Astronomical frequency comb for calibration of low and medium - - PowerPoint PPT Presentation

Astronomical frequency comb for calibration of low and medium resolution spectrographs

• innoFSPEC at AIP has several years expertise in astronomical instrumentation.
• innoFSPEC succesfully developed a new technique to generate an optical

source (called astro-comb) suitable for calibration of low and medium resolution spectrographs.

• The instrument is compact and easy to operate and has been tested on-sky

using the Potsdam Multi-aperture Spectrometer (PMAS) at the 3.5 m telescope at the Calar Alto Observatory (Spain).

• The instrument offers a continuous tune of the position of the calibration lines –

this brings the possibility of a complete characterization of the line spread function of the spectrograph at any resolution.

• We seek for partners at GEMINI that can use this instrument and could request

for additional features of our tunable calibration source.

The next slides describe the standing problems in wavelength calibration and results of the on- sky test with the frequency comb source developed at innoFSPEC Potsdam

Wavelength calibration

4

• Wavelength calibration sources:
• Lamps of He, U-Ne, Th/Ar, Hg.
• Iodine cells.
• Accurate wavelength calibration is required for:
• Search for extra-solar planets via the radial velocity method.
• Search for time variation of the fine structure constant (α).
• Measurement of the acceleration of the universe expansion.
• Stellar population.

Use of astronomical instrumentation relies heavily on accurate calibration procedures.

• Spectrographs are calibrated in: wavelength and flat field.
• Throughput: Response of whole telescope optics and spectrograph.

CCD pixel

science Calibration light

5

Calibration sources

Wavelength (Å) Flux

He lines

Wavelength

• Lamps characteristics limit the accuracy of wavelength calibration:
• Lines are not equally spaced nor equally intense.
• Lines are blended.
• Narrow spectral coverage.
• Lines’ frequencies are known with ∆ λ

/λ = 10-7 accuracy.

• ‘Ideal’ wavelength calibration source:
• Equally spaced and equally intense lines.
• Lines spaced 3-4 times the resolution of the spectrograph.
• Spectral coverage over the whole spectrograph operation range.
• Lines’ frequencies known with ∆ λ

/λ = 10-8 accuracy or better.

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Astro-comb

• Astronomical combs (astro-combs) have the characteristics
• f an ideal calibrator: but too tightly spaced lines.
• Comb mode spacing is increased by filtering with Fabry-Perot cavities.

after filtering

>20 GHz (80pm)



crosstalk

Before filtering

0.5 GHz (2pm)

Octave- spanning laser-comb Stabilization electronics

To spectral broadening Stabilization electronics

Fabry Perot cavities

Astro-combs are commercially available at 500-700 K Euro

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Any alternative to actual astro-combs?

a)

2000 4000 6000 8000 10000

1555,164 1555,166 1555,168 1555,170 1555,172

Wavelength (nm) Time (s)

b)

• Actual astro-combs are complex and expensive.
• We developed a tunable and compact astro-comb

for medium and low resolution spectrographs.

• The astro-comb stability is tracked with a wavemeter

having 0.3 pm resolution.

• The comb lines are stable within 0.025 pm during the astro-comb exposure time.

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Initial lasers 23 THz

• Broadband spectrum (400 nm

bandwidth) centered at 1580 nm.

• Optical-signal-to-noise ratio > 30 dB.
• 170 equally spaced lines are

generated out of 2 lasers.

• After frequency doubling, spectrum

is centered at 800 nm.

Comb generation bandwidth

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On-sky test with PMAS at Calar Alto

2 5 5

• PMAS: Integral Field Instrument at the Calar

Alto Observatory 3.5 m telescope:

• Wavelength coverage from 390 to 930 nm
• 4096×4096 CCD detector with 15 mm pixel size.
• Resolution of ~7000 (@800 nm).
• Neon: 20 randomly distributed lines with

very unequal intensities.

• Astro-comb: 300 lines evenly spaced in

frequency.

Neon Neon

Astro-comb Astro-comb

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Gaussian fitting

• A P3d data reduction software

allows analyzing the measured spectra.

• Each comb line is sampled by ~5

pixels: fitting uses a Gaussian function.

• Determination of the centroid and

the width of each comb line.

• Calculation is performed for each
• f the 400 fibers.

centroid width Line profile (width) measures the resolution of the spectrograph.

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Tunability of astro-comb

50 100 150 200 250 300 340 360 380 400 420

Comb line Frequency (THz)

a)

294.14 ± 0.005 GHz

20 40 60 80 100 120 140 340 350 360 370 380 390 400 410

Frequency (THz)

Comb line

454.34 ± 0.03 GHz

• The astro-comb can be set

for several comb spacing.

• Approriate for low-, medium- and

high-resolution spectrographs.

• Frequency equidistancy is verified

with 1 pm accuracy.

• Useful for assessing line spread function

characteristics.

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Sky lines exposures with PMAS

• Exposures of OH emission lines were

calibrated either with Neon (black) or astro-comb (green and red) light.

• The wavelengths of the OH doublets

are indicated with vertical lines.

• Due to the lack of Neon lines at some

spectral intervals the calibration is Inaccurate up to 20 pm.

• Astro-comb offers more accurate

wavelength calibration.

780 800 820 840 1500 3000 4500 6000 7500

Wavelength (nm)

Intensity (linear units)

a)

774,8 774,9 775,0 775,1 775,2 775,3 2400 3600 4800 6000

Wavelength (nm)

Intensity (linear units)

b)

834,2 834,3 834,4 834,5 834,6 834,7 1500 3000 4500 6000 7500

Intensity (linear units)

Wavelength (nm)

c)

842,8 842,9 843,0 843,1 843,2 843,3 1500 3000 4500 6000 7500

Intensity (linear units)

d)

Wavelength (nm)

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Ca-triplet exposures in stars

853,0 853,5 854,0 854,5 855,0 855,5 6000 8000 10000 12000 14000 16000

Wavelength (nm) Intensity (linear units)

HD3765 HD219538

b)

848 852 856 860 864 868 6000 8000 10000 12000 14000 16000 18000

Wavelength (nm) Intensity (linear units)

a)

• Exposures on several stars to analyze

Ca triplet absorption lines.

• Wavelength calibration is performed

with Neon and astro-comb.

• Examples with HD3765 and HD219538

show the ability of astro-combs to perform wavelength calibration perhaps with superior accuracy than Neon lamps.

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Conclusions

• Simple scheme for tunable astro-comb generation was demonstrated.
• Astro-comb provides much more calibration (stable) lines than lamps.
• On-sky test shows astro-comb suitability through examples with

OH emission lines detection and stellar exposures.

Contact information:

InnoFSPEC-VKS, Leibniz-Institut für Astrophysik, Potsdam, Germany

• Dr. Jose Boggio (jboggio@aip.de)
• Dr. Domenico Giannone (dgiannone@aip.de)