1. Measuring Seeing, The Differential Image Motion Monitor (DIMM)
Marc Sarazin
(European Southern Observatory)

2. List of Themes How to find the ideal site...and keep it good?
Optical Propagation through Turbulence
Mechanical and Thermal
Index of Refraction
Signature on ground based observations
Correction methods
Integral Monitoring Techniques
Seeing Monitoring
Scintillation Monitoring
Profiling Techniques
Microthermal Sensors
Scintillation Ranging
Modelling Techniques
July 2001
Zanjan, Iran

3. Why Differential Image Motion?
The tracking errors are automatically subtracted
The wind has no effect on the measurements
The telescope optical quality is not important (nevertheless circular images are required, i.e. no coma allowed)
Easy to implement with state of the art amateur astronomer detectors
The DIMM gives two statistical estimates of the same variable
July 2001
Zanjan, Iran

4. Optical Propagation The Signature of Atmospheric Turbulence
Measuring Seeing
Optical Propagation The Signature of Atmospheric Turbulence
Seeing: (radian, ^-0.2)
Fried parameter: ( meter, ^6/5)
July 2001
Zanjan, Iran

5. DIMM Principle
Two images of the same star are created on a CCD, corresponding to light having traveled through two parallel columns in the atmosphere
July 2001
Zanjan, Iran

6. DIMM Principle
The variance of the image motion through a circular aperture of diameter D depends on the seeing as:
The variance of the differential image motion through circular apertures of diameter D, separated by d is:
July 2001
Zanjan, Iran

7. DIMM Principle
The final estimate of the seeing is the average of both parallel and perpendicular motions
July 2001
Zanjan, Iran

8. Error Budget for a 10% accuracy goal
DIMM Principle
Error Budget for a 10% accuracy goal
The instrumental noise (sampling, centroiding) is measured in the lab on fixed sources. The constant part can be subtracted out, the noise is the remaining variance, about +/ pixel^2, or 5% relative error at 0.2” seeing. The plate scale is calibrated on double stars of known separation
The measurement noise might increase if the signal to noise ratio is too low: images with low SNR due to scintillation have to be rejected.
The statistical noise is inversely proportional to the square root of the number of samples in the time series. The relative error on the seeing is about 6% for 200 exposures.
The temporal under sampling due to too long exposure time: no way to correct for it because the velocity of the tilt is unknown. Interlacing two exposure times is the best way to control.
The very bad seeing (>2”) is over estimated because the stellar image breaks into speckles
July 2001
Zanjan, Iran

9. DIMM Precursor
A visual DIMM was used in the 60’s for site selection purposes in Chile and in Uzbekistan (photo: Maidanak Observatory).
See: J. Stock and G. Keller, 1960, in Stars and Stellar System, Vol. 1, Chicago University Press
July 2001
Zanjan, Iran

10. Portable DIMM Operation
Preparing for nighttime measurements on the high chilean sites (5200m) in the vicinity of the ALMA project
Source: Cornell Atacama project
July 2001
Zanjan, Iran

11. Portable DIMM Operation
Alignment of C11 telescope mount on a high chilean site (5200m) in the vicinity of the ALMA project
Pixel size=0.7”
Pupil Diameter=9cm
Pupil Separation=12cm
Exposure Time=10/20ms
50 frames/mn
Photo credit: P. Recabarren, Observatory of Cordoba, Argentina
July 2001
Zanjan, Iran

12. Portable DIMM Operation
1m high platform and daytime protection of the portable DIMM on the high chilean sites (5200m) in the vicinity of the ALMA project
Source: Cornell Atacama project
July 2001
Zanjan, Iran

13. Portable DIMM Operation
5m high tower and daytime protection of the portable DIMM at the observatory of Maidanak, Uzbekistan
The telescope stands in free air circulation to prevent build-up of local thermal pockets
July 2001
Zanjan, Iran

14. Automated DIMM Operation
Daytime protection of the automated DIMM at the VLT Observatory
The enclosure control is linked to the meteorological station (closes when wind>18m/s, Rh>80%)
July 2001
Zanjan, Iran

15. Automated DIMM Operation
35cm Telescope for the automated DIMM at the VLT Observatory
Pixel size=0.7”
Pupil Diameter=11cm
Pupil Separation=20cm
Exposure Time=5ms
600 frames/mn
July 2001
Zanjan, Iran

16. Automated DIMM Operation
The seeing is updated every minute for zenith observation at 0.5 micron wavelength
The accuracy is better than 10% above 0.2”
The natural atmospheric noise is about 10% of the seeing
July 2001
Zanjan, Iran

