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

Workshop: First look, Calibrations & RV standard IAP-05-11-24. Presentation of the Spectroscopic Global Iterative Solution The operations of SGIS Prototype & Perspectives of SGIS. An example of calibration: The wavelength calibration. Antoine Guerrier GEPI.

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

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  1. Workshop: First look, Calibrations & RV standard IAP-05-11-24 • Presentation of the Spectroscopic Global Iterative Solution • The operations of SGIS • Prototype & Perspectives of SGIS An example of calibration: The wavelength calibration Antoine Guerrier GEPI

  2. 1. Presentation of the SGIS concept IAP-05-11-24 1. The presentation ofthe Spectroscopic Global Iterative Solution (SGIS)

  3. 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

  4. 1.2. The reference sources IAP-05-11-24 • 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 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

  5. 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

  6. 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: • Derivation of the RV: • Wavelength calibrations used to calibrate raw spectra • Calibrated spectra used to derive RV • Calibration of the wavelength scale: • RV used to shift wavelengths of reference lines • 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-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

  8. 1.6. The scheme of SGIS IAP-05-11-24 INITIALIZATION SOURCE UPDATING  e.g. Radial Velocities REFERENCE SELECTION  Bright and stable stars Iterative processes CALIBRATION UPDATING  e.g. l-Pixel Relation ZERO POINT

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

  10. 2.1. The Source Updating step IAP-05-11-24 • First iterative step of the SGIS • Derivation of the Radial Velocity of the sources • by a classical cross-correlation algorithm: • Select a template spectrum (rest synthetic spectrum) • Apply wavelength calibrations on the raw spectrum • Shift the template spectrum according to a RV range • Compute cross-correlation coefficient between template & calibrated spectrum • 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-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

  12. 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

  13. 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 Cmnfor each calibration unit

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

  15. 3.1. The implementation of SGIS IAP-05-11-24 Data model Tools SGIS processing math spectrogis instrument initializing sourceUpdating sourceObs referenceSelection calibrationUpdating auxiliary control Test & Performance • 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

  16. 3.2. The diagnostics of errors IAP-05-11-24 initialization source-Updating reference-Selection calibration-Updating Wavelength calibration diagnostic Wavelength calibration diagnostic Centroïding diagnostics RV diagnostics Wavelength calibration diagnostic • Assess the behaviour & performance of the prototype

  17. 3.3.1 Results - Iteration 1 IAP-05-11-24

  18. 3.3.2 Results - Iteration 2 IAP-05-11-24

  19. 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!

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