The Absolute Reference Spectrograph at the VTTPrecision Solar Spectroscopy with a Laser Frequency Comb

H.-P. Doerr, T. J. Kentischer, M. Franz, W. Schmidt (KIS)T. Steinmetz, R. Probst, R. Holzwarth (MPQ, Garching)

CASSDA & SOLARNET MeetingFreiburg, 19 February 2014

Laser-Frequency-What?

Laser Frequency Combs (LFC)Optical frequency synthesizer with sub-Hz accuracyUltimate calibration sources for Astr. Spectrographs(Murphy et al. 2007, MNRAS 380)

First ever demonstration of an “Astro-Comb” at VTT in 2008(ESO/MPQ; Steinmetz et al. 2008, Sci 321)

Several groups working on astronomical applications(ESO/MPQ, MPQ/KIS, Harvard, Boulder)

LARS is an Absolute Reference SpectrographKIS received Leibniz grant to establish visible light LFCcalibrator at VTT in collaboration with MPQ, GarchingTarget instrument: VTT Echelle (R ≈ 106)Absolute wavelength calibration better than 1 m/s possible(Doerr et al. 2012, SPIE 8450)

Laser-Frequency-What?

Laser Frequency Combs (LFC)Optical frequency synthesizer with sub-Hz accuracyUltimate calibration sources for Astr. Spectrographs(Murphy et al. 2007, MNRAS 380)

First ever demonstration of an “Astro-Comb” at VTT in 2008(ESO/MPQ; Steinmetz et al. 2008, Sci 321)

Several groups working on astronomical applications(ESO/MPQ, MPQ/KIS, Harvard, Boulder)

LARS is an Absolute Reference SpectrographKIS received Leibniz grant to establish visible light LFCcalibrator at VTT in collaboration with MPQ, GarchingTarget instrument: VTT Echelle (R ≈ 106)Absolute wavelength calibration better than 1 m/s possible(Doerr et al. 2012, SPIE 8450)

Laser-Frequency-What?

Laser Frequency Combs (LFC)Optical frequency synthesizer with sub-Hz accuracyUltimate calibration sources for Astr. Spectrographs(Murphy et al. 2007, MNRAS 380)

First ever demonstration of an “Astro-Comb” at VTT in 2008(ESO/MPQ; Steinmetz et al. 2008, Sci 321)

Several groups working on astronomical applications(ESO/MPQ, MPQ/KIS, Harvard, Boulder)

LARS is an Absolute Reference SpectrographKIS received Leibniz grant to establish visible light LFCcalibrator at VTT in collaboration with MPQ, GarchingTarget instrument: VTT Echelle (R ≈ 106)Absolute wavelength calibration better than 1 m/s possible(Doerr et al. 2012, SPIE 8450)

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0.5

1

630.1 630.2 630.3

Inte

nsi

ty (

Arb

itra

ry U

nit

s)

Air Wavelength (nm)

Motivation: LARS solar + calibration spectrum

475.5775900(0) THz5.445 GHz

70

0 M

Hz

FWH

M

Instrument Schematics

GPS stabilized10 MHz Oscillator

High-power amplifier +pulse compressorFrequency conversion

Spectral broadening

CW Laser247.5 MHz

Mode-lockedfs-laser

1060 nm

Farby-Perot filter cavities

247.5 MHz

1 MHz

5.445 GHz 94 dB

247.5 MHz

1 MHz

5.445 GHz 94 dB

Fiber Switch

ContextImager

VTT fibercoupling unit

Light from VTT

Fiber from ChroTelFiber from Flatfield Lamp

To spectrograph fiber coupling unit

In Vivo

In Vivo

In Vivo

In Vivo

Calibration Performance

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80000

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0 512 1024 1536 2048

Inte

nsity

(Pho

toE

lect

rons

)

Wavelength (Pixel)

VTT Astro-Comb calibration spectrum @630 nm

1040 1080 1120

Calibration Performance

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-1

0

1

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Residua

lsfrom

linearfit

(m/s)

Residua

lsfrom

qubicfit

(m/s)

Wavelength (Pixel)

Calibration curve residuals

RMS qubic fit: 60 cm/sPhoton noise: 33 cm/s

Linear fitQubic fit

VTT Spectrograph stability

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ativ

ew

avel

engt

hdi

spla

cem

ent

(m/s

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Time - T0 (Seconds)

Short-term Spectrograph stability

short term: spectrograph seeing; ultra-short: vibrations?

