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Measuring Seeing, The Differential Image Motion Monitor (DIMM)
Measuring Seeing, The Differential Image Motion Monitor (DIMM)		 Measuring Seeing, The Differential Image Motion Monitor (DIMM)
Measuring Seeing, The Differential Image Motion Monitor (DIMM)		 DIMM Principle
DIMM Principle		 DIMM Precursor
DIMM Precursor		 Portable DIMM Operation
Portable DIMM Operation
Portable DIMM Operation
Portable DIMM Operation		 Portable DIMM Operation
Portable DIMM Operation
Portable DIMM Operation
Portable DIMM Operation		 Automated DIMM Operation
Automated DIMM Operation		 Automated DIMM Operation
Automated DIMM Operation		 Automated DIMM Operation
Automated DIMM Operation		 Automated DIMM Operation
Automated DIMM Operation
Automated DIMM Operation
Automated DIMM Operation		 Automated DIMM Operation
Automated DIMM Operation		 DIMM Seeing vs
DIMM Seeing vs		 Corrected DIMM Seeing vs
Corrected DIMM Seeing vs		 Corrected DIMM Seeing vs
Corrected DIMM Seeing vs
DIMM Seeing vs
DIMM Seeing vs		 DIMM Seeing vs
DIMM Seeing vs		 Monitoring Turbulence Height with the DIMM
Monitoring Turbulence Height with the DIMM		 Local Seeing: Ground Layer Turbulence at Paranal
Local Seeing: Ground Layer Turbulence at Paranal		 Local Seeing: Seeing Impact of Ground Layer
Local Seeing: Seeing Impact of Ground Layer
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Measuring Seeing, The Differential Image Motion Monitor (DIMM)

содержание презентации «Measuring Seeing, The Differential Image Motion Monitor (DIMM).ppt»
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1Measuring Seeing, The Differential 17Automated DIMM Operation. The system
Image Motion Monitor (DIMM). Marc Sarazin automatically switches to another star in
(European Southern Observatory). case of clouds The seeing is independent
2List of Themes How to find the ideal of cloudiness (although sometimes pretty
site...and keep it good? Optical good with high cirrus clouds) Aperture
Propagation through Turbulence Mechanical photometry alows to monitor the sky
and Thermal Index of Refraction Signature variability. July 2001. Zanjan, Iran. 17.
on ground based observations Correction 18Automated DIMM Operation. Aperture
methods Integral Monitoring Techniques photometry on ca 5000 DIMM short exposures
Seeing Monitoring Scintillation Monitoring allows to monitor the flux variability,
Profiling Techniques Microthermal Sensors equivalent to the extinction variability
Scintillation Ranging Modelling (June 2000 statistics below). The
Techniques. July 2001. Zanjan, Iran. 2. threshold for photometric sky is between
3Why Differential Image Motion? The 1% and 2% relative flux rms. July 2001.
tracking errors are automatically Zanjan, Iran. 18.
subtracted The wind has no effect on the 19DIMM Seeing vs. VLT Image Quality.
measurements The telescope optical quality DIMM converts image motion into large
is not important (nevertheless circular telescope seeing with the assumption of an
images are required, i.e. no coma allowed) infinite outer scale of the turbulence. UT
Easy to implement with state of the art images turned out about 10% better than
amateur astronomer detectors The DIMM predicted by DIMM, confirming the finite
gives two statistical estimates of the character of the outer scale. Comparison
same variable. July 2001. Zanjan, Iran. 3. of DIMM seeing (Y axis), with FORS Science
4Optical Propagation The Signature of Verification (SV) Image Quality (X axis)
Atmospheric Turbulence. Seeing: (radian, as processed by the SV team, corrected for
??^-0.2). Fried parameter: ( meter, zenith and 500nm. July 2001. Zanjan, Iran.
??^6/5). July 2001. Zanjan, Iran. 4. 19.
5DIMM Principle. Two images of the same 20Corrected DIMM Seeing vs. VLT Image
star are created on a CCD, corresponding Quality. DIMM converts image motion into
to light having traveled through two large telescope seeing with the assumption
parallel columns in the atmosphere. July of an infinite outer scale of the
2001. Zanjan, Iran. 5. turbulence. UT images turned out about 10%
6DIMM Principle. The variance of the better than predicted by DIMM, confirming
image motion through a circular aperture the finite character of the outer scale.
of diameter D depends on the seeing as: Correcting for that effect is possible by
The variance of the differential image removing from the DIMM the share of the
motion through circular apertures of tilt of an 8m aperture. Comparison of DIMM
diameter D, separated by d is: July 2001. seeing (Y axis) after correction for outer
Zanjan, Iran. 6. scale, with FORS Science Verification (SV)
7DIMM Principle. The final estimate of Image Quality (X axis) as processed by the
the seeing is the average of both parallel SV team, corrected for zenith and 500nm.
and perpendicular motions. July 2001. July 2001. Zanjan, Iran. 20.
