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

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

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

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

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

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

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

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 +/- 0.002 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

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

Portable DIMM Operation Preparing for nighttime measurements on the high chilean sites (5200m) in the vicinity of the ALMA project Source: Cornell Atacama project http://astrosun.tn.cornell.edu/atacama July 2001 Zanjan, Iran

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

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 http://astrosun.tn.cornell.edu/atacama July 2001 Zanjan, Iran

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

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

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

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

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

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

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

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

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

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

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

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

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

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