The Next Generation Transit Survey Becomes Operational at Paranal

Telescopes and Instrumentation

Richard G. West1 A new facility dedicated to the discov has the significant computer resources Don Pollacco1 ery of exoplanets has commenced required to reduce the data for each star Peter Wheatley1 science operations at Paranal. The in an image and search for brightness Michael Goad2 Next-Generation Transit Survey (NGTS) variations. In the case of WASP the Didier Queloz3 will deliver photometry at a precision reduced data products (star brightness Heike Rauer4 unprecedented for a ground-based as a function of time) grow at a rate of Christopher Watson5 wide-field survey, enabling the discov several GB per night. Over ten years Stéfane Udry6 ery of dozens of transiting exoplanets of operation, WASP has acquired more Nigel Bannister2 of the size of Neptune or smaller around than 16 million images covering 30 million Daniel Bayliss6 bright stars. NGTS is briefly described stars, a total of more than 0.5 × 1012 pho- François Bouchy6 and the survey prospects are outlined. tometric data points. Matthew Burleigh2 Juan Cabrera4 The ESA space mission CoRoT (Con Alex Chaushev2 Introduction vection, Rotation and planetary Transits) Bruno Chazelas6 and the National Aeronautics and Space Michel Crausaz6 Most of our knowledge of the properties Administration (NASA) satellite Kepler Szilard Csizmadia4 of individual exoplanets comes from those (and its continuation as K2) have obtained Philipp Eigmüller4 that transit their host star. Measurements photometry of sufficient accuracy to Anders Erikson4 of the transit lead to an estimate of the enable the detection of smaller, rocky Ludovic Genolet6 planet’s radius relative to that of the star planets. Given the relatively modest fields Ed Gillen3 and, since its orbital inclination can be of view of these instruments, most of their Andrew Grange2 recovered from the transit shape, then its planet candidates have been detected Maximilian Günther3 mass can be derived. Together these with host stars of V > 13 mag. Conse- Simon Hodgkin3 data provide the planet’s density, which quently, given their faintness and the James Kirk1 can be compared to theoretical models expected size of the reflex motion induced Grégory Lambert7 of its structure. Even in this new era of in the star, confirmation and measure- Tom Louden1 directly imaged planets (with high contrast ment of the planetary mass have gener- James McCormac1 imaging instruments such as the Spectro- ally been beyond our current observational Lionel Metrailler6 Polarimeter High-contrast Exoplanet capabilities, and just a handful of the Marion Neveu6 REsearch Instrument [SPHERE] on the brightest hosts with small planets have Alexis Smith4 Very Large Telescope, for example) and been studied. Andrew Thompson5 astrometric orbits from the European Roberto Raddi1 Space Agency (ESA) Gaia satellite, transit Simon R. Walker1 observations remain the only direct The need for another (red) survey James Jenkins8 method to determine accurate planetary Andrés Jordán9 radii. In order to characterise small planets we have two options: However, the detection of transiting plan- a) Employ an extremely wide-field satellite 1 Dept. of Physics, University of Warwick, ets is not trivial. As the transit probability enabling photometry of many bright United Kingdom diminishes rapidly with orbital period, stars. Both NASA and ESA are planning 2 Dept. of Physics and Astronomy, strong selection effects favour the detec- such missions, respectively the Transit- University of Leicester, United Kingdom tion of large planets (or, strictly speaking, ing Exoplanet Survey Satellite (TESS) 3 Institute of Astronomy, University of planets large relative to their host) with and the PLAnetary Transits and Oscilla- Cambridge, United Kingdom short periods. Consequently, transit sur- tions of stars (PLATO); 4 Institut für Planetenforschung, veys observe thousands of (Sun-like) b) Remembering that the transit depth Deutsches Zentrum für Luft- und stars to find just a handful of large planets. is dependent on the relative sizes of Raumfahrt, Berlin, Germany The two leading ground-based survey the planet and its star, observe smaller 5 Astrophysics Research Centre, Queen’s projects are HAT (Hungarian-made Auto- stars to detect smaller planets for a University Belfast, United Kingdom mated Telescope1) and WASP (Wide given photometric accuracy. There are 6 Observatoire de Genève, Sauverny, Angle Search for Planets2), which together several projects targeting individual Switzerland have discovered the majority of planets M-dwarfs that have recently achieved 7 ASTELCO Systems GmbH, Martinsried, that have accurately determined masses. stunning successes, such as the detec- Germany tion of GJ 1132b (Berta-Thompson et 8 Departamento de Astronomía, The instruments used in these surveys al. 2015) with MEarth3 and TRAPPIST-1 Universidad de Chile, Santiago, Chile are quite modest, but are capable of (Gillon et al., 2016) with the TRAnsiting 9 Departamento de Astronomía y repeatedly imaging hundreds of square Planets and PlanetesImals Small Tele- Astrofísica, Pontificia Universidad degrees and obtaining accurate photo scope (Jehin et al., 2011; TRAPPIST4). Católica de Chile, Santiago, Chile metry (better than 1% precision) for stars While M-dwarfs are common, they are brighter than V = 11 mag. Each facility intrinsically faint and spectroscopic

10 The Messenger 165 – September 2016 Grégory Lambert

Figure 1. Photograph of the NGTS facility at Paranal with the VLT in the background to the west.

