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

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

3. No on-board calibration device (e.g. calibration lamp)
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-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 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. How to use the reference sources observed?
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-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
SOURCE UPDATING
 e.g. Radial Velocities
REFERENCE SELECTION
 Bright and stable stars
CALIBRATION UPDATING
 e.g. l-Pixel Relation
INITIALIZATION
Iterative processes
ZERO POINT

9. 2. The main operations of SGIS
2. The operations of SGIS IAP
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 Cmn for each calibration unit

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

15. 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 processing
Data model
spectrogis
math
auxiliary
Tools
instrument
sourceObs
control
Test & Performance
referenceSelection
sourceUpdating
calibrationUpdating
initializing

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

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!