Workshop: First look, Calibrations & RV standard IAP-05-11-24

An example of calibration: The wavelength calibration

Antoine GuerrierAntoine Guerrier

GEPIGEPI

1. Presentation of the Spectroscopic Global Iterative Solution2. The operations of SGIS3. Prototype & Perspectives of SGIS

1. Presentation of the SGIS concept IAP-05-11-24

1. The presentation ofthe Spectroscopic Global Iterative

Solution (SGIS)

1.1. Problematic IAP-05-11-24

No on-board calibration device (e.g. calibration lamp)

No specific observation for the calibration Not possible to compare to an “instrumental” reference source

Need an alternative calibration method

Possible alternative

The Spectroscopic Global Iterative Solution (SGIS)= Wavelength self-calibration of the RVS

1.2. The reference sources IAP-05-11-24

Idea = Use sources observed by the RVS instrumentUse reference sources (i.e. bright and stable stars)

How many sources usable for the wavelength calibration? About 5.104 F8GK sources V<10 (about 4,6.105 V<12) (GEPI/GAIA-RVS/TN/017.01) About 80 epochs per star

Measure the evolution of the instrument with its own observations: Large number of stable reference sources + Same evolution of the characteristics of the reference sources = Evolution of the characteristics of the instrument

1.3. Analogy IAP-05-11-24 How to use the reference sources observed?

By analogy with the ground-based observations:

Classical ground-based spectrograph Use “reference lines”:

Known wavelengths in the laboratory reference frame from a calibration lamp

SGIS approach Use stellar reference lines:

Known wavelengths, little blended and identified in the spectra of the reference sources collected by the RVS

1.4. An iterative process IAP-05-11-24 Position of stellar reference lines depend upon 2 parameters:

the radial velocity of the sources (RV) the spectral dispersion law of the instrument

RV & Spectral dispersion law linked

RV & Spectral dispersion law have to be determined:1. Derivation of the RV:

i. Wavelength calibrations used to calibrate raw spectra

ii. Calibrated spectra used to derive RV2. Calibration of the wavelength scale:

i. RV used to shift wavelengths of reference linesii. Reference lines used to compute wavelength

calibrations An iterative approach is needed

Each iteration refine the RV & calibration data

1.5. Non iterative steps of SGIS IAP-05-11-24 • Initialisation step:

- Starting point of the iterative process

- Initialize spectral dispersion law with:• Ground calibrations or commissioning calibrations or calibrations from first look

• Zero point correction step:

- N+1 iteration of the SGIS

- RV expressed in relative reference frame

- To be usable, RV should be expressed in absolute reference frame

(e.g. barycentre of the Solar System)

- Ground-based standards used to derive relative-to-absolute reference frame transformations

- Transformations = Zero point corrections

1.6. The scheme of SGIS IAP-05-11-24

SOURCE UPDATING e.g. Radial Velocities

REFERENCE SELECTION Bright and stable stars

CALIBRATION UPDATING

e.g. -Pixel Relation

INITIALIZATION

Iterative processes

ZERO POINT

2. The operations of SGIS IAP-05-11-24

2. The main operations of SGIS

2.1. The Source Updating step IAP-05-11-24 First iterative step of the SGIS

Derivation of the Radial Velocity of the sourcesby a classical cross-correlation algorithm:

i. Select a template spectrum (rest synthetic spectrum) ii. Apply wavelength calibrations on the raw spectrum iii. Shift the template spectrum according to a RV rangeiv. Compute cross-correlation coefficient between template &

calibrated spectrumv. Compute the maximum of the cross-correlation coefficients

Maximum of cross-correlation coefficients = Best match between template & calibrated spectra= RV of the source

The RV of the source Updated

2.3. The Reference Selection step IAP-05-11-24

Selection of the reference source used in wavelength calibration

Reference source should be: Stable in radial velocity Of appropriate stellar type (i.e. about 20 lines unblended or little blended)

Check, source by source, the astrophysical characteristics of the source

Qualify or reject as a reference

2.4.1 The Calibration Updating step IAP-05-11-24

Calibrate the RVS spectral dispersion law: associate a mean wavelength to any sample

Wavelength dispersion law assumed constant over interval of time

Calibration units

Function F constrained for each calibration unit

€

s = F(s)

2.4.2 The Calibration Updating step - Prototype example IAP-05-11-24 2 simplifying assumptions:

FoV-to-focal-plane transformations constant over duration of calibration unit Same constant velocities in the FoV for each source

Mean-central-wavelength-to-sample function F expressed as function of FoV coordinates at the readout time of the sample:

Function F represented by a 2nd order polynomial fit

Wavelength calibration = Compute Cmn for each calibration unit

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s = F(η ,ζ )

€

s = Cmnηmζ n

n=0

2

∑m =0

2

∑

3. Prototype & Perspectives of SGIS IAP-05-11-24

3. Prototype & Perspectives of SGIS

3.1. The implementation of SGIS IAP-05-11-24 JAVA development of the first version of the SGIS prototype

Test of non-divergence of the prototype: Initializing the spectral dispersion law with the true values Over 100 days of mission With 1000 G5V stars (same charact., e.g. RV = 0km/s) With 10 epochs per star

SGIS processingData model

spectrogis

math

auxiliary

Tools

instrument

sourceObs

control

Test & Performance

referenceSelection

sourceUpdating

calibrationUpdating

initializing

3.2. The diagnostics of errors IAP-05-11-24

source-Updating

reference-Selection

calibration-Updating

initialization

Wavelength calibration diagnostic

RV diagnostics Centroïding diagnostics

Wavelength calibration diagnostic

Wavelength calibration diagnostic

Assess the behaviour & performance of the prototype

3.3.1 Results - Iteration 1 IAP-05-11-24

3.3.2 Results - Iteration 2 IAP-05-11-24

3.4. Conclusions & Perspectives IAP-05-11-24 1000 observations per calibration unit = Accuracy < 1km.s-1

The first series of tests Tests of non-divergence of the prototype (true spectral law) Results: divergence Problems localized!

Non-symmetric profiles of reference lines in the spectra Centroiding degradation in the Calibration Updating Solution: calibrate the centroiding method

New series of tests to valid the non-divergence of the prototype

Test of convergence: Not initialize the spectral dispersion law with true values Observe the behaviour of the prototype over iterations

GEPI/GAIA-RVS/TN/018 coming soon!