An Overview of Spatial Heterodyne Spectroscopy Overview of Spatial Heterodyne Spectroscopy Principle...

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  • An Overview ofSpatial Heterodyne Spectroscopy

    Principle OII SHS participants:

    Fred Roesler (University of Wisconsin)John Harlander (St. Cloud State University)Edwin Mierkiewicz (University of Wisconsin)Ronald J. Reynolds (University of Wisconsin)Kurt Jaehnig (University of Wisconsin)

  • G

    G

    B.S.

    A

    incident wavefront

    12 exiting wavefronts

    Imaging Detector

    input

    B.S. G

    P1

    P2

    Input

    Output

    a) b)

    G

    The Spatial Heterodyne SpectrometerTransmitting SHS Properties

    SHS is basically a Michelsoninterferometer with the return mirrorsreplaced by fixed diffraction gratings G.

    For each wavenumber in the wavefrontentering the interferometer, twowavefronts exit the system with awavenumber-dependent crossing anglebetween them.

    This produces a superposition of Fizeaufringes with wavenumber-dependentspatial frequencies localized near thegratings.

    A position sensitive detector records theFizeau fringe pattern produced by theinterferometer.

    x

    x

  • The Spatial Heterodyne Spectrometer

    The heterodyne concept is evoked by the factthat the dispersive elements may be tuned toplace zero spatial frequency at a selectedwavenumber o, where o is the Littrowwavenumber of the diffraction gratings(2osin=m/d)

    For a system tuned to o, adjacent spectralelements o+, o+2,..o+n produce1,2,n-cycle spatial frequencies across thedetector.

    As each spectral element produces a uniquespatial frequency, the Fourier transform of therecorded spatial frequencies provides thespectrum within a limited spectral range(determined by the detector sampling) aboutthe heterodyne wavelength.

    x

    Zero spatial frequency at the Littrow wavenumber o

  • G

    G

    B.S.

    A

    incident wavefront

    12 exiting wavefronts

    Imaging Detector

    input

    B.S. G

    P1

    P2

    Input

    Output

    a) b)

    G

    The Spatial Heterodyne SpectrometerTransmitting SHS Additional SHS Properties

    No mechanical part is moved in thisprocess.

    The resolving power is the diffraction-limited resolving power of the gratingcombination.

    The throughput is that characteristic ofinterference spectrometers at the achievedresolving power.

    SHS can be field-widened with fixedprisms in each arm, giving SHS anenormous throughput gain overconventional systems of similar size andresolving power.

    x

    x

  • G

    G

    B.S.

    A

    incident wavefront

    12 exiting wavefronts

    Imaging Detector

    input

    B.S. G

    P1

    P2

    Input

    Output

    a) b)

    G

    The Spatial Heterodyne SpectrometerTransmitting SHS

    x

    x

  • The instrument described thus farproduces identical output for inputwavenumbers o+ and o-. Thisambiguity can be avoided by adding aslight y-tilt to one of the gratings.

    Wavenumbers > o are rotatedclockwise, while < o are rotatedcounter clockwise.

    3726.062 A3728.81 A

  • Diffuse [OII] 372.7nm from the Warm Ionized Medium

    Science Motivation: The warm ionized medium was surveyed with WHAM at 656.3nm Strong evidence for previously unrecognized energy sources has

    emerged [OII] 372.7nm emission is expected to verify their existence

    Why SHS?: WHAM does not work at 372.7nm Fabry-Perot efficiencies are low and tolerances high below 400nm The field-widened SHS is highly efficient in the NUV SHS is more tolerant of defects by >15 compared to a FP

    Status: Observations are underway with the OII system at PBO

  • Pine Bluff, WI (89o 40 W, 43o 04 N)

  • Ne 3727.105 APix 733

    Ne 3719.8 APix 441

    Ne 3713.08 APix 160

    R~20,000

  • OII SHS on

    OII SHS off

  • Slanger, Keck order 96 (high res)

    OII SHS on

  • Diffuse [OII] 372.7nm Emission from the Warm ISM

    SHIMMERSpatial Heterodyne Imager for Mesospheric RadicalsMeasuring mesospheric OH at 308.0nmSTS 112 and STPSat-1

    Diffuse CIV 155.0mn Emission from the Hot ISM

    Interplanetary Hydrogen 121.6nm

    Comet C/Neat (2001 Q4) OH and OI

    Daysky OI, R ~300,000

  • the end

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    (Harlander et al., 2004)

    Single well isolated lineat 307.59 nm; fivefringe tilt perpendicularto the dispersion plane

    Several emission featuresin the bandpass; provideswavelength calibrationand spectral resolution(~0.12 A)

    Continuum source,spectral shape dominatedby the prefilter; allspectral informationlocalized near zero path