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An Overview of Spatial 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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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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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 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

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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.

x

Zero spatial frequency at the Littrow wavenumber σo

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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 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

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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

x

x

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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

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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

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Pine Bluff, WI (89o 40’ W, 43o 04’ N)

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Ne 3727.105 APix 733

Ne 3719.8 APix 441

Ne 3713.08 APix 160

R~20,000

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OII SHS on

OII SHS off

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Slanger, Keck order 96 (high res)

OII SHS on

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• 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

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the end

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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