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Optics in Astronomy - Interferometry -

Optics in Astronomy - Interferometry -. Oskar von der Lühe Kiepenheuer-Institut für Sonnenphysik Freiburg, Germany. Contents. Fundamentals of interferometry Concepts of interferometry (contd.) Practical interferometry. Concepts of interferometry (contd.) Differential delay tracking

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Optics in Astronomy - Interferometry -

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  1. Optics in Astronomy- Interferometry - Oskar von der Lühe Kiepenheuer-Institut für Sonnenphysik Freiburg, Germany

  2. Contents • Fundamentals of interferometry • Concepts of interferometry (contd.) • Practical interferometry • Concepts of interferometry (contd.) • Differential delay tracking • Observables • Sensitivity • Practical interferometry • Todays Interferometers and Science cases Optics in Astronomy

  3. Differential delay tracking Optics in Astronomy

  4. Differential delay tracking • Goals of DDT: • off-source phase referencing • narrow-angle astrometry Source of figures: PTI Optics in Astronomy

  5. Measured Quantities and Observables The measured quantity of interest is the correlated flux at wavelength  and angle frequency Fourier component of source intensity: Complex visibility: Optics in Astronomy

  6. Observables I • Group Delay : • Delay for which interference contrast is maximised • Group delay depends on • baseline • instrumental fixed delays • source position • delay errors: optics, vibrations, atmosphere • High precision measurement of permit relative position measurements with 1mas accuracy over wide angles (many degrees) and with ~10µas accuracy over narrow angles (arcminutes):Mark III, PTI • Calibration with reference stars or optical truss anchored to earth crust: NPOI •  Astrometry! Optics in Astronomy

  7. Observables II • Visibility Amplitude : • maximum contrast in interferogram • visibility as function of delay depends on spatio-spectral content of source and system throughput • essential for imaging with photometric fidelity • calibration through rapid switching between program and reference sources with known visibility, monitoring of system parameters •  Maps, Images! Optics in Astronomy

  8. Observables III • Referenced phase : • argument of complex visibility: • can be referenced to off-set calibrator source by differential delay measurements • can be referenced to program source at different wavelength: GI2T • essential for imaging • Raw visibility phases are no good observables due to uncontrolled errors: • Visibility phase can be re-transformed by changing origin of coordinate system without affecting the morphology of the reconstructed image •  Maps, Images! Optics in Astronomy

  9. Observables IV • Closure phase : • triple product of complex visibility: • good observable provided there are no baseline-dependent error sources • insensitive to source position • fewer independent closure relations than baselines: vs. • essential for imaging if there are no referenced phases •  Maps, Images! Optics in Astronomy

  10. i l k Phase closure Optics in Astronomy

  11. Signal-to-noise ratio of a visibility measurement S system Strehl,  = 1 with mode stop,  = 0 w/o mode stop Np detected number of source photons Nbdetected number of background photons Nddetector noise, expressed as equivalent rms no. of photons Nredredundancy of baseline considered Nteltotal number of telescopes in array V intrinsic visibility of source K number of incoherently averaged visibility measurements Optics in Astronomy

  12. Signal-to-noise ratio of a visibility measurement Optics in Astronomy

  13. Milestones in Optical Interferometry Optics in Astronomy

  14. Today‘s Interferometers Optics in Astronomy

  15. Keck Interferometer Array, USA Optics in Astronomy

  16. image courtesy Bertrand Koehler Optics in Astronomy

  17. images courtesy Keck Observatory Optics in Astronomy

  18. VLT Interferometer, EUR Optics in Astronomy

  19. VLTI Delay Lines Optics in Astronomy

  20. VINCI - VLTI Commissioning Instrument Optics in Astronomy

  21. VLTI - Mid-Infrared Instrument (MIDI) Optics in Astronomy

  22. MIDI principle and optical design Optics in Astronomy

  23. Fundamental Stellar Parameters Distribution of apparent diameters for various spectral classes for stars seen from Paranal, Chile Fringe contrast as function of apparent diameter Optics in Astronomy

  24. SUSI, Narrabri, Australia, IOTA, Mt. Hamilton, USA Optics in Astronomy

  25. Stellar Surfaces Three maps of  Ori (Betelgeuse) taken in Nov. 1997 700 nm (WHT) 905 nm (COAST) 1290 nm (COAST) pictures courtesy COAST Optics in Astronomy

  26. Multiple Stars Maps of Capella taken 15 days apart Optics in Astronomy

  27. COAST, Cambridge, UK Optics in Astronomy

  28. Observations of Mizar with NPOI on May 1 - 4, 29, June 1, 1996 Optics in Astronomy

  29. NPOI, Flagstaff, USA Optics in Astronomy

  30. Stellar Environments Optics in Astronomy

  31. GI2T, Calern, France Optics in Astronomy

  32. Palomar Testbed Interferometer (PTI), USA Optics in Astronomy

  33. Optics in Astronomy

  34. CHARA Array Optics in Astronomy

  35. Conclusions • Optical interferometry has matured and becomes a regular tool for astronomy • eight interferometers with apertures up to 1.5m operational on regular basis • three arrays involving 10m class telescopes nearing completion • unique science being produced Optics in Astronomy

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