide variety of conditions that are pre-sent at the ALMA site: total fluctuationsare different by about two orders of mag- Figure 4. Test observa- tion of a sub-mm brightnitude between the two observations. quasar on a roughly 650-It can be seen that the total (peak-to- metre baseline withpeak) fluctuations on the upper panel of ALMA. The red line is theFigure 3 are about 50 μm on timescales phase (in degrees) of the observed (complex) visi-of about five minutes; this is significantly bility on this baseline —less than 350 μm, the shortest wave- note that for a quasar (orlength at which ALMA will observe. On other point-like) source atthe lower panel of Figure 3, the fluctua- the tracking centre of the interferometer we expecttions are greater than 3.5 mm, i.e, they a constant phase in time.are larger than the longest wavelength at The blue line is the visibil-which ALMA will initially observe. ity phase after correction of the data based on theFigures 4 and 5 show two examples WVR signals and using the wvrgcal program.of WVR phase correction at work. In bothplots, the red trace represents the phaseof the recorded visibilities while observ- Figure 5. Like Figure 4,ing a quasar. In the absence of atmos- this is a test observation of a strong quasar, butpheric and instrumental phase errors we on a baseline of aroundwould expect this phase to be constant 60 m and during stablein time — the variations actually observed weather. The red line isare due to the combination of atmos- again the uncorrected observed phase (inpheric effects and instrumental errors. degrees) of the visibility,The uncorrected phase in Figure 4 is vary- while the blue line is theing by more than 360 degrees, i.e., by phase after WVR-baseda full rotation, which means that in these correction. Note the change of vertical scaleconditions it would not be possible to between Figure 4 andmake any measurements on faint sources. this figure.The blue line shows the phase aftercorrection using our wvrgcal software.14 The Messenger 143 – March 2011
  15. 15. –44°05o00 02 02 04 04 06J2000 Declination J2000 Declination 06 08 08 10 10 12 12 14 14 –44°05o00 16 05 38 51 .0 h m s 50 .6 50 .4 50 .2 50 .0 s s s s 49 .8 s 49 .6 s 05 h38 m51s.0 50 s.6 50 s.4 50 s.2 50 s.0 49 s.8 49 s.6 J2000 Right Ascension J2000 Right Ascension Figure 6. Un-deconvolved images of a quasar (which It can be seen that the fringes in the map to use, so they can automatically remove is unresolved) with ALMA in a very heterogeneous made from corrected data (right panel) the distorting effects of the atmosphere configuration: four of the antennas were very close together while the fifth was about 650 metres away. are much sharper and have much higher and allow them to focus on the novel sci- The heterogeneity leads to the rapid modulation contrast compared to the map made ence in their ALMA datasets. Nonethe- in the north–south direction, which corresponds to from uncorrected data. After deconvolu- less, ALMA already has made great pro- the long baseline. The image on the panel on the tion (and completing the baseline cover- gress towards this goal, and can already left was made with no phase correction while the image in the panel on the right has had WVR phase age) this increase in sharpness directly claim to have an effective and ground- correction applied. It can be seen that the phase corresponds to an increase in resolution breaking adaptive optics system. fluctuations, when uncorrected, lead to an almost and fidelity. complete wash-out of fringes on the long baseline. Acknowledgements Future challenges The results shown in these plots are of course the The phase errors can be seen to be result of the efforts of many tens, if not hundreds, of reduced by an order of magnitude, to a The data presented in this article repre- people who have been involved in ALMA over the level where meaningful averaging of the sent by far the most extensive tests of years. Phase correction tests require everything in the ALMA system to be working perfectly, so a great data can be done. the capabilities of 183 GHz phase correc- deal of credit is due to all of those involved, from tion ever attempted. They demonstrate the designers of the systems to those keeping the The example shown in Figure 5 is less that the technique should increase signifi- observatory running in Chile. The specific work extreme — the uncorrected data have cantly the sensitivity of ALMA, by reduc- described here, including the analysis of test data and development of the wvrgcal program has a phase root-mean-square deviation ing the decorrelation caused by phase been carried out by the Astrophysics Group at the of about 20 degrees. However, even in errors, and increase the fidelity of ALMA Cavendish Laboratory, University of Cambridge, these much more stable conditions, images by ensuring visibility phases are as part of the ALMA Enhancement Programme, an application of WVR phase correction more accurately measured. In addition, enhancement to the baseline ALMA project. This work is funded by the European Union’s Sixth leads to much improved phase stability. they should improve the efficiency of Framework Programme. Also notable in this example is that ALMA operations, by permitting observa- variations in uncorrected phase at longer tions to take place when atmospheric timescales are also very effectively re- instabilities cause rapid large amplitude References duced by WVR phase correction. phase fluctuations. Evans, N. et al. 2003, Site properties