04_RVS-IAP-Calyyyyyyyyyyyyyyyyyyyuu2.ppt

Presentation

1. Workshop: First look, Calibrations & RV standard IAP-05-11-24
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-05-11-24
1. The presentation of
the 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 5.10
4
F8GK sources V<10 (about 4,6.10
5
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:
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-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 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:
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-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-samplefunction 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
mnfor 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
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

16. 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

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!