La spectroscopie à transformée de Fourier Le FTS de Herschel-SPIRE et ses potentialités scientifiques Kjetil Dohlen
Plan de l’exposée Objectifs et potentialites scientifiques Vue globale de l’instrument L’évolution de l’instrument Pourquoi un FTS Les bases du FTS Les performances de l’instrument Résolution spectrale Echantillonnage et résolution spatiale Expérience personnelle
Le télescope Herschel Télescope de 3.5m diamètre 3.3m pupille entrée Lancé en 2007 par Ariane 5 Orbite autour de L2
Les instruments de Herschel Trois instruments montes dans le cryostat HiFi: Spectrométrie hétérodyne, 156-625 µm PACS: Imagerie et spectroimagerie 60-210 µm SPIRE: Imagerie et spectroimagerie 200-670 µm Refroidies par 2000 litres de He liquide Plus que 3 ans opération
Galaxies – normal, starburst and AGN SPIRE Scientific Goals 10 100 1000 1012L 0.5 5 3 1 Z = 0.1 l (mm) Galaxies – normal, starburst and AGN Statistics and physics of galaxy formation in the early universe 10 1 0.1 0.01 5s, 1h R=40 R=3 Flux density (Jy) l (mm) Star formation and interstellar matter Solar system: giant planets, comets and solid bodies 1000 100 10 Flux density (Jy) SPIRE PACS
Potentialités du FTS Spectre complet dans le domaine 200 – 670 mm Resolution variable Raies, R=1000 Continu, R=40 Spectro-imagerie Imagerie dans des raies (atomiques et moléculaires) Etude des conditions physiques dans différents milieux Large domaine spectral Possibilité d’observer plusieurs transitions d’une même molécule Etude des conditions physiques
Instrument Design Drivers • Photometer - Deep mapping with highest efficiency and largest possible field of view - Multi-band coverage with simultaneous observation - Point and compact source observation with high efficiency • Spectrometer - Sensitivity optimised for point/compact source spectroscopy - Imaging spectroscopy with maximum available field of view - Wide wavelength coverage - Variable spectral resolution (few x 10 to few x 100) • Both - Thermal background dominated by the Herschel telescope - Simplicity, affordability, reliability, ease of operation - Complementary to other Herschel instruments and other facilities
Light-tight baffles at strap entry points SPIRE Focal Plane Unit Light-tight baffles at strap entry points Central optical bench panel Spectrometer side 690 mm Photometer side Thermally isolating supports 2-K thermal straps
Herschel focal surface Photometer Layout and Optics Beam steering sirror Herschel focal surface 2-K cold stop M3 M4 M5 M6 M7 M8 Beam steering mirror Offner relay Dichroics and arrays M9 3He cooler 2-K box M4 M6 M8 M5 M7 M3 SPIRE optical bench (4 K) Detector array modules
FTS Layout and Optics 4-K box Fore-optics shared with photometer Output port Telescope input port Baffle Beam divider Calibrator input port Detector array modules Intensity beam dividers Mirror mechanism Output port 2nd-port calibrator
Evolution of the instrument (1) Original proposal for the BOL instrument: Double Fabry-Perot Abandoned because of its design complexity February 1997 Separation of photometer and spectrometer channels « SpecBOL » « PhotBOL » Scanning flat grating spectrometer working in multiple orders Included lenses
Evolution of the instrument (2) March 1997: All-reflective flat grating design June 1997 Prospect of bolometric array detectors Study of static, all-reflective cross-dispersed design Concave « holographic » main grating Offner-type concentric cross-dispersion spectrograph
Evolution of the instrument (3) November 1997: The ultimate grating design Concave « holographic » grating Reimaged pupil for cold stop Simultaneous detection in several orders, allowing: sufficient wavelength range with limited grating scan range multiplex advantage Advantage for LAM: ISO-LWS heritage for grating mechanism Problems: R ~ few 100 No imagery Extremely stray light sensitive
Evolution of the instrument (3) November 1997: The ultimate grating design Concave « holographic » grating Reimaged pupil for cold stop Simultaneous detection in several orders, allowing: sufficient wavelength range with limited grating scan range multiplex advantage Advantage for LAM: ISO-LWS heritage for grating mechanism Problems: R ~ few 100 No imagery Extremely stray light sensitive
« Mais alors, pourquoi pas un FTS ? » R~1000 possible Imagerie Bande continue Moins sensible a la lumière parasite Et une nouvelle chasse aux designs commença...
