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Measurements of the wave-front outer scale at Paranal

Measurements of the wave-front outer scale at Paranal. Influence of this parameter in interferometry. R. Conan, A. Ziad, J. Borgnino and F. Martin UMR 6525 Astrophysique, Université de Nice-Sophia Antipolis, F-06108 Nice Cedex 2, France A. Tokovinin

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Measurements of the wave-front outer scale at Paranal

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  1. Measurements of the wave-front outer scale at Paranal Influence of this parameter in interferometry R. Conan, A. Ziad, J. Borgnino and F. Martin UMR 6525 Astrophysique, Université de Nice-Sophia Antipolis, F-06108 Nice Cedex 2, France A. Tokovinin Sternberg Astronomical Institute, Universitestsky prosp., 13, 119899 Moscow, Russia

  2. The Generalized Seeing Monitor • Four telescopes (Ø=10cm, L-shape layout) • Simultaneous measurement of one component of the angle of arrival • Real-time computation of : • the Fried’s parameter : r0 • the spatial coherence outer scale : L0 • the isoplanatic patch : q0 SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  3. SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  4. GSM at Paranal27 Nov./20 Dec. 1998 • GSM has worked during 19 clear nights • 1884 measurements of each parameter : • median seeing : 0.9 as, • median L0 : 22 m, • median q0 : 1.9 as. SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  5. Co-phasing & outer scaleTheory • Standard deviation of the Optical Path Difference (OPD) : where is the structure function and is the spectrum of the phase fluctuations (Von Karman). SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  6. Co-phasing & outer scaleTheory • Solutions to the structure function • without aperture filtering : • with aperture filtering : • 1. • 2. • 3. SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  7. Co-phasing & outer scaleTheory • Structure function of the phase fluctuations SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  8. Co-phasing & outer scaleTheory • Isolines of the OPD in units of wavelength as a function of B and L0 • D = 10m and r0 = 10cm SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  9. Co-phasing & outer scaleApplication to VLTI • OPD in units of the wavelength • Diameter : 8.2m • Baselines : 40, 100 and 200 m • r0 and L0 measured by the GSM SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  10. OPD residual after correctionby a fringe tracker • Structure function of the residual phase (residual piston) : with : SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  11. OPD residual after correctionby a fringe tracker • OPD residual = with and SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  12. SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

  13. Summary • Longest set of continuous nights of outer scale measurements at Paranal (median L0 : 22 m) • Derivation of mathematical formulas for OPD with and without the filtering effect of a finite size aperture • OPD values expected at Paranal (D=8.2m, B=40, 100 and 200m) are 10 timeslower than for a model with an infinite L0. The OPD is independent of baseline values. SPIE's International Symposium - Astronomical Telescopes and Instrumentation 2000

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