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Transcript of cedarweb.vsp.ucar.educedarweb.vsp.ucar.edu/wiki/images/0/0a/POW_Mierkerwicz... · 2009. 2. 27. ·...
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)
θ
θ
G
G
B.S.
A
incident wavefront
12 exiting wavefronts
Imaging Detector
input
B.S. θG
P1
P2
Input
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.
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 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.
x
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
P2
Input
Output
a) b)
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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
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
rom
atic
Sourc
e (Z
n)
Em
issi
on L
ine
Sourc
e (M
nN
e)Bro
ad B
and
Sourc
e (D
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