Post on 09-Feb-2016
description
Measuring Seeing, The Differential Image Motion Monitor (DIMM)
Marc Sarazin(European Southern Observatory)
July 2001 Zanjan, Iran 2
List of ThemesHow 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
Fried parameter: ( meter, ^6/5)
Seeing: (radian, ^-0.2) 0
98.0)(r
FWHM
53
0
22
0 )()sec(2423.0)(
dhhCr n
Optical Propagation
The Signature of Atmospheric Turbulence
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:35
0312222 36.022 rDyxxy
• The variance of the differential image motion through circular apertures of diameter D, separated by d is:
35
0231312
350
231312
145.018.02
097.018.02
rdD
rdD
July 2001 Zanjan, Iran 7
DIMM Principle
The final estimate of the seeing is the average of both parallel and perpendicular motions
FWHMFWHMFWHM 21
July 2001 Zanjan, Iran 8
DIMM PrincipleError 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 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 OperationPreparing 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/ataca
ma
July 2001 Zanjan, Iran 11
Portable DIMM OperationAlignment of C11
telescope mount on a high chilean site (5200m) in the
vicinity of the ALMA project
• Pixel size=0.7”Pixel size=0.7”• Pupil Diameter=9cm Pupil Diameter=9cm • Pupil Separation=12cm Pupil Separation=12cm • Exposure Time=10/20msExposure Time=10/20ms• 50 frames/mn50 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 http://astrosun.tn.cornell.edu/atacama
July 2001 Zanjan, Iran 13
Portable DIMM Operation
5m high tower and 5m high tower and daytime protection daytime protection
of the portable of the portable DIMM at the DIMM at the
observatory of observatory of Maidanak, Maidanak, UzbekistanUzbekistanThe telescope stands
in free air circulation to prevent build-up of local thermal pockets
July 2001 Zanjan, Iran 14
Automated DIMM Operation
Daytime protection Daytime protection of the automated of the automated DIMM at the VLT DIMM at the VLT
ObservatoryObservatoryThe enclosure control is The enclosure control is
linked to the linked to the meteorological station meteorological station
(closes when (closes when wind>18m/s, Rh>80%)wind>18m/s, Rh>80%)
July 2001 Zanjan, Iran 15
Automated DIMM Operation
35cm Telescope for 35cm Telescope for the automated the automated
DIMM at the VLT DIMM at the VLT ObservatoryObservatory
• Pixel size=0.7”Pixel size=0.7”• Pupil Diameter=11cm Pupil Diameter=11cm • Pupil Separation=20cm Pupil Separation=20cm • Exposure Time=5msExposure Time=5ms• 600 frames/mn600 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 cloudsThe 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 OperationAperture 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
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.
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.
July 2001 Zanjan, Iran 20
Corrected DIMM Seeing vs. VLT Image Quality
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.
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.
July 2001 Zanjan, Iran 21
Corrected DIMM Seeing vs. VLT Image Quality
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.
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.
July 2001 Zanjan, Iran 22
DIMM Seeing vs. Large Telescope Image Quality
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 m).
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:
July 2001 Zanjan, Iran 23
Monitoring Turbulence Height with the DIMM
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.
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).
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
Conclusion
Intercalibration of the site monitoring instruments is
recommended