Transcript A visible-light AO system for the 4.2 m SOAR telescope

MASS-DIMM
and SODAR
at Cerro Pachon
A. Tokovinin
Gemini, May 3, 2005
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Outline
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“Seeing” and delivered image quality
MASS-DIMM instrument
Free atmosphere and Ground layer
Detailed GL models from SODAR
Operational use of MASS-DIMM data
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Contributors to DIQ
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What is “seeing”?
Seeing is
the HF power
of optical
turbulence (J)
expressed
in “arcseconds”
[Kolmogorov]
” = (J / 6.8x10-13)3/5
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Atmospheric IQ at Pachon
Seeing
worst 25%
median
best 25%
L0 =25m
Why do we use “seeing”? – to avoid a mess!
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CP site monitor
Dec 2004-Apr 2005
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What is MASS?
6-layer model
Weighting functions
PROFILE
4 normal indices
6 diff. indices
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Restoration of profile
 Response is
“triangular”
 Good integrals
(βf, θ0)
 S/N~10% (better
sensitivity at low
turbulence)
http://www.ctio.noao.edu/~atokovin/profiler
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Combine MASS with DIMM!
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MASS-DIMM
instruments
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MASS-DIMM + Meade
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MASS-DIMM: details
5.5mm
diam.
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Measured parameters
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Seeing β (Fried parameter)
Free-atmosphere seeing βf
Isoplanatic angle θ0
AO time constant τ0 (without ground layer, but…)
Low-resolution profile: 6 layers at 0.5,1,2,4,8,16km
NOT MEASURED: Outer scale, detailed profile, wind
MASS database: http://mass.ctio.noao.edu
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MASS-DIMM expansion
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TMT site-testing program (6)
Chilean observatories (3)
ESO 1 (+5)
Antarctica (1)
Palomar (1+)
…
2002-2005: 3 years
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What can go wrong with CP site monitor?
DIMM:
Bad focus
(over-estimate seeing)
Pointing problems
Wind shake
Un-friendly control
MASS:
Low flux
Cirrus cloud
Dirty optics
Bad alignment
Wrong param.
Overshoots
Join DIMM and MASS data in the common database!
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“Overshoots”
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Strong scintillation
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DIMM
MASS-DIMM: CP, 2005
MASS
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Histograms
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FA and GL are independent!
artifacts
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FA: calm or turbulent?
Turbulence comes
in bursts
When free atmosphere
is calm, it is stable!
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Free atmosphere in detail
Strong turbulence = low altitude
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SODAR campaign at CP
Goal: define the thickness of
the Ground Layer
Duration: Oct. 30 to Nov. 15, 2004
Equipment: XFAS from Scintec
Altitude range: 40m – 800m
Vertical resolution: 20m
Time resolution: 20 min.
Results: 505 profiles (168 hours)
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SODAR profiles
Nov 3
Nov 4
Nov 6
Nov 5
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SODAR vs. MASS-DIMM
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GL model
worst 25%
typical
best 25%
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GL model: balloons
J, 10-13 m1/3 h(1/e), m
Good
0.9-1.2
15-30
Typical
1.6-3
20-40
Bad
6.5-10
50-100
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CP model
The model is an independent
combination of FA and GL.
FA: one layer with changing
strength and altitude.
25%
50%
75%
FA
0.29
0.41
0.56
GL
0.41
0.53
0.67
Total
0.63
0.77
0.91
GL: exponential model
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Operational use of MASS-DIMM data
Periods of calm (<0.3”) FA (25%): extreme value for AO!
(can be predicted from jet stream?)
MASS gives on-line isoplanatic angle and AO time constant
select critical/non-critical AO programs
Periods of bad (>1”) FA seeing (5%): hopeless!
use telescope as a “light bucket” for spectroscopy
GL seeing can be as bad as 1” (under strong wind?)
will be corrected by GLAO!
Match DIQ to site seeing (critical in IR)
Good seeing: product of FA and GL probabilities
Bad seeing: sum of FA and GL probabilities
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Last FA-calm night: May1/2, 2005
The “sweet spot” for AO!
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