Telescopes and Instrumentation MUSE Commissioning Roland Bacon1 The Multi Unit Spectroscopic Explorer reach MUSE from the star’s surface, but Joel Vernet2 (MUSE) is now in Paranal and was it is also the duration of the project, which Elena Borisiva3 installed on the VLT’s Unit Telescope 4 started in 2001 when the consortium Nicolas Bouché4 in January 2014. MUSE enters science answered the ESO call for ideas for sec- Jarle Brinchmann5 operations in October. A short summary ond generation VLT instrumentation. Marcella Carollo3 of the commissioning activities and Sci- David Carton5 ence Verification are presented. Some Joseph Caruana7 examples of the first results achieved Commissioning Susana Cerda2 during the two commissioning runs are Thierry Contini4 highlighted. After some technical half nights to finalise Marijn Franx5 the instrument’s alignment on the tele- Marianne Girard4 scope, the first commissioning period Adrien Guerou4,2 The Multi Unit Spectroscopic Explorer started. During 15 nights, from 7 to Nicolas Haddad2 (MUSE) is a second generation Very 21 February 2014, functional tests and George Hau2 Large Telescope (VLT) instrument for performance assessments were per- Christian Herenz7 integral field spectroscopy in the optical formed. From the first minute, MUSE Juan Carlos Herrera2 range (460–940 nm). It has two modes worked as expected and the whole run Bernd Husemann2 with different fields of view: 1 by 1 arc- went very smoothly, with no down time. Tim-Oliver Husser6 minutes at a spatial sampling of 0.2 arc- Aurélien Jarno1 seconds; and, when coupled to the The performance, as anticipated by the Sebastian Kamann6 Adaptive Optics Facility, a laser tomogra- laboratory measurements, is excellent Davor Krajnovic7 phy adaptive optics assisted mode with a and in general, much better than the Simon Lilly3 field of 7.5 by 7.5 arcseconds (with original specifications. The MUSE image Vincenzo Mainieri2 25 milliarcsecond pixels). The instrument quality is so good that it is limited by the Thomas Martinsson5 concept is described in Bacon et al. 0.2-arcsecond sampling. Definitive proof Ralf Palsa2 (2006) and an update on construction is of this key performance statistic was Vera Patricio1 given in Bacon et al. (2012). obtained in the third commissioning run Arlette Pécontal1 when an exceptionally good seeing of Roser Pello4 After its successful Preliminary Accept- less than 0.4 arcseconds on a stellar Laure Piqueras1 ance in Europe in September 20131, cluster was simultaneously recorded by Johan Richard1 MUSE was shipped to Paranal. A large set the telescope guide probe and measured Christer Sandin7 of boxes representing 24 tonnes and 200 on the final datacube provided by MUSE. Ilane Schroetter4 cubic metres of material arrived in Paranal The other important performance assess- Fernando Selman2 in early October and the reintegration pro- ment is the throughput of the instrument, Maryam Shirazi3 cess started. From October to December which peaks at 55 % at 700 nm. When Alain Smette2 2013 the consortium engineers, with the including the telescope and atmosphere Kurt Soto3 support of ESO Paranal and Garching transmission, this translates to an end- Ole Streicher7 staff, proceeded to the reintegration and to-end peak efficiency of 35 %, an out- Tanya Urrutia7 final alignment of the instrument in the standing achievement for such a complex Peter Weilbacher7 new Paranal integration hall. Early in Jan- instrument. This makes MUSE the most Lutz Wisotzki7 uary 2014, the instrument, fully aligned, efficient spectrograph on the VLT in the Gerard Zins8,2 was moved to Unit Telescope 4 (UT4). 500–850 nm wavelength range. Since MUSE was too large to enter In parallel with the functional tests and the 1 CRAL, Observatoire de Lyon, Saint- through the dome entrance door, it had performance assessment, we obtained a Genis-Laval, France to be lifted by a dedicated crane to pass series of showcase observations to dem- 2 ESO through the dome slit — a scary moment! onstrate the capabilities of MUSE on a 3 Institute of Astronomy, ETH Zentrum, On 19 January 2014, MUSE successfully number of different astronomical targets. Zurich, Switzerland “landed” on the Nasmyth platform B of At the end of this first commissioning run, 4 IRAP, Observatoire Midi Pyrénées, UT4. After a few checks, it was connected half a billion spectra had been obtained. Toulouse, France to the tele scope system and some fine- This impressive number of spectra was 5 Leiden Observatory, Leiden University, tuning of the alignment was performed. successfully ingested by the Paranal the Netherlands On 31 January 2014, a few minutes after computer infrastructure and successfully 6 Institute for Astrophysics Göttingen, dome