SITE PARAMETERS RELEVANT FOR

HIGH RESOLUTION IMAGING

Marc Sarazin

European Southern Observatory

2Zanjan, July 2001

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?

3Zanjan, July 2001

Fried parameter: ( meter, ^6/5)

Seeing: (arcsec, ^-0.2) 0

98.0)(r

FWHM

53

0

22

0 )()sec(2

423.0)(

dhhCr n

Optical Propagation

The Signature of Atmospheric Turbulence

Easy to remember: r0=10cmFWHM=1” in the visible (0.5m)

4Zanjan, July 2001

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 “

0I

IS

0rFWHM

Seeing = FWHM

Strehl Ratio

5Zanjan, July 2001

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

v

r031.0

6Zanjan, July 2001

53

0

22

0 )()()sec(2

91.2)( 35

dhhChv n

53

0

22

0 )()sec(2

91.2)( 35

dhhCh n

Correlation time:

Isoplanatic angle:

High Resolution Imaging

7Zanjan, July 2001

High Resolution Imaging

Active or Adaptive optics?

dc 0.01 0.1 1 10 100 Temporalfrequency

(Hz)

Spatial

frequency

1/D

10/D

GravityThermal

Local

air

Wind

Atmosphere

D: Diameter ofthe telescope

Figuring

Active Optics Adaptive optics

0.1/D

Active optics can correct large amplitudes on slowly varying effects of smaller spatial frequency

8Zanjan, July 2001

High Resolution Imaging

Correction Methods based on

Adaptive Optics use

natural or artificial

reference stars for wave front

sensing

9Zanjan, July 2001

High Resolution Imaging

Small Field Correction by

adaptive optics(simulation by M. Le Louarn, ESO)

10Zanjan, July 2001

High Resolution Imaging

Correction Methods

based on

Adaptive Optics:

Anisoplanatismsets a limit to the

distance of the reference star

Strehl=0.38 at =0

Reference Star

Télescope

ScienceObject

Turbulence

Common Atmospheric Path

11Zanjan, July 2001

Atmospheric Turbulence

Poor sky coverage with natural guide stars

12Zanjan, July 2001

High Resolution Imaging

13Zanjan, July 2001

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

14Zanjan, July 2001

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

15Zanjan, July 2001

High Resolution Imaging

Wide Field Correction by

adaptive optics(simulation by

R.Rigaut, Gemini project)

(C)

Multiple guide stars

(one per field corner)

16Zanjan, July 2001

Atmospheric Turbulence

19Zanjan, July 2001

Performances of Adaptive Optics Correction

Finite number of actuator (fitting error):

2fit=0.34 (D/ro)5/3

Finite number of sub apertures (spatial aliasing)

2al=0.17 (D/ro)5/3

Finite lag between measure and actions:

2sl~ (fg/f3dB)5/3 fg=1/ o

Noise in the measurements:

2m~1/(Nph.ro

2.o)5/3

Wave front from object and guide star cross different layers sections (anisoplanatism): 2

isop~(/ o)5/3

Total error variance: 2tot =2

fit +2al +2

sl +2m +2

isop

Strehl~exp(- 2tot)

Atmospheric Turbulence

20Zanjan, July 2001

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, 10-11.4m 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

21Zanjan, July 2001

The new tools for site surveys

The development of new automated monitoring instruments is necessary, in particular: