site parameters relevant for high resolution imaging marc sarazin european southern observatory
TRANSCRIPT
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: