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SITE PARAMETERS RELEVANT FOR HIGH RESOLUTION IMAGING

SITE PARAMETERS RELEVANT FOR HIGH RESOLUTION IMAGING. Marc Sarazin European Southern Observatory. List of Themes. Optical Propagation through Turbulence Mechanical and Thermal effects Index of Refraction Signature on ground based observations Correction methods

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SITE PARAMETERS RELEVANT FOR HIGH RESOLUTION IMAGING

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  1. SITE PARAMETERS RELEVANT FOR HIGH RESOLUTION IMAGING Marc Sarazin European Southern Observatory

  2. List of Themes • Optical Propagation through Turbulence • Mechanical and Thermal effects • Index of Refraction • Signature on ground based observations • Correction methods • Integral monitoring Techniques • Seeing Monitoring • Scintillation Monitoring • Profiling Techniques • Instrumented Masts • Balloon Borne Sensors • Scintillation Ranging • Modelling Techniques • Conclusions - How to find the ideal site...and keep it good?

  3. Optical PropagationThe Signature of Atmospheric Turbulence Seeing: (arcsec, ^-0.2) Fried parameter: ( meter, ^6/5) Easy to remember: r0=10cmFWHM=1” in the visible (0.5m)

  4. Optical PropagationThe Signature of Atmospheric Turbulence S= 0.7 à 2.2 um FWHM=0.056 “ Seeing = FWHM Strehl Ratio S=0.3 à 2.2 um FWHM=0.065 “

  5. Optical PropagationThe Signature of Atmospheric Turbulence A Speckle structure appears when the exposure is shorter than the atmosphere coherence time  0 1ms exposure at the focus of a large telescope

  6. High Resolution Imaging Correlation time: Isoplanatic angle:

  7. High Resolution Imaging Active or Adaptive optics? Active optics can correct large amplitudes on slowly varying effects of smaller spatial frequency

  8. High Resolution Imaging Correction Methods based on Adaptive Optics use natural or artificial reference stars for wave front sensing

  9. High Resolution Imaging Small Field Correction by adaptive optics (simulation by M. Le Louarn, ESO)

  10. High Resolution Imaging Science Object Reference Star Turbulence Common Atmospheric Path Télescope Correction Methods based on Adaptive Optics: Anisoplanatism sets a limit to the distance of the reference star Strehl=0.38 at =0

  11. Atmospheric Turbulence Poor sky coverage with natural guide stars

  12. High Resolution Imaging

  13. High Resolution Imaging Wide Field Correction by adaptive optics (simulation by R.Rigaut, Gemini project) (A) Uncorrected Field, showing speckle structure and global image motion

  14. High Resolution Imaging Wide Field Correction by adaptive optics (simulation by R.Rigaut, Gemini project) (B) Single guide star in the center of the field

  15. High Resolution Imaging Wide Field Correction by adaptive optics (simulation by R.Rigaut, Gemini project) (C) Multiple guide stars (one per field corner)

  16. Atmospheric Turbulence

  17. Performances of Adaptive Optics Correction Atmospheric Turbulence • Finite number of actuator (fitting error): s2fit=0.34 (D/ro)5/3 • Finite number of sub apertures (spatial aliasing) s2al=0.17 (D/ro)5/3 • Finite lag between measure and actions: s2sl~ (fg/f3dB)5/3 fg=1/ to • Noise in the measurements: s2m~1/(Nph.ro2.to)5/3 • Wave front from object and guide star cross different layers sections (anisoplanatism): s2isop~(q/ qo)5/3 Total error variance: s2tot =s2fit +s2al +s2sl +s2m +s2isop Strehl~exp(- s2tot)

  18. The new tools for site surveys • The development of new automated monitoring instruments is necessary, in particular: • Sky monitor • -CONCAM (Kitt Peak): cloud imager, fisheye lens, ST8 CCD on a fixed mount • -IR All-Sky camera (APO-SLOAN):cloud imager, scanning mirror, 10-11.4m filter, pyroelectric detector • -All Sky Imager (ESO project): wide field photometry of reference stars, 50mm lens, BVI filters, 2kx2k CCD on a scanning mount. • Sodium Layer Monitor? • Portable (single star?) turbulence profiler

  19. The new tools for site surveys The development of new automated monitoring instruments is necessary, in particular:

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