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

2. Zanjan, July 2001 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. Zanjan, July 2001 3 Fried parameter: ( meter,  ^6/5) Seeing: (arcsec,  ^-0.2) Optical Propagation The Signature of Atmospheric Turbulence Easy to remember: r0=10cm  FWHM=1” in the visible (0.5  m)

4. Zanjan, July 2001 4 Optical Propagation The Signature of Atmospheric Turbulence S= 0.7 à 2.2 um FWHM=0.056 “ S=0.3 à 2.2 um FWHM=0.065 “ Seeing = FWHM Strehl Ratio

5. Zanjan, July 2001 5 Optical Propagation The 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. Zanjan, July 2001 6 Correlation time: Isoplanatic angle: High Resolution Imaging

7. Zanjan, July 2001 7 High Resolution Imaging Active or Adaptive optics? Active optics can correct large amplitudes on slowly varying effects of smaller spatial frequency

8. Zanjan, July 2001 8 High Resolution Imaging Correction Methods based on Adaptive Optics use natural or artificial reference stars for wave front sensing

9. Zanjan, July 2001 9 High Resolution Imaging Small Field Correction by adaptive optics (simulation by M. Le Louarn, ESO)

10. Zanjan, July 2001 10 High Resolution Imaging Correction Methods based on Adaptive Optics: Anisoplanatism sets a limit to the distance of the reference star Strehl=0.38 at  =  0 Reference Star Télescope Science Object Turbulence Common Atmospheric Path

11. Zanjan, July 2001 11 Atmospheric Turbulence Poor sky coverage with natural guide stars

12. Zanjan, July 2001 12 High Resolution Imaging

13. Zanjan, July 2001 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. Zanjan, July 2001 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. Zanjan, July 2001 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. Zanjan, July 2001 16 Atmospheric Turbulence

19. Zanjan, July 2001 19 Performances of Adaptive Optics Correction Finite number of actuator (fitting error):  2 fit =0.34 (D/ro) 5/3 Finite number of sub apertures (spatial aliasing)  2 al =0.17 (D/ro) 5/3 Finite lag between measure and actions:  2 sl ~ (f g /f 3dB ) 5/3 f g =1/  o Noise in the measurements:  2 m ~1/(N ph.r o 2.  o ) 5/3 Wave front from object and guide star cross different layers sections (anisoplanatism):  2 isop ~(  /  o ) 5/3 Total error variance:  2 tot =  2 fit +  2 al +  2 sl +  2 m +  2 isop Strehl~exp(-  2 tot ) Atmospheric Turbulence

20. Zanjan, July 2001 20 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

21. Zanjan, July 2001 21 The new tools for site surveys The development of new automated monitoring instruments is necessary, in particular: