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HARPS ... North

HARPS ... North. Principles. HARPS-N PDR Harvard CfA, 06/07-12-2007. What’s HARPS?. Fiber fed, cross-disperser echelle spectrograph Spectral resolution: geometrical 84’000, optical 115’000 Field: 1 arcsec on the sky (HARPS-N: 0.9 arcsec!)

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HARPS ... North

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  1. HARPS ... North • Principles HARPS-N PDR Harvard CfA, 06/07-12-2007

  2. What’s HARPS? • Fiber fed, cross-disperser echelle spectrograph • Spectral resolution: geometrical 84’000, optical 115’000 • Field: 1 arcsec on the sky (HARPS-N: 0.9 arcsec!) • Wavelength range: 383 nm - 690 nm • Sampling: 4 px per geometrical SE (3.3 real) • Environmental control • Drift measurement via simultaneous thorium

  3. The Doppler measurement cross-correlation mask

  4. Error sources • Stellar noise (or any other object) • Contaminants (Earth’s atmosphere, moon, etc.) • Instrumental noise • Calibration accuracy (any technique) • Instrumental stability (from calibration to measurement) • Photon noise

  5. Stellar “noise”: p-modes

  6. Stellar “noise”:p-modes

  7. Stellar “noise”: Activity

  8. Contaminants: Atmosphere

  9. Photon “noise” • Is NOT only SNR !!!! • Spectral resolution • Spectral type • Stellar rotation

  10. Contaminants: Close-by objects

  11. Photon “noise”: Spectral information Flux

  12. Photon “noise”: Spectral resolution

  13. Photon “noise”: Stellar rotation

  14. Instrumental errors • External • Illumination of the spectrograph • Internal • “Motion” of the spectrum on the detector

  15. Limitations:Telescope centering and guiding Stored guiding image for QC Slit spectrograph 1 arcsec Δ RV

  16. Limitations:Light-feeding Guiding error: 0.5’’ → 2-3 m/s for a fiber-fed spectrograph Fiber-fed spectrograph Fiber entrance Image scrambler Fiber exit

  17. Instrumental stability ΔRV = 1 m/s Δλ = 0.00001 A 15 nm 1/1000 pixel ΔRV =1 m/s ΔT = 0.01 K Δp = 0.01 mBar Vacuum operation Temperature control

  18. Design Elements • Fiber feed (mandatory for this techniques) • Stable enviroment (gravity, vibrations, etc.) • Image Scrambling • No moving or sensitive parts after fiber • SIMPLE and ROBUST optomechanics • “Best” (reasonably) achievable env. control • Vacuum operation • Thermal control • High spectral resolution

  19. Instrumental stability

  20. Line (and Instrumental) stability Absolute position on the CCD of a Th line over one month

  21. Simultaneous reference Object ThAr

  22. 0 RV 0 RV Object spectrum ThAr spectrum Wavelength calibration Object fiber ThAr reference

  23. RV (object) = RV (measured) RV(drift) - 0 RV 0 RV RV (measured) RV(drift) Object spectrum ThAr spectrum Measurement Object fiber ThAr reference

  24. Simultaneous reference

  25. The wavelength calibration px

  26. Instrumental errors: Calibration • pixel-position precision • photon noise • blends • pixel inhomogeneities, block stitching errors • accuracy of the wavelength standard • systematic errors, Atlas, RSF • instabilities (time, physical conditions: T, p, I) • accuracy of the fit algorithm

  27. Calibration: The problem of blends Isolated lines are very rare! Fit neighbouring lines simultaneously with multiple Gaussians

  28. But HARPS-N is also ... • ... a software concept delivering full precision observables: • Scheduling many observations efficiently • Full quality pipeline available at the telescope • Fully automatic, in “near” realtime, RV computation • Link to data analysis • Continuous improvements and follow-up

  29. Limiting factors and possible improvements • New calibration (and sim. reference) source • Perfect guiding and/or scrambling, good IQ needed • Improve detector stability (mounting, thermal control)

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