An Overview of Spatial Heterodyne Spectroscopy Overview of Spatial Heterodyne Spectroscopy Principle...
Transcript of An Overview of Spatial Heterodyne Spectroscopy Overview of Spatial Heterodyne Spectroscopy Principle...
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)
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B.S.
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incident wavefront
12 exiting wavefronts
Imaging Detector
input
B.S. θG
P1
P2
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Output
a) b)
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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.
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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 σo, where σo is the Littrowwavenumber of the diffraction gratings(2σosinθ=m/d)
For a system tuned to σo, adjacent spectralelements σo+δσ, σo+2δσ,..σo+nδσ 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.
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Zero spatial frequency at the Littrow wavenumber σo
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θ
G
G
B.S.
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incident wavefront
12 exiting wavefronts
Imaging Detector
input
B.S. θG
P1
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Input
Output
a) b)
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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
θ
θ
G
G
B.S.
A
incident wavefront
12 exiting wavefronts
Imaging Detector
input
B.S. θG
P1
P2
Input
Output
a) b)
θ
G
The Spatial Heterodyne SpectrometerTransmitting SHS
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x
The instrument described thus farproduces identical output for inputwavenumbers σo+δσ and σo-δσ. Thisambiguity can be avoided by adding aslight y-tilt to one of the gratings.
Wavenumbers σ > σo are rotatedclockwise, while σ < σo 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
Monoch
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Sourc
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2)
(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