Zanjan, Iran. 7. 21Corrected DIMM Seeing vs. VLT Image
8DIMM Principle. Error Budget for a 10% Quality. DIMM converts image motion into
accuracy goal. The instrumental noise large telescope seeing with the assumption
(sampling, centroiding) is measured in the of an infinite outer scale of the
lab on fixed sources. The constant part turbulence. UT images turned out about 10%
can be subtracted out, the noise is the better than predicted by DIMM, confirming
remaining variance, about +/- 0.002 the finite character of the outer scale.
pixel^2, or 5% relative error at 0.2” Correcting for that effect is possible by
seeing. The plate scale is calibrated on removing from the DIMM the share of the
double stars of known separation The tilt of an 8m aperture. Comparison of DIMM
measurement noise might increase if the seeing (Y axis) after correction for outer
signal to noise ratio is too low: images scale, with UT1 Science Verification (SV)
with low SNR due to scintillation have to Image Quality (X axis) as processed by the
be rejected. The statistical noise is SV team from Test Camera long exposures,
inversely proportional to the square root corrected for zenith and at 500nm. July
of the number of samples in the time 2001. Zanjan, Iran. 21.
series. The relative error on the seeing 22DIMM Seeing vs. Large Telescope Image
is about 6% for 200 exposures. The Quality. DIMM converts image motion into
temporal under sampling due to too long large telescope seeing with the assumption
exposure time: no way to correct for it of an infinite outer scale of the
because the velocity of the tilt is turbulence. Assuming that the outer scale
unknown. Interlacing two exposure times is larger than the telescope aperture, a
the best way to control. The very bad first order correction is obtained by
seeing (>2”) is over estimated because removing the one axis image jitter
the stellar image breaks into speckles. (Gradient tilt) variance from the long
July 2001. Zanjan, Iran. 8. exposure FWHM: Outer scale correction
9DIMM Precursor. A visual DIMM was used coefficient to apply to the DIMM estimates
in the 60’s for site selection purposes in of the image quality of a 8m telescope
Chile and in Uzbekistan (photo: Maidanak limited by the atmosphere, for 0 and 60
Observatory). See: J. Stock and G. Keller, degree zenith angle, as a function of the
1960, in Stars and Stellar System, Vol. 1, observing wavelength (the following
Chicago University Press. July 2001. central wavelength of the bands [U, B, V,
Zanjan, Iran. 9. R, I, J, H, K, L, M, N] corresponding to
10Portable DIMM Operation. Preparing for [0.36, 0.44, 0.55, 0.64, 0.79, 1.25, 1.65,
nighttime measurements on the high chilean 2.2, 3.4, 5.0, 10] in mm). July 2001.
sites (5200m) in the vicinity of the ALMA Zanjan, Iran. 22.
project Source: Cornell Atacama project 23Monitoring Turbulence Height with the
http://astrosun.tn.cornell.edu/atacama. DIMM. Scintillation through DIMM apertures
July 2001. Zanjan, Iran. 10. of 10-12cm diameter can be related to the
11Portable DIMM Operation. Alignment of isoplanatic angle (Loos & Hogge, Appl.
C11 telescope mount on a high chilean site Opt. 18, 15; 1979) and then to the
(5200m) in the vicinity of the ALMA normalized 5/3rd moment of the turbulence
project Pixel size=0.7” Pupil Diameter=9cm height (Hbar). The atmospheric seeing
Pupil Separation=12cm Exposure (black lower curve, in arcsec) is the
Time=10/20ms 50 frames/mn Photo credit: P. cumulative effect of several turbulent
Recabarren, Observatory of Cordoba, layers at various altitudes: monitoring
Argentina. July 2001. Zanjan, Iran. 11. the characteristic altitude of the
12Portable DIMM Operation. 1m high turbulence (red upper curve, in km) is
platform and daytime protection of the necessary for planning adaptive optics
portable DIMM on the high chilean sites instrumentation. In this example, the bad
(5200m) in the vicinity of the ALMA seeing is located at low altitude while
project Source: Cornell Atacama project good conditions are produced by a few
http://astrosun.tn.cornell.edu/atacama. layers at high altitude. July 2001.
July 2001. Zanjan, Iran. 12. Zanjan, Iran. 23.
13Portable DIMM Operation. 5m high tower 24Local Seeing: Ground Layer Turbulence
and daytime protection of the portable at Paranal. Measurement of the
DIMM at the observatory of Maidanak, microthermal activity and Seeing at
Uzbekistan. The telescope stands in free Paranal (GSM Campaign, Nice University)
air circulation to prevent build-up of during a night presenting variable
local thermal pockets. July 2001. Zanjan, conditions (F. Martin, R. Conan, A.
Iran. 13. Tokovinin, A. Ziad, H. Trinquet, J.
14Automated DIMM Operation. Daytime Borgnino, A. Agabi and M. Sarazin; Optical
protection of the automated DIMM at the parameter relevant for high angular
VLT Observatory The enclosure control is resolution at Paranal from GSM instrument
linked to the meteorological station and surface layer contribution; Astron.
(closes when wind>18m/s, Rh>80%). Astrophys. Supplement, v.144, p.39-44;
July 2001. Zanjan, Iran. 14. June 2000). July 2001. Zanjan, Iran. 24.
15Automated DIMM Operation. 35cm 25Local Seeing: Seeing Impact of Ground
Telescope for the automated DIMM at the Layer. Measurement of the microthermal
VLT Observatory Pixel size=0.7” Pupil activity and Seeing at Paranal (GSM
Diameter=11cm Pupil Separation=20cm Campaign, Nice University): The
Exposure Time=5ms 600 frames/mn. July contribution of the layer 7-21m above
2001. Zanjan, Iran. 15. ground is marginal both during good and
16Automated DIMM Operation. The seeing bad seeing conditions . July 2001. Zanjan,
is updated every minute for zenith Iran. 25.
observation at 0.5 micron wavelength The 26Conclusion. Intercalibration of the
accuracy is better than 10% above 0.2” The site monitoring instruments is
natural atmospheric noise is about 10% of recommended. July 2001. Zanjan, Iran. 26.
the seeing. July 2001. Zanjan, Iran. 16.
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