An Astronomer’s View of Optical Turbulence The characterization, understanding and use of an astronomical site of an astronomical site is a challenging affair René Racine Université de Montréal and Association of Canadian Universities for Research in Astronomy TMT site testing hardware

Summary Futility Futility Utility Utility Challenges Challenges Dreams Dreams

Futility

Declarations The average integrated turbulence and turbulence profile above a site are highly predictable The average integrated turbulence and turbulence profile above a site are highly predictable Newton 1730 “Atop the Tallest Mountains, above the Grosser Clouds” Newton 1730 “Atop the Tallest Mountains, above the Grosser Clouds” Racine 2005 [simple-minded model of FWHM(altitude, elevation)] Racine 2005 [simple-minded model of FWHM(altitude, elevation)] Their measurement for the purpose of site selection is thus a Their measurement for the purpose of site selection is thus a futile activity futile activity Factors besides optical turbulence can be decisive for site selection. Factors besides optical turbulence can be decisive for site selection. Politics : “landlords”, funding agencies, sponsors… Politics : “landlords”, funding agencies, sponsors… Enviro-cultural debates protracted delays Enviro-cultural debates protracted delays Costs Costs This further enhances the futility of site testing and modeling for selection purpose. This further enhances the futility of site testing and modeling for selection purpose.

Demonstrations

44 pre-2005 and TMT night-time seeing campaigns. Uniform, cross-calibrated TMT data less scatter The first few meters above ground are important Altitude dependence is weak Note: open circles are for Dome C

TMT data Average deviation from model = 0.054” “The accuracy of the model is therefore estimated to be ±0.04".” Racine, R PASP 117, 401 ( 0.037” w/o Tolar ) The accuracy of the TMT campaigns is therefore ( – ) 1/2 ~ 0.03”) (w/o Tolar ~ 0.00”)

Turbulence profiles differ less between sites than between nights or times. 7 sites, 83 night ~1000 profiles

Utility

What is important to astronomers is what the turbulence profile is or will be (now-casting, forecasting) What is important to astronomers is what the turbulence profile is or will be (now-casting, forecasting) And what the resulting telescopic PSF will be And what the resulting telescopic PSF will be OT spectral index, outer scale effect OT spectral index, outer scale effect To adjust science programs to atmospheric conditions To adjust science programs to atmospheric conditions Flexible, queue scheduling maximizes productivity Flexible, queue scheduling maximizes productivity To adjust instrumental parameters to turbulence profile To adjust instrumental parameters to turbulence profile Conjugate optics for AO, GLAO, MCAO… Conjugate optics for AO, GLAO, MCAO…

Observatory managers care because telescope time is expensive One clear ELT night $ €

Astronomers care because the natural seeing is the most crucial determinant of the science SNR

Challenges

Challenge no. 1a : Forecast time dependent variations for the next night AO SNR Note again better seeing at higher elevation M. Sarazin 2006

The first few meters above ground are important. Is wind the main driver of ground OT ?

Challenge no. 1b : Forecast time dependent variations over the loooong term 6 months of DIMM seeing data at a TMT site AO SNR

Challenge no. 2 : Explain scatter between sub-sites : very local effects are important DIMM seeing (”) Strictly simultaneous seeing measurements (8-m elevation, 2 min. samples) from two sub sites of same altitude separated by 600 m can differ very significantly. Seeing forecasts for that summit are largely irrelevant unless local effects are suitably modeled. A fitting interpretation is that each site is affected by uncorrelated sporadic local OT enhancements of probability of occurrence / mean amplitude : 50% / 0.15” Site A 25% / 0.65” Site B Analysis shows that differences in the DIMMs’ line-of sight (different stars) through the planetary boundary layer and the free atmosphere contribute a relative seeing dispersion of ~15%. Site A Site B 93 nights

One of many episodes of strong differences in seeing between two neighboring sites

Challenge no. 3: Explain “Dome and Mirror” OT Evolution of median image quality at the CFHT

CFHT image quality vs  T mirror  T dome Racine et al. 1991, PASP 103, 1020

All this local stuff may be “small stuff” to expert atmosphere dynamicists… All this local stuff may be “small stuff” to expert atmosphere dynamicists… But it is of paramount importance to astronomers But it is of paramount importance to astronomers Thus far, astronomers and their engineer colleagues have dealt largely empirically with these “mysterious” behaviors Thus far, astronomers and their engineer colleagues have dealt largely empirically with these “mysterious” behaviors Astronomers can take advantage of the research developed so far in OT. Close collaboration between astronomers and OT physicists, and coordinated benchmark tests should be very beneficial. Astronomers can take advantage of the research developed so far in OT. Close collaboration between astronomers and OT physicists, and coordinated benchmark tests should be very beneficial.

TMT enclosure and telescope structure: Efforts to minimize dome seeing mirror seeing wind buffeting

Dreams

What astronomers dream of Clear skies ! Clear skies ! Excellent (<0.5”) seeing (integrated OT) Excellent (<0.5”) seeing (integrated OT) Foreknowledge of seeing Foreknowledge of seeing Mitigation of ground layer turbulence Mitigation of ground layer turbulence Mitigation of “dome and mirror seeing” Mitigation of “dome and mirror seeing” Low altitude natural turbulence or Low altitude natural turbulence or Foreknowledge of dominant layers’ altitudes Foreknowledge of dominant layers’ altitudes Low (<2 m /sec.) local winds (esp. for ELTs) Low (<2 m /sec.) local winds (esp. for ELTs) Low (<2 mm) PWV (for mid IR work) Low (<2 mm) PWV (for mid IR work)

Helping make these dreams come true Clear skies : high altitude tropical (or polar?) site Clear skies : high altitude tropical (or polar?) site reliable weather forecasts reliable weather forecasts Excellent seeing : high altitude site Excellent seeing : high altitude site reliable integrated OT forecast reliable integrated OT forecast Ground layer (< 30 m) turbulence Ground layer (< 30 m) turbulence model ground-air interface, forecast how local OT arises model ground-air interface, forecast how local OT arises Dome and mirror seeing Dome and mirror seeing ab initio quantitative models, mitigation measures ab initio quantitative models, mitigation measures Foreknowledge of dominant layers’ altitudes Foreknowledge of dominant layers’ altitudes Reliable turbulence profile forecasts Reliable turbulence profile forecasts Low (<2 mm) PWV Low (<2 mm) PWV High altitude site High altitude site Low (<2 m /sec.) local winds (esp. for ELTs) Low (<2 m /sec.) local winds (esp. for ELTs) Low altitude site…or imaginative structures and wind shields Low altitude site…or imaginative structures and wind shields

An obvious double-blind test A DIMM in installed at a site, any site… A DIMM in installed at a site, any site… A team of OT experts A team of OT experts Inspects and instruments the site with ‘meteo’ sensors Inspects and instruments the site with ‘meteo’ sensors Uses the data to calculate the zenith DIMM seeing Uses the data to calculate the zenith DIMM seeing While an independent DIMM observer While an independent DIMM observer Simultaneously measures the zenith seeing. Simultaneously measures the zenith seeing. OT experts and DIMM observer do not communicate. OT experts and DIMM observer do not communicate. OT experts and DIMM observer transmit their time-flagged results to an independent reporter. OT experts and DIMM observer transmit their time-flagged results to an independent reporter. The calculations are improved until < 0.05”. Then the OT experts forecast the seeing for the next night… Then the OT experts forecast the seeing for the next night…

Astronomers are all aware that these dreams are all aware that these dreams can only become reality through close collaboration with through close collaboration with atmospheric physicists To repeat…