An Astronomer’s An Astronomer’s View View

of Optical of Optical Turbulence Turbulence

The characterization, understanding The characterization, understanding and useand use

of an astronomical site of an astronomical site

is a challenging affairis a challenging affair

René Racine

Université de Montréaland

Association of Canadian Universities for Research in Astronomy

TMT site testing hardware

SummarySummary

FutilityFutility UtilityUtility ChallengesChallenges DreamsDreams

FutilityFutility

DeclarationsDeclarations

The The averageaverage integrated turbulence and turbulence profile integrated turbulence and turbulence profile above a site are above a site are highly predictablehighly predictable Newton 1730 “Newton 1730 “Atop the Tallest Mountains, above the Grosser Atop the Tallest Mountains, above the Grosser

CloudsClouds”” Racine 2005 Racine 2005 [simple-minded model of FWHM(altitude, elevation)] [simple-minded model of FWHM(altitude, elevation)]

Their measurement Their measurement for the purpose of sitefor the purpose of site selectionselection is thus is thus a a

futile activityfutile activity

Factors besides optical turbulence can be decisive for site Factors besides optical turbulence can be decisive for site selection. selection. Politics : “landlords”, funding agencies, sponsors…Politics : “landlords”, funding agencies, sponsors… Enviro-cultural debates protracted delaysEnviro-cultural debates protracted delays CostsCosts This further enhances the futility of site testing and modeling for selection This further enhances the futility of site testing and modeling for selection

purposepurpose..

DemonstrationsDemonstrations

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30° latitude equivalent altitude (m )

44 pre-2005 and 7 44 pre-2005 and 7 2005 - 20072005 - 2007 TMT TMT night-timenight-time seeing campaignsseeing campaigns..

Uniform, cross-calibrated Uniform, cross-calibrated TMT data TMT data less less scatterscatter

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elevation (m)

The first few meters above ground are important

Altitude dependence is weak

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elev

atio

n (m

)Note: open circles are for Dome C

2005-2007 TMT data2005-2007 TMT dataAverage deviation from model = 0.054”Average deviation from model = 0.054”

“The accuracy of the model is therefore estimated to be ±0.04".” Racine, R. 2005 PASP 117, 401

( 0.037” w/o Tolar )

The accuracy of the TMT campaigns is therefore (0.0542 – 0.042)1/2 ~ 0.03”) (w/o Tolar ~ 0.00”)

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asec

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model

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Tolar

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alti

tud

e (

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AS

L )

average deviation of log (Cn2)

tropopause,highly variable

boundary layer,weakly variable

stratosphere,moderately variable

night to night

site to site

10 min to 10 min

mid troposphere,strongly variable

variability of turbulence profiles

Turbulence profiles differ less between sites Turbulence profiles differ less between sites than between nights or times. than between nights or times.

7 sites, 83 night~1000 profiles

UtilityUtility

What What isis important to astronomers is what the important to astronomers is what the turbulence profile turbulence profile isis or or will bewill be (now-casting, (now-casting, forecasting)forecasting)

And what the resulting telescopic PSF And what the resulting telescopic PSF will bewill be OT spectral index, outer scale effectOT spectral index, outer scale effect

To adjust science programs to atmospheric To adjust science programs to atmospheric conditionsconditions

Flexible, queue scheduling maximizes Flexible, queue scheduling maximizes productivityproductivity To adjust instrumental parameters to To adjust instrumental parameters to

turbulence profileturbulence profile Conjugate optics for AO, GLAO , MCAO…Conjugate optics for AO, GLAO , MCAO…

Observatory managers Observatory managers care becausecare because

telescope time is telescope time is expensiveexpensive

One clear ELT night

$ 300 000

€ 200 000

AstronomersAstronomers care because care becausethe natural seeing is the most crucial determinant the natural seeing is the most crucial determinant

of the science SNRof the science SNR

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rela

tive

SN

R

adaptive optics,8-m, 150 modes

natural seeing

ChallengesChallenges

Challenge no. 1a : Challenge no. 1a : ForecastForecast time time dependent variationsdependent variations

for the next nightfor the next night

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Note again better seeing at higher elevation

M. Sarazin 2006

The first few meters above ground The first few meters above ground areare importantimportant..

Is wind the main driver of ground OT ?Is wind the main driver of ground OT ?

