ELISA, a Small Experiment For a Large Sub-millimeter Survey

Abstract

This paper presents the scientific objectives and technical aspects ofthe balloon-borne Experiment for Large Infrared Survey Astronomy(ELISA). The emphasis is put upon the synergies existing between theELISA project and future space missions, both with respect toscientific and technical aspects. ELISA is a small balloon projectfor an experiment dedicated to measure the Far-Infrared toSub-millimeter continuum emission of Galactic dust over a largefraction of the sky, with unprecedented sensitivity and angularresolution. The primary mirror of the telescope, similar to the oneused for the Top-Hat mission, will have a diameter of 1 m, ensuring anangular resolution of about 3.5'. Bolometer arrays will be used infour photometric bands centered at 170, 240, 400, and 650 μm andproviding a 22' × 45' instantaneous field of view per channel. A liquid He cryostat will host the cold optics, including thesecondary mirror of the telescope, as well as the detectors, whichwill be cooled to 0.3 K using an ³He close-cycle fridge. Mapping ofthe sky will be accomplished by rotating the gondola over a largeazimuth range (up to 60° amplitude). The pointing of theexperiment will be maintained to a constant elevation during theazimuth scans through a feed back loop using the signal from a largeformat, fast stellar sensor, operating day and night. The scientificgoal of the experiment is to map the diffuse Sub-millimeter emissionalong a large fraction of the Milky Way. The astronomical dataobtained will be used to derive the emission properties of the dustgrains in the Interstellar Medium (ISM), such as their temperature andemissivity. It will also allow to systematically measure thepolarization of the dust emission. It should also lead to thedetection of a few thousand point sources such as newly formed starsand distant galaxies. In addition to these goals, the ELISA projectwill serve as a test bed for the detector technology that will be usedfor the HERSCHEL space mission to be launched in 2007. The ELISA datawill be made available to help calibrate the observations ofHERSCHEL and PLANCK and to plan the large-scale surveys to beundertaken with HERSCHEL. Owing to these objectives, 3 flights of theELISA experiment, are foreseenin the period from 2004 to 2006. The ELISA project is carried out byan international collaboration including France (CESR, IAS, CEA,CNES),The Netherlands (SSD/ESTEC), Denmark (DSRI), UK (Cardiff Univ.).