follow up of these targets is challeng- NGTS has its roots in the hugely suc- and excellent photometric conditions ing. An alternative approach is to cessful WASP project2 but has been were paramount in this decision. optimise the system for the detection developed through two prototypes. The of planets around K-dwarfs, which are first was set up on La Palma in 2010 and more luminous. The Next Generation was used to look at the potential use of The NGTS facility Transit Survey is our attempt to realise deep-depleted charge-coupled device this survey. (CCD) technology for the detectors and of NGTS5 consists of a cluster of twelve wider-aperture astrographs. This first pro- identical telescope units, each unit com- The scientific goal of NGTS is to discover totype led to the final design which was prising a 20-cm f/2.8 astrograph and a population of Neptune-sized planets constructed at Geneva Observatory in a 2k × 2k deep-depleted CCD camera around bright stars. In order to achieve 2012. This second prototyping phase was mounted on an independently steerable this, a photometric precision unprece- then used to further optimise the design, fork mount. Each unit has a field of view dented in wide-field ground-based sur- and to develop the control software. of approximately 3 × 3 degrees, yielding veys (better than 0.1%) is required. NGTS a total field of view for the whole facility has therefore been carefully designed After discussions with ESO it was agreed of approximately 100 square degrees to minimise the instrumental effects to site the NGTS at Paranal Observatory. (roughly equivalent to that of the Kepler which have limited the precision of previ- While astronomical seeing was not a satellite). A photograph of the NGTS facil- ous surveys. prime factor, atmospheric water content ity at Paranal is shown in Figure 1.

The Messenger 165 – September 2016 11 Telescopes and Instrumentation West R. G. et al., The Next Generation Transit Survey Becomes Operational

In general, NGTS utilises commercial Figure 2. Light curves of off-the-shelf products which have been single transits of the Jupiter-sized exoplanet modified to optimise their performance. WASP-4b measured For example, the detectors are deep- with NGTS (upper) and depleted e2v (CCD-42) devices packaged WASP (lower). into a camera by Andor Technology @F L

(model iKon-L). Andor manufactured the 6 original WASP cameras so we already had a good working relationship with this company. Optimisation of these l devices proved more demanding than we expected and the company expended considerable effort to produce devices suitable for the extreme accuracies we were trying to achieve. Similarly, the telescopes are H-series astrographs from @F Astro Systeme Austria (ASA), but fitted L 6 with a custom corrector optic designed to NGTS specifications. The NGTS enclosure was designed and fabricated l by GR-PRO in the UK; it is of glass- reinforced plastic (GRP) composite con- l l l struction, and has a footprint of around 3HLDLHM 10 × 15 metres. ated with El Niño, NGTS has already Saturn’s (6–22 Earth radii). NGTS is most The NGTS facility is fully robotic and acquired more than 3.5 million science sensitive to planets with orbits of less operates unsupervised, following a images. The quality of the data produced than 20 days. pre-generated schedule that is prepared by NGTS is exceptional, and the instru- on a daily basis. The survey data are ment has proven itself well capable of transferred to the NGTS Data Centre at delivering the required sub-millimagnitude NGTS Consortium the University of Warwick, UK, to be photometric precision. As an example analysed using sophisticated automated of the step-change improvement that The NGTS Consortium consists of algorithms that search for the tell-tale NGTS represents over previous ground- the following institutes: University of signatures of an exoplanet transiting its based surveys, Figure 2 compares the Warwick, UK; Observatoire de Genève, host star. Detections made by these light-curve of a single transit of a known Switzerland; Deutsches Zentrum für algorithms are further vetted by auto- exoplanet (the “hot Jupiter” WASP-4b Luft- und Raumfahrt (DLR), Germany; matic and manual means, and the high- [Wilson et al., 2008]) taken with NGTS, University of Leicester, UK; Queen’s est quality candidates are passed for with the best quality transit in the WASP University Belfast, UK; and the University photometric and spectroscopic follow-up survey data. The WASP data show instru- of Cambridge, UK. The Chilean astronomi- using larger facilities (e.g., the CORALIE mental effects that have a magnitude cal community is currently represented spectrograph on the EULER telescope similar to that of the transit signal, so by the Pontificia Universidad Católica de at La Silla). Processed NGTS light-curves WASP required coverage of many transits Chile and the Universidad de Chile. will be made available to the community in order to make a significant detection. via the ESO Science Archive Facility after NGTS on the other hand could have a proprietary period (two years for the detected WASP-4b with a single transit. References first release, one year for subsequent Berta-Thompson, Z. K. et al. 2015, Nature, 527, 204 releases). A preliminary analysis of the data from Gillon, M. et al. 2016, Nature, 533, 221 the first six months of the full NGTS Jehin, E. 2011, The Messenger, 145, 2 survey has yielded several dozen candi- Wilson, D. M. et al. 2008, ApJ, 675, L113 First results and the future date planets which are we currently following up. Recent simulations based Links First light at Paranal was achieved with on the actual performance of NGTS have the first NGTS telescope in January shown that the full NGTS survey can be 1 HAT Exoplanet Surveys: https://hatsurveys.org/ 2 2015 and the transit survey began with expected to detect several super-Earths WASP: https://wasp-planets.net/ 3 MEarth project: https://www.cfa.harvard.edu/ fivetelescopes in August 2015. The full (planets with a radius less than twice MEarth/Welcome.html complement of twelve telescopes that of the Earth), tens of Neptune-sized 4 TRAPPIST Telescope Network: http://www.orca. became operational in February 2016 planets (2–6 Earth radii), and more than ulg.ac.be/TRAPPIST/Trappist_main/News.html 5 and, despite the poor weather associ- 200 planets with a radius larger than NGTS: http://www.ngtransits.org

12 The Messenger 165 – September 2016