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Telescopes and Instrumentation TRAPPIST: TRAnsiting Planets and PlanetesImals Small Telescope Emmanuël Jehin1 Michaël Gillon1 Didier Queloz2 E. Jehin/ESO E. Pierre Magain1 Jean Manfroid1 Virginie Chantry1 Monica Lendl2 Damien Hutsemékers1 Stephane Udry2 1 Institut d’Astrophysique de l’Université de Liège, Belgium 2 Observatoire de l’Université de Genève, Switzerland TRAPPIST is a 60-cm robotic telescope that was installed in April 2010 at the ESO La Silla Observatory. The project is led by the Astrophysics and Image Pro- cessing group (AIP) at the Department of Figure 1. The TRAPPIST telescope in its 5­metre time is obviously needed to monitor the Astrophysics, Geophysics and Ocean- enclosure at the La Silla Observatory, Chile. activity of several comets with a fre­ ography (AGO) of the University of Liège, quency of a few times per week. Some in close collaboration with the Geneva TRAPPIST is an original project using a comets are known, but others appear Observatory, and has been funded by single telescope that has been built and serendipitously. For the latter, telescope the Belgian Fund for Scientific Research optimised to allow the study of those two availability is crucial if we want to react (F.R.S.-FNRS) and the Swiss National aspects of the growing field of astrobiol­ rapidly to observe those targets at the Science Foundation (SNF). It is devoted ogy. It provides high quality photometric appropriate moment and for several hours to the detection and characterisation of data of exoplanet transits and allows or nights in a row; this strategy can pro­ exoplanets and to the study of comets the gaseous emissions of bright comets vide unique datasets impossible to obtain and other small bodies in the Solar Sys- to be monitored regularly. The project otherwise. tem. We describe here the goals of the is centred on three main goals: (1) the project and the hardware and present detection of the transits of new exoplan­ some results obtained during the first six ets; (2) the characterisation of known Telescope and instrumentation months of operation. transiting planets, in particular the pre­ cise determination of their size; and (3) For low cost operations and high flexibil­ the survey of the chemical composition of ity, TRAPPIST (see Figure 1) had to be The science case bright comets and the evolution of their a robotic observatory. The observation activity during their orbit. programme, including the calibration The hundreds of exoplanets known today plan, is prepared in advance and submit­ allow us to place our own Solar System ted daily to a specific software installed in the broad context of our own Galaxy. A dedicated robotic telescope on the computer controlling the obser­ In particular, the subset of known exo­ vatory. This computer controls all the planets that transit their parent stars are The basic project concept is a robotic tel­ technical aspects of the obser vations: key objects for our understanding of escope fully dedicated to high precision dome control, pointing, focusing, image the formation, evolution and properties of exoplanet and comet time-series pho­ acquisition, astrometry and software planetary systems. The objects of the tometry, providing the large amount of guiding, calibrations, data storage... It is Solar System are, and will remain, exqui­ observing time requested for those in sleep mode during daytime and wakes site guides for helping us understand research projects. Exoplanet transits typi­ up one hour before sunset, opening the the mechanisms of planetary formation cally last several hours, up to a full night. dome and starting to cool the CCD. This and evolution. Comets, in particular, are There are now many known transiting process is made possible thanks to a most probably remnants of the initial planets, and many more candidates found collection of computer programs working population of planetesimals of the outer by transit surveys which need to be together and interacting with the tele­ part of the protoplanetary disc. Therefore confirmed and characterised. Moreover scope, dome, CCD camera, filter wheels the study of their physical and chemical these targets need to be observed at and meteorological station. Such a properties allows the conditions that very specific times, during eclipses, put­ complete and rapid integration, using prevailed during the formation of the four ting even more constraints on telescope mostly off­the­shelf solutions, would have giant planets to be probed. availability. Similarly a lot of observing been impossible a few years ago and 2 The Messenger 145 – September 2011 WASP—43b TRAPPIST I+z 1 0.99 Flux 0.98 0.97 σ = 0.0003 0.96 –0.04 –0.02 0 0.02 0.04 dT (days) aluminium components, it weighs only Figure 3. TRAPPIST I + z transit photometry of the 65 kg and was allied to a compact Ger­ planet WASP­43b, period­folded