opening, MUSE saw its first light: reduced on the pipeline and offline work- Germany the high proper motion sub-dwarf M star, stations. Figure 1 shows just a sample 7 Leibniz Institut für Astrophysik Potsdam, VZ Pictoris, at a distance of 4 pc, known of some of the targets observed during AIP, Germany as Kapteyn’s star. The first light target the commissioning from Jupiter, nearby 8 IPAG, Observatoire de Grenoble, France was a symbolic choice because 13 years star clusters and nebulae to nearby gal- is not only the time spent by the light to axies and a more distant galaxy cluster. The Messenger 157 – September 2014 13 Telescopes and Instrumentation Bacon R. et al., MUSE Commissioning a) c) b) d) e) 25.0 Figure 1. A gallery of examples selected from the 120 MUSE commissioning observations. a) The transit of Europa over Jupiter. The image was 20.0 reconstructed in the methane absorption band of the planet (865 nm) to increase the contrast between the 100 satellite and the planet. 15.0 b) Reconstructed colour image of the central part of the globular cluster NGC 6397, demonstrating the high multiplex power of MUSE. MUSE can recover 10.0 the spectra of thousands of individual stars in just a 80 few minutes, enabling important studies in such crowded environments. 5.0 s) c) This beautiful image of the centre region of the Orion Nebula was obtained with a large number of km/ ( pointings across the nebula. Five million spectra 60 0.0 ty cseconds were recorded in less than three hours telescope loci Ar time (including overheads) to build this mosaic. Ve -5.0 Gas morphology (d) and kinematics (e) of the plane- tary nebula NGC 3132 from a mosaic of three expo- –1 40 sures. Note that an accuracy of 1 km s is achieved -10.0 in the measured velocity field. –15.0 20 –20.0 0 –25.0 0102030405060 Arcseconds 14 The Messenger 157 – September 2014 f) g) MUSE Velocity NGC 5813 Hα 24.8 30 100 30 24.0 20 20 23.2 50 10 10 22.4 0 0 0 21.6 α H cseconds Ar 20.8 –10 –10 –50 20.0 –20 –20 19.2 –30 –100 –30 18.4 0 –30 –20 –10 0 10 20 30 –30 –20 –10 0 10 20 30 Arcseconds h) i) j) f) The high spatial resolution of MUSE is well demon- strated in the stellar velocity field of the central part of the elliptical galaxy NGC 5813 which exhibits a well-known kinematically decoupled core. g) The same exposure shows the spectacular Hα emission morphology of the galaxy. h) Motion and morphology of the gas in the NGC 4650A ring galaxy is represented in this colour picture (relative gas velocities are colour coded in green–red). i) More distant objects have also been observed, such as the lensing massive cluster of galaxies SMACSJ2031.8-4036. The colour picture is a zoom of five highly magnified images (in blue) of the known z = 3.5 Lyman-α emission galaxy located in the background of the cluster. j) Another example of interaction processes in galax- ies is shown in this reconstructed image of AM353- 272, alias the Dentist’s Chair. MUSE will be an ideal tool to study the complex kinematics and physical conditions in these types of galaxies. The Messenger 157 – September 2014 15 Telescopes and Instrumentation Bacon R. et al., MUSE Commissioning a) b) c) d) e) Figure 2. A series of images of the planetary nebula the puzzling image shown in (e) was revealed to be NGC 4361, observed to test the impact of the dither- the Hα image of a background galaxy at z = 0.02. ing strategy on the line spread function, illustrate the Given that this galaxy is just located behind the neb- discovery potential of MUSE. The reconstructed ula and that its Hα emission is two orders of magni- images of the nebula in white light (a) and different tude fainter than the planetary nebula Hα emission emission lines ([O III] in (b), Hα in (c), [N II] in (d) dis- at this location, any imager, including the Hubble play various morphologies which are indicative of the Space Telescope, would not be able to separate the physical conditions (ionisation, density and tempera- galaxy emission, while MUSE uncovered the galaxy ture, as well as reddening) of the gas. While further in just a few minutes of integration. exploring the datacube at the telescope around Hα, The quality of the datacubes produced, as to measure the performance of MUSE The MUSE SV consisted of two observ- demonstrated by the examples in Figure 1, for this type of science. MUSE also has a ing runs: 20–29 June and 18–24 August is also a good demonstration of the instru- high discovery potential as demonstrated 2014. Thanks to the high efficiency and ment quality and the maturity of the data in Figure 2. This figure shows a set of reliability of the instrument, all but two reduction system. The hard work of all images of the planetary nebula NGC 4361 programmes were fully completed.