Challenge no. 1b : Challenge no. 1b : ForecastForecast time time dependent variationsdependent variationsover the over the loooongloooong term term

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6 months of DIMM seeing data at a TMT site

AO SNR

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Challenge no. 2 : Challenge no. 2 : ExplainExplain scatterscatter between sub-sites :between sub-sites :

very local effects are importantvery local effects are important

DIMM seeing (”)

DIM

M s

eei

ng

(”)

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

Sit

e B

93 nights

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

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one-hour intervals

Challenge no. 3: Challenge no. 3: ExplainExplain “Dome and “Dome and Mirror” OTMirror” OT

Evolution of median image quality at the CFHTEvolution of median image quality at the CFHT

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1980 1985 1990 1995

log(FWHM")

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0.63"

0.80"

CFHT image quality vs CFHT image quality vs TTmirror mirror TTdomedome

Racine et al. 1991, PASP 103, 1020

All this local stuff may be “All this local stuff may be “small stuffsmall stuff” to expert ” to expert atmosphere dynamicists…atmosphere dynamicists…

But it is of But it is of paramount importanceparamount importance to astronomers to astronomers

Thus far, astronomers and their engineer Thus far, astronomers and their engineer colleagues have dealt colleagues have dealt largely empiricallylargely empirically with with these “mysterious” behaviorsthese “mysterious” behaviors

Astronomers can take advantage of the research Astronomers can take advantage of the research developed so far in OT. Close collaboration developed so far in OT. Close collaboration between astronomers and OT physicists, and between astronomers and OT physicists, and coordinated benchmark tests should be very coordinated benchmark tests should be very beneficial.beneficial.

TMT enclosure and telescope structure:

Efforts to minimize dome seeingmirror seeingwind buffeting

DreamsDreams

What astronomers dream ofWhat astronomers dream of Clear skies !Clear skies ! Excellent (<0.5”) seeing (integrated OT)Excellent (<0.5”) seeing (integrated OT) Foreknowledge of seeingForeknowledge of seeing Mitigation of ground layer turbulenceMitigation of ground layer turbulence Mitigation of “dome and mirror seeing”Mitigation of “dome and mirror seeing” Low altitude natural turbulence orLow altitude natural turbulence or Foreknowledge of dominant layers’ Foreknowledge of dominant layers’

altitudes altitudes Low (<2 m /sec.) local winds (esp. for Low (<2 m /sec.) local winds (esp. for

ELTs)ELTs) Low (<2 mm) PWV (for mid IR work)Low (<2 mm) PWV (for mid IR work)

Helping make these dreams Helping make these dreams come truecome true

Clear skies : high altitude tropical (or Clear skies : high altitude tropical (or polar?) sitepolar?) site reliable weather forecastsreliable weather forecasts

Excellent seeing : high altitude siteExcellent seeing : high altitude site reliable integrated OT forecastreliable integrated OT forecast

Ground layer (< 30 m) turbulenceGround layer (< 30 m) turbulence model ground-air interface, forecast how local OT model ground-air interface, forecast how local OT

arisesarises Dome and mirror seeingDome and mirror seeing

ab initioab initio quantitative models, mitigation measures quantitative models, mitigation measures Foreknowledge of dominant layers’ Foreknowledge of dominant layers’

altitudes altitudes Reliable turbulence profile Reliable turbulence profile forecastsforecasts

Low (<2 mm) PWVLow (<2 mm) PWV High altitude siteHigh altitude site

Low (<2 m /sec.) local winds (esp. for Low (<2 m /sec.) local winds (esp. for ELTs)ELTs) Low altitude site…or imaginative structures and wind Low altitude site…or imaginative structures and wind

shieldsshields

An obvious double-blind testAn 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 expertsA team of OT experts

Inspects and instruments the site with ‘meteo’ sensorsInspects and instruments the site with ‘meteo’ sensors Uses the data to Uses the data to calculatecalculate the zenith DIMM seeing the zenith DIMM seeing

While an independent DIMM observerWhile an independent DIMM observer Simultaneously Simultaneously measuresmeasures the zenith seeing. 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-OT experts and DIMM observer transmit their time-

flagged results to an independent reporter.flagged results to an independent reporter.

The calculations are improved until <|O – C|> < The calculations are improved until <|O – C|> < 0.05”.0.05”.

Then the OT experts forecast the seeing for the Then the OT experts forecast the seeing for the nextnext night…night…

AstronomersAstronomers

are all aware that these dreams are all aware that these dreams

can only become realitycan only become reality

through close collaboration with through close collaboration with

atmospheric physicistsatmospheric physicists

To repeat…