Evolution of the instrument (4) December 1997: SWIFT Swinging arms FTS Martin-Puplett polarized design Advantage for LAM: ISO-LWS heritage for mechanism Retained for the ESA proposal Problems: R~500 50% maximum efficiency
Evolution of the instrument (5) October 1998: Polarizing Mach-Zehnder Martin-Puplett polarized design with dual inputs and outputs Potentially 100% efficiency Up to R~1000 Problems: Extremely cumbersome Difficult alignment No ISO-LWS heritage for mechanism Mechanism concept from GSFC proposed
Evolution of the instrument (6) February 1999: Mach-Zehnder with 50/50 beam splitter Wide-band beamsplitter developed by QMW (P. Ade) Metal-mesh filter technology Much more compact No more need for input and output polarizers Potentially 100% efficiency Up to R~1000 Problems: What problems?
Evolution of the instrument (7) Ah-oui, le mécanisme, fallait quand-même le faire... Développement sous responsabilité LAM (Pascal Dargent et al.) Principe GSFC retenu Course Stabilité Modifications importantes Passage du faisceau Masse Tenu vibrations
Evolution of the instrument (8) Pour ne pas parler du contrôle commande Développement sous responsabilité LAM (Didier Ferrand et al.) Senseur de position Heidenhein Spatialisé avec ObsPM (G. Michel) et CEA
Comment ça marche, un FTS ? Interférogramme = FT(Spectre) Une ligne en émission = Signal en Cosinus Spectre large = Somme de Cosinus Spectre = FT(Interférogramme)
Resolution maximale de SPIRE Résolution d’un FTS La résolution d’un FTS est définie comme R = l/dl = 1/(l ds) ou s = 1/l est le « nombre d’onde », proportionnel à la fréquence Plus l’interférogramme est long (OPD grand), plus la résolution spectrale (ds) est fine: ds = 1/(2 OPD) On a donc que: R = 2 OPD/l Alors que dans le cas d’un réseau limité par la fente: Rslit l Pour SPIRE OPD = 4*course = 4*31.25mm = 125mm Pour l = 250mm, on a donc R250mm = 1000 Resolution maximale de SPIRE
Detector Arrays (2Fl Feedhorns) 45 mm PLW 43 detectors PMW 88 detectors 22 mm SLW 19 detectors SSW 37 detectors Photometer Spectrometer Coincident beam centres PSW 139 detectors 500 mm 350 mm 250 mm 315-670 mm 200-315 mm PLW Array
Image sampling Gaussian mode feedhorn detectors PLW Array Gaussian mode feedhorn detectors PSF on the sky has ~ Gaussian profile FWHM ~ l/D, slightly broader than Airy profile Pixels separated by 2l/D Image is undersampled « Jiggling » of the image required 16 pointings for full sampling
FTS Observing Modes • Ds = 0.04 - 2 cm-1 (R250mm = 1000 - 20) by adjusting scan length • Continuous scan: - Mirror scan rate = 0.5 mm s-1 - Signal frequency range = 3 - 10 Hz - Calibrator in 2nd port nulls telescope background • Step-and-integrate: - 2nd port calibrator is off - Mirror stepped with integration at each position - BSM chops on sky • Point source spectroscopy/spectrophotometry - Telescope pointing fixed - Background characterised by adjacent pixels • Imaging spectroscopy - Beam steering mirror adjusts pointing between scans to acquire fully-sampled spectral image
Sensitivity Estimates: Spectrometer Sensitivity Estimates: Spectrometer R250 = 1000 R250 = 40 Line spectroscopy (s = 0.04 cm-1) Low-resolution spectrophotometry (s = 1 cm-1) Map Map Flux density 5 1 hr (mJy) Line flux 5 1 hr (W m-2 x1E-17) Point source Point source (m) (m)
Science avec le FTS une expérience personnelle FTS statique HFTS: Holographic FTS HHS: Heterodyne holographic spectrometer SHS: Static heterodyne spectrometer ... Compact, portable Environnement Atmosphère Végétation Géologie Avantage de l’étendue Absence de fente
Science avec le FTS NO2 dans l’air de Londres Model simplifié de expérience NO2 observé en laboratoire Les raies de Fraunhofer permettent de calibrer (H) (Fe) lPeak NO2 observé dans l’atmosphère 7 x 10-6 estimé