Download Site Parameters Relevant For High Resolution Imaging

SITE PARAMETERS RELEVANT FOR
HIGH RESOLUTION IMAGING
Marc Sarazin
European Southern Observatory
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?
2
Zanjan, July 2001
Optical Propagation
The Signature of Atmospheric Turbulence
Seeing:
(arcsec, ^-0.2)
FWHM ( )  0.98

r0



 2 
2
Fried parameter: r0 ( )  0.423
 sec( )  Cn (h)dh
( meter, ^6/5)
  


0
2
3
5
Easy to remember: r0=10cmFWHM=1” in the visible (0.5m)
3
Zanjan, July 2001
Optical Propagation
The Signature of Atmospheric Turbulence
Seeing = FWHM
S= 0.7 à 2.2 um
FWHM=0.056 “

FWHM 
r0
Strehl Ratio
I
S
I0
S=0.3 à 2.2 um
FWHM=0.065 “
4
Zanjan, July 2001
Optical Propagation
The Signature of Atmospheric Turbulence
A Speckle structure appears when the exposure is
shorter than the atmosphere coherence time  0
r0
  0.31
v
1ms exposure at the focus of a large telescope
5
Zanjan, July 2001
High Resolution Imaging
Correlation time:



5
 2 
2
 0 ( )  2.91
 sec( )  v 3 (h)  Cn (h)dh
  


0
2
3
5
Isoplanatic angle:


5
 2 
2
 0 ( )  2.91
 sec( )  h 3 Cn (h)dh
  


0
2
Zanjan, July 2001

3
5
6
High Resolution Imaging
Active or Adaptive optics?
Active optics can correct large amplitudes on slowly
varying effects of smaller spatial frequency
Spatial
Active Optics
frequency
10/D
1/D
0.1/D
Adaptive optics
Atmosphere
F
i
g
u
r
i
n
g
Local
air
D: Diameter of
the telescope
Gravity
Thermal
dc
Wind
0.01
0.1
1
10
100
Temporal
frequency
(Hz)
7
Zanjan, July 2001
High Resolution Imaging
Correction Methods
based on
Adaptive Optics use
natural or
artificial
reference stars
for wave front
sensing
8
Zanjan, July 2001
High Resolution Imaging
Small Field
Correction by
adaptive optics
(simulation by M.
Le Louarn, ESO)
9
Zanjan, July 2001
High Resolution Imaging
Reference Star
Correction Methods
based on
Adaptive Optics:
Anisoplanatism
Science
Object
Turbulence
sets a limit to the
distance of the
reference star
Strehl=0.38 at =0
Zanjan, July 2001
Common
Atmospheric
Path
Télescope
10
Atmospheric Turbulence
Poor sky
coverage
with
natural
guide stars
11
Zanjan, July 2001
High Resolution Imaging
12
Zanjan, 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
13
Zanjan, 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
14
Zanjan, 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)
15
Zanjan, July 2001
Atmospheric Turbulence
16
Zanjan, July 2001
Atmospheric Turbulence
Performances of Adaptive Optics Correction





Finite number of actuator (fitting error):
s2fit=0.34 (D/ro)5/3
Finite number of sub apertures (spatial aliasing)
s2al=0.17 (D/ro)5/3
Finite lag between measure and actions:
s2sl~ (fg/f3dB)5/3 fg=1/ o
Noise in the measurements:
s2m~1/(Nph.ro2.o)5/3
Wave front from object and guide star cross different layers sections
(anisoplanatism):
s2isop~(/  o)5/3
Total error variance: s2tot =s2fit +s2al +s2sl +s2m +s2isop
Strehl~exp(- s2tot)
19
Zanjan, 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, 1011.4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
20
Zanjan, July 2001
The new tools for site surveys
The development of new automated monitoring instruments
is necessary, in particular:
21
Zanjan, July 2001