Workshop: First look, Calibrations

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

• An example of calibration
• The wavelength calibration

1. Presentation of the Spectroscopic Global
   Iterative Solution
2. The operations of SGIS
3. Prototype Perspectives of SGIS

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

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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 Vlt10 (about 4,6.105
  Vlt12)
• (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

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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

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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

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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
• - N1 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

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

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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

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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

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

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

• Assess the behaviour performance of the
  prototype

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3.3.1 Results - Iteration 1 IAP-05-11-24
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3.3.2 Results - Iteration 2 IAP-05-11-24
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3.4. Conclusions Perspectives
IAP-05-11-24

• 1000 observations per calibration unit
  Accuracy lt 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!