VTT Spectrograph stability

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759

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ft (

m/s

)

Pre

ssure

(hPa

)

T - T0 = 06Mar2013 01:27 (Hours)

Spektrograph Drift vs air pressure (slit)

DriftPressure

long-term: air pressure (+temperature)

Instrument Characteristics

Single-mode fiber feed (SMFF; 1, 3, 10 arcsec FOV)Operating wavelength range ≈ 460 − 650 nmSpectral resolution ≈ 750 000Frequency solution accurate at ≈ 10 cm s−1 per exposure(Seeing limits repeatability; ≈ 1m/s, but averages out)Virtually no scattered lightPSF comes for free!“Real” flat-fields from continuum lamp; excellent fidelity

ApplicationsLarge-scale solar structures; Lab measurementsNot for: small-scale velocity fieldsBut: we can cross-calibrate co-observing instruments!

Instrument Characteristics

Single-mode fiber feed (SMFF; 1, 3, 10 arcsec FOV)Operating wavelength range ≈ 460 − 650 nmSpectral resolution ≈ 750 000Frequency solution accurate at ≈ 10 cm s−1 per exposure(Seeing limits repeatability; ≈ 1m/s, but averages out)Virtually no scattered lightPSF comes for free!“Real” flat-fields from continuum lamp; excellent fidelity

ApplicationsLarge-scale solar structures; Lab measurementsNot for: small-scale velocity fieldsBut: we can cross-calibrate co-observing instruments!

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0.5

1

6300 6300.5 6301 6301.5 6302 6302.5 6303

Norm

aliz

ed

Inte

nsi

ty

Wavelength (A)

LARS fidelity in comparison

Kitt-Peak FTSLiegeLARS

0.95

1

6300 6300.5 6301

0.94

0.96

0.98

6301.8 6302

Instrument Characteristics

Single-mode fiber feed (SMFF; 1, 3, 10 arcsec FOV)Operating wavelength range ≈ 460 − 650 nmSpectral resolution ≈ 750 000Frequency solution accurate at ≈ 10 cm s−1 per exposure(Seeing limits repeatability; ≈ 1m/s, but averages out)Virtually no scattered lightPSF comes for free!“Real” flat-fields from continuum lamp; excellent fidelity

ApplicationsLarge-scale solar structures; Lab measurementsNot for: small-scale velocity fieldsBut: we can cross-calibrate co-observing instruments!

Example: Telluric O2 Wavelengths

1.30

1.32

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1.42

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1.46

1.48

1.50

1.52

02Aug12h

03Aug00h

03Aug12h

04Aug00h

04Aug12h

05Aug00h

05Aug12h

06Aug00h

1

1.2

1.4

1.6

1.8

2

2.2

Depart

ure

fro

m P

ierc

e B

reck

inri

dge (

mÅ

)

Air

mass

Date

LARS wavelengths of telluric O2 6298.5 Å line

wavelengthairmass

Example: Telluric O2 Wavelengths

-25

-20

-15

-10

-5

0

5

0 1 2 3 4 5 6 7

rela

tive lin

e c

ente

r sh

ift

(m/s

)

Time - 2013-12-03 9:44 UT (hours)

Relative shift, telluric O2 6298.46

Example: Telluric O2 Wavelengths

Pierce & Breckinridge (1973), LARS preliminary(!) (Aug 2013)

P & B LARS ∆ (mÅ)

6298.4571 6298.45849 ± 0.00007 1.396299.2296 6299.23074 ± 0.00006 1.146302.0005 6302.00158 ± 0.00007 1.086302.7629 6302.76520 ± 0.00004 2.30

Caveat: defintion of wavelength of an asymmetric lineLARS wavelengths: parabolic fit ± 10mÅP&B: “lowest 5 - 10%”