and stringency, ALMA Memo Series, 471, The ALMA Project As an illustration of the effect of WVR- However, much work remains to be done. Laing, R. et al. 2010, The Messenger, 141, 41 based phase correction on imaging, It is vital that ALMA can demonstrate Nikolic, B. et al. 2008, The Messenger, 131, 14 in Figure 6 we show an un-deconvolved that its phase correction strategy works (“dirty”) map of a point source with to specification in a wide range of atmos- Links ALMA in an unusual configuration with pheric conditions and on baselines all 1 north-south baselines much longer the way out to the maximum allowed by Source code for wvrgcal available at: http://www. than the others. We have made the map the configuration designs. In addition, 2 Mailing list for wvrgcal updates: both with the raw data, and with the it remains a challenge to ensure that the data after WVR-based phase correction. software tools are easy for astronomers The Messenger 143 – March 2011 15
  16. 16. Telescopes and InstrumentationGRAVITY: Observing the Universe in MotionFrank Eisenhauer1 Frédéric Chapron 2, 10 GRAVITY is the second generation VeryGuy Perrin 2, 10 Udo Neumann 3 Large Telescope Interferometer instru-Wolfgang Brandner 3 Leander Mehrgan 9 ment for precision narrow-angle as-Christian Straubmeier 4 Oliver Hans1 trometry and interferometric imaging.Karine Perraut 5 Gérard Rousset 2, 10 With its fibre­fed integrated optics,António Amorim 6 Jose Ramos 3 wavefront sensors, fringe tracker, beamMarkus Schöller 9 Marcos Suarez 9 stabilisation and a novel metrologyStefan Gillessen1 Reinhard Lederer1 concept, GRAVITY will push the sensi-Pierre Kervella 2, 10 Jean-Michel Reess 2, 10 tivity and accuracy of astrometry andMyriam Benisty 3 Ralf-Rainer Rohloff 3 interferometric imaging far beyond whatConstanza Araujo-Hauck 4 Pierre Haguenauer 9 is offered today. Providing precisionLaurent Jocou 5 Hendrik Bartko1 astrometry of order 10 microarcseconds,Jorge Lima 6 Arnaud Sevin 2, 10 and imaging with 4-milliarcsecondGerd Jakob 9 Karl Wagner 3 resolution, GRAVITY will revolutioniseMarcus Haug1 Jean-Louis Lizon 9 dynamical measurements of celestialYann Clénet 2, 10 Sebastian Rabien1 objects: it will probe physics close toThomas Henning 3 Claude Collin 2, 10 the event horizon of the Galactic CentreAndreas Eckart 4 Gert Finger 9 black hole; unambiguously detect andJean-Philippe Berger 5, 9 Richard Davies1 measure the masses of black holesPaulo Garcia 6 Daniel Rouan 2, 10 in massive star clusters throughout theRoberto Abuter 9 Markus Wittkowski 9 Milky Way; uncover the details of massStefan Kellner1 Katie Dodds-Eden1 accretion and jets in young stellarThibaut Paumard 2, 10 Denis Ziegler 2, 10 objects and active galactic nuclei; andStefan Hippler 3 Frédéric Cassaing 7, 10 probe the motion of binary stars, exo-Sebastian Fischer 4 Henri Bonnet 9 planets and young stellar discs. TheThibaut Moulin 5 Mark Casali 9 instrument capabilities of GRAVITY areJaime Villate 6 Reinhard Genzel1 outlined and the science opportunitiesGerardo Avila 9 Pierre Lena 2 that will open up are summarised.Alexander Gräter1Sylvestre Lacour 2, 10Armin Huber 3 1 Max-Planck Institute for Extraterrestrial Fundamental measurements over a wideMichael Wiest 4 Physics, Garching, Germany range of fields in astrophysicsAxelle Nolot 5 2 LESIA, Observatoire de Paris, CNRS,Pedro Carvas 6 UPMC, Université Paris Diderot, Much as long-baseline radio interfer-Reinhold Dorn 9 Meudon, France ometry has tone, GRAVITY infrared (IR)Oliver Pfuhl1 3 Max-Planck Institute for Astronomy, astrometry, with an accuracy of orderEric Gendron 2, 10 Heidelberg, Germany 10 microarcseconds and phase-referencedSarah Kendrew 3 4 Physikalisches Institut, University of imaging with 4-milliarcsecond resolution,Senol Yazici 4 Cologne, Germany will bring a number of key advancesSonia Anton 6, 8 5 UJF–Grenoble 1/CNRS-INSU, Institut (Eisenhauer et al., 2008). GRAVITY willYves Jung 9 de Planétologie et d’Astrophysique de carry out the ultimate empirical test toMarkus Thiel1 Grenoble, France show whether or not the Galactic CentreÉlodie Choquet 2, 10 6 Laboratório de Sistemas, Instrumen- harbours a black hole (BH) of four millionRalf Klein 3 tação e Modelação em Ciências e solar masses and will finally decide ifPaula Teixeira 6, 9 Tecnologias do Ambiente e do Espaço the near-infrared flares from Sgr A* origi-Philippe Gitton 9 (SIM), Lisbon and Porto, Portugal nate from individual hot spots close toDavid Moch1 7 ONERA, Optics Department (DOTA), the last stable orbit, from statistical fluc-Frédéric Vincent 2, 10 Châtillon, France tuations in the inner accretion zone orNatalia Kudryavtseva 3 8 Centro de Investigação em Ciências from a jet. If the current hot-spot interpre-Stefan Ströbele 9 Geo-Espaciais, Porto, Portugal tation of the near-infrared (NIR) flaresEckhard Sturm1 9 ESO is correct, GRAVITY has the potential toPierre Fédou 2, 10 10 Groupement d’Intérêt Scientifique directly determine the spacetime metricRainer Lenzen 3 PHASE (Partenariat Haute résolution around this BH. GRAVITY may evenPaul Jolley 9 Angulaire Sol Espace) between be able to test the theory of general rela-Clemens Kister1 ONERA, Observatoire de Paris, CNRS tivity in the presently unexplored strongVincent Lapeyrère 2, 10 and Université Paris Diderot field limit. GRAVITY will also be able toVianak Naranjo 3 unambiguously detect intermediate massChristian Lucuix 9 BHs, if they exist. It will dynamicallyReiner Hofmann1 measure the masses of supermassive16 The Messenger 143 – March 2011