and binned per two minute intervals, with the best-fit transit model man equatorial mount, the New Technol­ superimposed. The residuals of the fit, shifted along ogy Mount NTM-500, from the same the y-axis for the sake of clarity, are shown below company. This robust mount uses direct and their standard deviation is 300 parts per million drive technology to avoid the well­known (ppm). This light curve results from the global analy­ sis of 20 transits observed by TRAPPIST for this exo­ periodic errors found on the usual equa­ planet. Figure 2. Close­up of the 60­cm TRAPPIST torial mounts for small telescopes and telescope. therefore permits accurate pointing and tracking. The accuracy of the tracking (1.3 arcseconds per pixel) and a 10% allowed us to set up the experiment in allows an exposure time of four minutes accuracy are B­band 16.2, V­band less than two years. maximum, which is usually enough for 16.4, Rc­band 16.4, Ic­band 15.5 and our bright targets. Each frame is cali­ I + z­band 15.6; and in 200 seconds, The observatory is controlled through a brated in right ascension and declination B­band 19.7, V­band 19.4, Rc­band 19.2 VPN (Virtual Private Network) connection and software guiding runs continuously to and Ic-band 18.1. between La Silla and Liège University. keep the target centred on the same few The telescope and each individual sub­ pixels for the whole exposure sequence. The camera is fitted with a double filter system can be used from anywhere in wheel specifically designed for the pro­ the world, provided an internet connec­ The CCD camera was built by Finger ject and allowing a total of 12 different tion is available. In case of a low­level Lakes Instrumentation, with thermo­ 5 × 5 cm filters and one clear position. mechanical failure, we can count on the electric cooling and a CCD of the latest One filter wheel is loaded with six broad­ help of the Swiss technician on site or generation. This is a thinned broad­ band filters (Johnson-Cousins BVRcIc, the La Silla staff. band backside­illuminated Fairchild chip Sloan z’, and a special I + z filter for exo­ with 2048 × 2048 15-µm pixels providing planet transits) and the other filter wheel Hundreds of images, amounting to a field of view of 22 by 22 arcminutes is loaded with six narrowband filters 2–15 GB, are produced every night. and a plate scale of 0.6 arcseconds per for the comet programme. The comet Reduction pipelines run on a dedicated pixel. The sensitivity is excellent over filters were designed by NASA for the computer installed in the control room. all the spectral range, with a peak of 98% international Hale–Bopp campaign For the exoplanet programme, only tables at 750 nm, declining to around 80% (Farnham et al., 2000). Four filters iso- and plots with the final results are trans­ at 550 nm and 60% at 300 nm. It is opti­ lating the main molecular emission lines ferred to Liège, while for the comet pro­ mised for low fringe level in the far red present in cometary spectra (OH [310 nm], gramme, it is often necessary to transfer and achieves a sensitivity of 40% at CN [385 nm], C3 [405 nm], C2 + NH2 dozens of frames in order to perform 950 nm. The gain is set to 1.1 e­/ADU. [515 nm]) are permanently mounted, more interactive tasks on the images. There are three different readout modes: while the two other filters of the set + + Every third month, a backup disk is sent a low noise readout mode (readout noise (CO [427 nm] and H2O [705 nm]) are to Belgium and transferred to the archive [RON] 9.7 e­ in 8s), a fast mode (RON also available. In addition two narrow­ machine. 14 e­ in 4s) and a very fast readout of 2s band filters, isolating “continuum win­ using two quadrants. The cooling is dows” (BC [445 nm] and GC [525 nm]) for The telescope is a 60­cm f/8 Ritchey– –55 deg below ambient, usual operation the estimation of the solar spectrum Chrétien design built by the German being at –35 °C with a dark count of reflected by the dust of the comet, are ASTELCO company (see Figure 2). Owing 0.11 e­/s/pixel. Typical magnitudes mounted. to its open design with carbon fibre and reached in 20s with a 2 × 2 binning The Messenger 145 – September 2011 3 Telescopes and Instrumentation Jehin E. et al., TRAPPIST : TRAnsiting Planets and PlanetesImals Small Telescope Installation, first light and start of allowing us to constrain its bulk compo­ Characterisation of known transiting operations sition. Furthermore, the special geometry planets of the orbit makes the study of impor­ Once a transiting planet is detected, it is The telescope was installed in April 2010 tant properties of the planet (e.g., atmos­ of course desirable to characterise it in the T70 Swiss telescope building pheric composition, orbital obliquity, etc) thoroughly with high precision follow­up belonging to Geneva University (Figure 1).
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