Wavelength Standards for IR Spectrographs at ESO Supporting high resolution spectroscopy Florian Kerber, ESO

Wavelength Standards for IR Wavelength Standards for IR Spectrographs at ESOSpectrographs at ESO

Supporting high resolution spectroscopy

Florian Kerber, ESO

F. Kerber (ESO) ESAC, March 22nd, 2007 2

Wavelength Calibration - Near IRWavelength Calibration - Near IR

Night Sky OH Meinel Bands (Abrams et al., 1994; Rousselot et al. 2000)– Present on science exposure

– Number of lines at given wavelength and resolution

– Variability

– Stability at high resolution ? (< 25 m s-1)

Lack of good sources – Lamps - Ne, Kr– Gas cells


Th-Ar hollow cathode lampTh-Ar hollow cathode lamp

Anode

Cathode

Kerber et al. 2007, ASP Conf. Series. 364, p.461


Hollow cathode lampHollow cathode lamp

Introduced by Paschen (1916) – Study of hyperfine structure

Atomic Absorption (AA) Spectroscopy (Walsh 1950s)– Lifetime - Getter– Quantitative chemical analysis– Great variety of materials– Mass production for commercial market


Th-Ar lamp:Visible and Near IRTh-Ar lamp:Visible and Near IR

Why Thorium ?– Nature: 232Th - no isotope splitting – zero nuclear spin - no hyperfine structure

Established standard source in Visual– Palmer & Engleman (1983) 278 - 1000 nm– FEROS, FLAMES, HARPS, UVES, X-shooter


CRIRESCRIRES

Cryogenic High Resolution InfraRed Echelle Spectrometer (CRIRES) at VLT– 950 - 5000 nm, – Resolution / 100,000– Project to establish

wavelength standards (NIST)


Th-Ar Lamp ProjectTh-Ar Lamp Project

Physical Modelling Group (ESO INS)– F. Kerber, P. Bristow

NIST Atomic Spectroscopy Group– J. Reader, G. Nave, C.J. Sansonetti

Space Telescope European Coordinating Facility (ST-ECF)– M.R. Rosa

ESO La Silla-Paranal Observatory– G. Lo Curto


Th-Ar lamp:Visible and Near IRTh-Ar lamp:Visible and Near IR

Th-Ar is work in progress for near-IR – Hinkle et al. (2001), 1000 - 2500 nm, 500 lines

• Gaps in wavelength coverage• FTS and grating spectrometer data

– Engleman et al. (2003), 1000 - 5000 nm, > 5000 lines

• High current source, rich Th spectrum• Differences in spectrum but …


Visible/IR FTS at NISTVisible/IR FTS at NIST


NIST Experimental WorkNIST Experimental Work

2-m UV/Visible/IR FTS – CaF2 beamsplitter, silver coated mirrors, InSb detectors

Two different settings optimized for wavelength– 800 - 2000 nm, resolution 0.01 cm-1

– > 2000 nm, resolution 0.005 cm-1

Data acquisition times: up to 20 hours Radiometric standard lamp


Th-Ar


CRIRES

FTS


Th-Ar at NIST: ResultsTh-Ar at NIST: Results

Total ~2500 lines between 750 nm and 4800 nm – accuracy ~ 0.001 cm-1 for strong lines

Absolute accuracy in wavelength: ~1.4 x 10-8 – calibration using laser measurements of Th lines

(DeGraffenreid & Sansonetti 2002) – 5 x 10-8 1/100 of a pixel in CRIRES

Engleman et al. (2003) and our work agree:– Th: (6 ± 2) x 10-9 (weighted mean)– Ar: (-2 ± 0.2) x 10-8


ResultsResults

The absolute accuracy in wavelength is about 1.4 10-8 – calibration using laser measurements of Th lines (DeGraffenreid &

Sansonetti 2002, JOSA, B19, 1715) Engleman et al (2003) and our work agree:

– Th: 6 x 10-9 (weighted mean)

– Ar: 2 x 10-8

CRIRES wavelength calibration: 5 x 10-8 corresponds to ~1/100 of a pixel

Th-Ar lamp is able to support CRIRES science


Additional MeasurementsAdditional Measurements

Parabolic Mirror

Integrating Sphere

FTS

Elliptical Mirror

Sample Compartment

At ESO


Spectrum - Operating CurrentSpectrum - Operating Current

0

1

2

3

4

5

6

7

8

2 6 10 14 18 22

Lamp operating current [mA]

Intensity [normalised to 10 mA]

ArgonThorium


Gas Cells - Status and next StepsGas Cells - Status and next Steps

CRIRES commissioning: efficient use of the N2O gas cells (l=18 cm) as calibration source: ~2200 - 4100 nm

primary calibration molecule from Heterodyne frequency measurements at NIST (Maki & Wells)– Fundamental mode: 523 – 2845 cm -1 (1912 – 3515 nm)

Characterize spectrum of the CRIRES gas cell (P,T) Q2 2007– optimal choice of fill gas pressure - measurements using ESO’s FTS

Wavelength Standards for first overtone transitions of N2O Possible use of OCS (carbonyl sulfide) as calibrator (Q2 07)

– separate gas cell or in a mixture with N2O Long-term Stability of spectrum (Q4 2007)


Gas Cells - NGas Cells - N22O O


Summary: CRIRES Calibration Summary: CRIRES Calibration

• Calibration Reference Data– Established Wavelength Standards

Th-Ar: 950 - 2200 nm, ~5000 lines; N2O: 2200 - 4100 nm– Laboratory “ground truth” Accuracy few

parts in 10-8; ≤ 1/100 pixel

• Properties of Sources– Optimize use: Th-Ar - current; N2O - gas pressure

• Paradigma for wavelength calibration in near IR– very similar to the UV-visible region


OutlookOutlook

• CRIRES Science Verification: – Nissen et al. astro-ph/0702689: Sulphur abundances

• CRIRES operational: April 1st, 2007– ESO archive

• Spectral Data Model - Simple Line Access Protocol• Publish IR data set for Th: VOTable - metadata

– Q2/Q3 2007– linking astronomical and atomic physics communities

Documents

Wavelength Standards for IR Spectrographs at ESO Supporting high resolution spectroscopy Florian Kerber, ESO