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)
0
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
0
20000
40000
60000
80000
100000
120000
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
-100
-50
0
50
100
150
200
0 512 1024 1536 2048-3
-2
-1
0
1
2
3
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
-50
-40
-30
-20
-10
0
10
20
30
0 10 20 30 40 50 60 70 80 90 100
Rel
ativ
ew
avel
engt
hdi
spla
cem
ent
(m/s
)
Time - T0 (Seconds)
Short-term Spectrograph stability
short term: spectrograph seeing; ultra-short: vibrations?
VTT Spectrograph stability
-200
-100
0
100
200
300
400
500
600
700
800
0 1 2 3 4 5 6 7
759
760
761
762
763
764
Dri
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!
0
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
1.34
1.36
1.38
1.40
1.42
1.44
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%”