1. MASS-DIMM and SODAR at Cerro Pachon
A. Tokovinin
Gemini, May 3, 2005

2. Outline “Seeing” and delivered image quality MASS-DIMM instrument
Free atmosphere and Ground layer
Detailed GL models from SODAR
Operational use of MASS-DIMM data
Gemini, May 3, 2005

3. Contributors to DIQ
Gemini, May 3, 2005

4. What is “seeing”? ” = (J / 6.8x10-13)3/5 Seeing is the HF power
of optical
turbulence (J)
expressed
in “arcseconds”
[Kolmogorov]
” = (J / 6.8x10-13)3/5
Gemini, May 3, 2005

5. Atmospheric IQ at Pachon
Seeing
worst 25%
median
best 25%
L0 =25m
CP site monitor
Dec 2004-Apr 2005
Why do we use “seeing”? – to avoid a mess!
Gemini, May 3, 2005

6. What is MASS? 6-layer model Weighting functions 4 normal indices
6 diff. indices
PROFILE
Gemini, May 3, 2005

7. Restoration of profile
Response is “triangular”
Good integrals
(βf, θ0)
S/N~10% (better sensitivity at low turbulence)
Gemini, May 3, 2005

8. Combine MASS with DIMM!
Gemini, May 3, 2005

9. MASS-DIMM instruments
Gemini, May 3, 2005

10. MASS-DIMM + Meade
Gemini, May 3, 2005

11. MASS-DIMM: details
5.5mm
diam.
Gemini, May 3, 2005

12. Measured parameters 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:
Gemini, May 3, 2005

13. MASS-DIMM expansion TMT site-testing program (6)
Chilean observatories (3)
ESO 1 (+5)
Antarctica (1)
Palomar (1+) …
: 3 years
Gemini, May 3, 2005

14. What can go wrong with CP site monitor?
MASS:
Low flux
Cirrus cloud
Dirty optics
Bad alignment
Wrong param.
Overshoots
DIMM:
Bad focus
(over-estimate seeing)
Pointing problems
Wind shake
Un-friendly control
Join DIMM and MASS data in the common database!
Gemini, May 3, 2005

15. “Overshoots”
Gemini, May 3, 2005

16. Strong scintillation
Gemini, May 3, 2005

17. MASS-DIMM: CP, 2005
DIMM
MASS
Gemini, May 3, 2005

18. Histograms
Free
Ground
Total
Gemini, May 3, 2005

19. FA and GL are independent!
artifacts
Gemini, May 3, 2005

20. FA: calm or turbulent? Turbulence comes in bursts When free atmosphere
is calm, it is stable!
Gemini, May 3, 2005

21. Free atmosphere in detail
Strong turbulence = low altitude
Gemini, May 3, 2005

22. 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)
Gemini, May 3, 2005

23. SODAR profiles
Nov 3
Nov 4
Nov 6
Nov 5
Gemini, May 3, 2005

24. SODAR vs. MASS-DIMM
Gemini, May 3, 2005

25. GL model
worst 25%
typical
best 25%
Gemini, May 3, 2005

26. 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
Gemini, May 3, 2005

27. CP model
The model is an independent
combination of FA and GL.
FA: one layer with changing
strength and altitude.
GL: exponential model
25%
50%
75% FA
0.29
0.41
0.56 GL
0.53
0.67
Total
0.63
0.77
0.91
Gemini, May 3, 2005

28. 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
Gemini, May 3, 2005

29. Last FA-calm night: May1/2, 2005
The “sweet spot” for AO!
Gemini, May 3, 2005