21ST INTERNATIONAL SYMPOSIUM ON SPACE TERAHERTZ TECHNOLOGY, OXFORD, 23-25 MARCH, 2010 Herschel/HIFI In-flight Commissioning and Performance David Teyssier 1,* , Christophe Risacher 2 , Pieter Dieleman 2 , Wouter Laauwen 2 , A. de Jonge 2 , J. Braine 3 , M. Caris 4 , O. Coeur- Joly 5 , B. Delforge 6 , F. Helmich 2 , F. Herpin 3 , R. Higgins 7 , W. Jellema 2 , T. Klein 4 , J. Kooi 8 , C. Leinz 4 , K. Edwards 2 , A. Marston 1 , P. Morris 9 , M. Olberg 10 , V. Ossenkopf 11 , J. Pearson 12 , P. Roelfsema 2 , R. Shipman 2 1 European Space Astronomy Centre, ESA, Madrid, Spain 2 SRON Netherlands Institute for Space Research, Groningen, the Netherlands 3 Universite de Bordeaux, Laboratoire d’Astrophysique de Bordeaux, 33000 Bordeaux, France 4 Max Planck Institute für Radio Astronomie, Auf dem Hügel 69, 53121 Bonn, Germany 5 Centre d’etude Spatiale des Rayonnements, Universite de Toulouse [UPS], 31062 Toulouse Cedex 9, France 6 Laboratoire d’Etudes du Rayonnement et de la Matière en Astrophysique, UMR 8112 CNRS/INSU, OP, ENS, UPMC, UCP, Paris, France 7 National University of Ireland, Maynooth, Department of experimental physics, County Kildare, Ireland 8 Physics Department, California Institute of Technology, Pasadena, California, USA 9 Infrared Processing and Analysis Center, California Institute of Technology, MC 100-22, 770 South Wilson Avenue, Pasadena, CA 91125 USA 10 OSO, Chalmers University of Technology, SE-412 96 Göteborg, Sweden 11 KOSMA, I. Physik. Institut, Universität zu Köln, Zülpicher Str. 77, D 50937 Köln, Germany 29 12 Jet Propulsion Laboratory, 4800 Oak Grove Drive, MC 264-782, Pasadena, CA 91109 USA *Contact: David.Teyssier@esa.int, phone +34-91-813 1355 Abstract— The Herschel Space Observatory was successfully launched on the 14 th of May, 2009, lifting into space the Heterodyne Instrument for the Far-Infrared (HIFI). After the standard early orbit operations on the Space-craft, the instrument was switched on for the first time on the 24 th of May, kicking off an instrument Commissioning period of more than a month and a half. We present here the outcome of this Commissioning campaign, including the measured in-flight performance of the instrument. Among the challenges encountered during this period was the need to optimize the stability of the signal provided by the Local Oscillator (LO) chains in some of the frequency bands, in particular that covering the singly Ionized Carbon (C+) line at the upper edge of the HIFI range, and one of the backbones of the science program of the instrument. The same concept also allowed significant improvements in the upper end of the 3b LO chain. Another noticeable achievement was the very good co-alignment measured between the mixer beams in the respective H and V polarizations offered by HIFI, allowing for more than a simple build-in redundancy, as the observing efficiency will really benefits from the combination of the two mixers. I. INTRODUCTION Herschel [1] is the fourth ‘cornerstone’ mission in the ESA science programme. With a 3.5 m Cassegrain telescope it became on the 14 th of May 2009 the largest space telescope ever launched. It allows photometry and spectroscopy observations in approximately the 55-672 µm range, bridging the gap between earlier infrared space missions and ground- based facilities. Aboard Herschel, the Heterodyne Instrument for the Far-Infrared (HIFI [2]) offers very high-resolution (up to R>10 7 ) spectroscopy with a continuous coverage between 488 and 1272 GHz (SIS mixers), and 1430 and 1902 GHz (HEB mixers), as well as a simultaneous coverage of up to 4 GHz with its two backend spectrometers (an Acousto-Optic Spectrometer, and a Digital Auto-correlation Spectrometer). HIFI is suitable for investigating interstellar gas, for example in star-forming areas and in the gas expelled by dying stars. In the far-infrared and the submillimetre areas where HIFI is working, a wide range of molecules absorb radiation, including water. HIFI is suitable for capturing this radiation and can unravel it into a spectrum in which the characteristic fingerprints of all the different molecules can be seen. This instrument can be used to determine exactly what the gases are composed of and the temperature and pressure they are at. Furthermore, HIFI is highly suitable for measuring the speed of gas. In this paper, we present the early phases of the HIFI operations that focussed on the Commissioning of the instrument, and the assessment of its performances. II. THE HIFI COMMISSIONING CAMPAIGN A. The Early Mission Phases Immediately after its successful launch on the 14 th of May 2009, Herschel entered the so-called Launch and Early Operation Phase (LEOP) during which most of the Spacecraft functionalities are verified. HIFI was switched on exactly 10 days after the launch, and started a Commissioning Phase which, shared with the other two instruments on-board Herschel (PACS and SPIRE), took about almost two months. About 440 hours of instrument data were collected during this phase. 40