17. Automated DIMM Operation
The system automatically switches to another star in case of clouds
The seeing is independent of cloudiness (although sometimes pretty good with high cirrus clouds)
Aperture photometry alows to monitor the sky variability
July 2001
Zanjan, Iran

18. Automated DIMM Operation
Aperture photometry on ca 5000 DIMM short exposures allows to monitor the flux variability, equivalent to the extinction variability (June 2000 statistics below)
The threshold for photometric sky is between 1% and 2% relative flux rms
July 2001
Zanjan, Iran

19. DIMM Seeing vs. VLT Image Quality
DIMM converts image motion into large telescope seeing with the assumption of an infinite outer scale of the turbulence. UT images turned out about 10% better than predicted by DIMM, confirming the finite character of the outer scale.
Comparison of DIMM seeing (Y axis), with FORS Science Verification (SV) Image Quality (X axis) as processed by the SV team, corrected for zenith and 500nm.
July 2001
Zanjan, Iran

20. Corrected DIMM Seeing vs. VLT Image Quality
DIMM converts image motion into large telescope seeing with the assumption of an infinite outer scale of the turbulence. UT images turned out about 10% better than predicted by DIMM, confirming the finite character of the outer scale. Correcting for that effect is possible by removing from the DIMM the share of the tilt of an 8m aperture.
Comparison of DIMM seeing (Y axis) after correction for outer scale, with FORS Science Verification (SV) Image Quality (X axis) as processed by the SV team, corrected for zenith and 500nm.
July 2001
Zanjan, Iran

21. Corrected DIMM Seeing vs. VLT Image Quality
DIMM converts image motion into large telescope seeing with the assumption of an infinite outer scale of the turbulence. UT images turned out about 10% better than predicted by DIMM, confirming the finite character of the outer scale. Correcting for that effect is possible by removing from the DIMM the share of the tilt of an 8m aperture.
Comparison of DIMM seeing (Y axis) after correction for outer scale, with UT1 Science Verification (SV) Image Quality (X axis) as processed by the SV team from Test Camera long exposures, corrected for zenith and at 500nm.
July 2001
Zanjan, Iran

22. DIMM Seeing vs. Large Telescope Image Quality
DIMM converts image motion into large telescope seeing with the assumption of an infinite outer scale of the turbulence. Assuming that the outer scale larger than the telescope aperture, a first order correction is obtained by removing the one axis image jitter (Gradient tilt) variance from the long exposure FWHM:
Outer scale correction coefficient to apply to the DIMM estimates of the image quality of a 8m telescope limited by the atmosphere, for 0 and 60 degree zenith angle, as a function of the observing wavelength (the following central wavelength of the bands [U, B, V, R, I, J, H, K, L, M, N] corresponding to [0.36, 0.44, 0.55, 0.64, 0.79, 1.25, 1.65, 2.2, 3.4, 5.0, 10] in mm).
July 2001
Zanjan, Iran

23. Monitoring Turbulence Height with the DIMM
Scintillation through DIMM apertures of 10-12cm diameter can be related to the isoplanatic angle (Loos & Hogge, Appl. Opt. 18, 15; 1979) and then to the normalized 5/3rd moment of the turbulence height (Hbar).
The atmospheric seeing (black lower curve, in arcsec) is the cumulative effect of several turbulent layers at various altitudes: monitoring the characteristic altitude of the turbulence (red upper curve, in km) is necessary for planning adaptive optics instrumentation. In this example, the bad seeing is located at low altitude while good conditions are produced by a few layers at high altitude.
July 2001
Zanjan, Iran

24. Local Seeing: Ground Layer Turbulence at Paranal
Measurement of the microthermal activity and Seeing at Paranal (GSM Campaign, Nice University) during a night presenting variable conditions (F. Martin, R. Conan, A. Tokovinin, A. Ziad, H. Trinquet, J. Borgnino, A. Agabi and M. Sarazin; Optical parameter relevant for high angular resolution at Paranal from GSM instrument and surface layer contribution; Astron. Astrophys. Supplement, v.144, p.39-44; June 2000).
July 2001
Zanjan, Iran

25. Local Seeing: Seeing Impact of Ground Layer
Measurement of the microthermal activity and Seeing at Paranal (GSM Campaign, Nice University): The contribution of the layer 7-21m above ground is marginal both during good and bad seeing conditions .
July 2001
Zanjan, Iran

26. Intercalibration of the site monitoring instruments is recommended
Conclusion
Intercalibration of the site monitoring instruments is recommended
July 2001
Zanjan, Iran