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

September 2002Optics in Astronomy2 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

September 2002Optics in Astronomy3 Differential delay tracking

September 2002Optics in Astronomy4 Differential delay tracking Goals of DDT: off-source phase referencing narrow-angle astrometry Source of figures: PTI

September 2002Optics in Astronomy5 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:

September 2002Optics in Astronomy6 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!

September 2002Optics in Astronomy7 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!

September 2002Optics in Astronomy8 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!

September 2002Optics in Astronomy9 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!

September 2002Optics in Astronomy10 Phase closure i k l

September 2002Optics in Astronomy11 Signal-to-noise ratio of a visibility measurement Ssystem Strehl,  = 1 with mode stop,  = 0 w/o mode stop N p detected number of source photons N b detected number of background photons N d detector noise, expressed as equivalent rms no. of photons N red redundancy of baseline considered N tel total number of telescopes in array Vintrinsic visibility of source Knumber of incoherently averaged visibility measurements

September 2002Optics in Astronomy12 Signal-to-noise ratio of a visibility measurement

September 2002Optics in Astronomy13 Milestones in Optical Interferometry

September 2002Optics in Astronomy14 Today‘s Interferometers

September 2002Optics in Astronomy15 Keck Interferometer Array, USA

September 2002Optics in Astronomy16 image courtesy Bertrand Koehler

September 2002Optics in Astronomy17 images courtesy Keck Observatory

September 2002Optics in Astronomy18 VLT Interferometer, EUR

September 2002Optics in Astronomy19 VLTI Delay Lines

September 2002Optics in Astronomy20 VINCI - VLTI Commissioning Instrument

September 2002Optics in Astronomy21 VLTI - Mid-Infrared Instrument (MIDI)

September 2002Optics in Astronomy22 MIDI principle and optical design

September 2002Optics in Astronomy23 Fundamental Stellar Parameters Fringe contrast as function of apparent diameter Distribution of apparent diameters for various spectral classes for stars seen from Paranal, Chile

September 2002Optics in Astronomy24 SUSI, Narrabri, Australia, IOTA, Mt. Hamilton, USA

September 2002Optics in Astronomy25 Stellar Surfaces 700 nm (WHT)905 nm (COAST)1290 nm (COAST) Three maps of  Ori (Betelgeuse) taken in Nov pictures courtesy COAST

September 2002Optics in Astronomy26 Multiple Stars Maps of Capella taken 15 days apart

September 2002Optics in Astronomy27 COAST, Cambridge, UK

September 2002Optics in Astronomy28 Observations of Mizar with NPOI on May 1 - 4, 29, June 1, 1996

September 2002Optics in Astronomy29 NPOI, Flagstaff, USA

September 2002Optics in Astronomy30 Stellar Environments

September 2002Optics in Astronomy31 GI2T, Calern, France

September 2002Optics in Astronomy32 Palomar Testbed Interferometer (PTI), USA

September 2002Optics in Astronomy33

September 2002Optics in Astronomy34 CHARA Array

September 2002Optics in Astronomy35 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