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Workshop: First look, Calibrations & RV standard IAP-05-11-24 An example of calibration: The wavelength calibration Antoine Guerrier GEPI 1.Presentation.

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Presentation on theme: "Workshop: First look, Calibrations & RV standard IAP-05-11-24 An example of calibration: The wavelength calibration Antoine Guerrier GEPI 1.Presentation."— Presentation transcript:

1 Workshop: First look, Calibrations & RV standard IAP An example of calibration: The wavelength calibration Antoine Guerrier GEPI 1.Presentation of the Spectroscopic Global Iterative Solution 2.The operations of SGIS 3.Prototype & Perspectives of SGIS

2 1. Presentation of the SGIS concept IAP The presentation of the Spectroscopic Global Iterative Solution (SGIS)

3 1.1. Problematic IAP 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

4 1.2. The reference sources IAP  Idea = Use sources observed by the RVS instrument Use reference sources (i.e. bright and stable stars)  How many sources usable for the wavelength calibration?  About F8GK sources V<10 (about 4, 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

5 1.3. Analogy IAP  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

6 1.4. An iterative process IAP  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 RV 2.Calibration of the wavelength scale: i.RV used to shift wavelengths of reference lines ii.Reference lines used to compute wavelength calibrations  An iterative approach is needed  Each iteration refine the RV & calibration data

7 1.5. Non iterative steps of SGIS IAP 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

8 1.6. The scheme of SGIS IAP SOURCE UPDATING  e.g. Radial Velocities REFERENCE SELECTION  Bright and stable stars CALIBRATION UPDATING  e.g. -Pixel Relation INITIALIZATION Iterative processes ZERO POINT

9 2. The operations of SGIS IAP The main operations of SGIS

10 2.1. The Source Updating step IAP  First iterative step of the SGIS  Derivation of the Radial Velocity of the sources by 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 range iv.Compute cross-correlation coefficient between template & calibrated spectrum v.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

11 2.3. The Reference Selection step IAP  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

12 2.4.1 The Calibration Updating step IAP  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

13 2.4.2 The Calibration Updating step - Prototype example IAP  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 C mn for each calibration unit

14 3. Prototype & Perspectives of SGIS IAP Prototype & Perspectives of SGIS

15 3.1. The implementation of SGIS IAP  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 processing Data model spectrogis math auxiliary Tools instrument sourceObs control Test & Performance referenceSelection sourceUpdating calibrationUpdating initializing

16 3.2. The diagnostics of errors IAP source- Updating reference- Selection calibration- Updating initialization Wavelength calibration diagnostic RV diagnostics Centro ï ding diagnostics Wavelength calibration diagnostic  Assess the behaviour & performance of the prototype

17 3.3.1 Results - Iteration 1 IAP

18 3.3.2 Results - Iteration 2 IAP

19 3.4. Conclusions & Perspectives IAP  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!


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