Physical Modelling of Instruments

1. Physical Modelling of Instruments Activities in ESO’s Instrumentation Division Florian Kerber, Paul Bristow

2. Our Partners • INS, TEC, DMD, LPO, … • Instrument Teams (CRIRES, X-shooter …) • Space Telescope European Coordinating Facility (ST-ECF) • M.R. Rosa • Atomic Spectroscopy Group (NIST) • J. Reader, G. Nave, C.J. Sansonetti • CHARMS (NASA, Goddard SFC) • D.B. Leviton, B.J. Frey

3. Outline • Instrument Modelling - Concept • Instrument Modelling - Basics • Instrument Modelling - Details • Input for the Model • Discussion

4. Building & Operating an Instrument • Science Requirements • Optical Design (code V, Zemax) • Engineering Expertise • Testing and Commissioning • Operation and Data Flow • Calibration of Instrument • Scientific Data and Archive

5. From Concept to Application • M. Rosa: Predictive calibration strategies: The FOS as a case study (1995) • P. Ballester, M. Rosa: Modeling echelle spectrographs (A&AS 126, 563, 1997) • P. Ballester, M. Rosa: Instrument Modelling in Observational Astronomy (ADASS XIII, 2004) • Bristow, Kerber, Rosa: four papers in HST Calibration Workshop, 2006 • UVES, SINFONI, FOS, STIS, VLTI, ETC

6. Physical Model • Optical Model (Ray trace) • High quality Input Data • Simulated Data • Close loop between Model and Observations • Optimizer Tool (Simulated Annealing)

7. STIS-CE Lamp Project Echelle, c  251.3 nm • Pt-Ne atlas, Reader et al. (1990) done for GHRS • STIS uses Pt/Cr-Ne lamp • Impact of the Cr lines strongest in the NUV • List of > 5000 lines •  accurate to < 1/1000 nm # of lines: Pt-Ne 258 # of lines: Pt-Ne 258 vs Pt/Cr-Ne 1612

8. STIS

9. STIS Science Demo Case: Result 10-4 nm 1 pixel Standard:=(3.3 ± 1.9) STIS Model:=(0.6 ± 1.7)

10. Traditional Wavelength Calibration • Data collected for known wavelength source (lamp or sky): • Match observed features to wavelengths of known features • Fit detector location against wavelength => polynomial dispersion solution

11. Physical Model Approach • Essentially same input as the polynomial: • x,y location on detector • Entrance slit position (ps) & wavelength () • Require that the model maps: for all observed features.

12. CRIRES • 950 - 5000 nm • Resolution / 100,000 • ZnSe pre-disperser prism • Echelle 31.6 lines/mm • 4 x Aladdin III 1k x1k InSb array • Commissioning June 06

13. Model Kernel

14. Model Kernel • Speed • Streamlined (simplistic) description • Fast - suitable for multiple realisations • Spectrograph (CRIRES - cold part only) • Tips and tilts of principal components • Dispersive behaviour of prism and grating • Detector layout • This is not a full optical model

15. Operating Modes (foreseen) • General optimisation (calibration scientist, offline) • Grating & prism optimisation (automatic) • Data reduction (pipeline) • Data simulation (interactive, offline)

17. Operating Modes (foreseen) • General optimisation (calibration scientist, offline) • Grating & prism optimisation (automatic) • Data reduction (pipeline) • Data simulation (interactive, offline)

18. Operating Modes (foreseen) • General optimisation (calibration scientist, offline) • Grating & prism optimisation (automatic) • Data reduction (pipeline) • Data simulation (interactive, offline)

19. Operating Modes (foreseen) • General optimisation (calibration scientist, offline) • Grating & prism optimisation (automatic) • Data reduction (pipeline) • Data simulation (interactive, offline)

21. Simulated Stellar Spectrum

23. Optimisation Strategy • Take limits from design and construction • One order/mode - rich spectra • Optimise detector layout • Multiple order/modes (detector layout fixed) • Optimise all except prism/grating • All order/modes (all parameters fixed except prism/grating) • Optimise prism/grating settings for each mode

24. Near IR Wavelength Standards 1270–1290 nm Ne Kr Th-Ar

25. Th-Ar lamp:Visible and Near IR • Established standard source in Visual • Palmer & Engleman (1983) 278 - 1000 nm • FEROS, FLAMES, HARPS, UVES, Xshooter • Cryogenic High Resolution Echelle Spectrometer (CRIRES) at VLT • 950 - 5000 nm, Resolution ~100,000 • Project to establish wavelength standards (NIST) • UV/VIS/IR 2 m Fourier Transform Spectrometer (FTS)

27. Measurements with FTS at ESO

28. Spectrum - Operating Current

29. Th-Ar in the near IR: Summary • > 2000 lines as wavelength standards in the range 900 - 4500 nm • insight into the properties of Th-Ar lamps, variation of the spectral output/continuum as a function of current • Th-Ar hollow cathode lamps - a standard source for wavelength calibration for near IR astronomy

30. CRIRES pre-disperser prism - ZnSe n(,T) from CHARMS, (GSFC, NASA) Leviton & Frey, 2004

31. ZnSe Prism: Temperature 73 - 77 K • Measured line shifts • Physical Model • Th-Ar line list • n(,T) & dn/dT of ZnSe 1124 1138 Wavelength [nm]

32. Location of Th-Ar lines - Temperature

33. Conclusions - Physical Model • Preserve know how about instrument • Replace empirical wavelength calibration • High quality input data is essential • Predictive power • Support instrument development • assess expected performance • reduce risk • Calibration data is still required!

34. Conclusions - Physical Model • The resulting calibration is predictive and expected to be more precise • The process of optimising the model is somewhat more complex than fitting a polynomial • Understanding of physical properties and their changes • CRIRES will be the first ESO instrument to utilise this approach to calibration