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

Measuring Seeing, The Differential Image Motion Monitor (DIMM)

Marc Sarazin(European Southern Observatory)

July 2001 Zanjan, Iran 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


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


Fried parameter: ( meter, ^6/5)

Seeing: (radian, ^-0.2) 0

98.0)(r

FWHM

53

0

22

0 )()sec(2

423.0)(

dhhCr n

Optical Propagation

The Signature of Atmospheric Turbulence


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


DIMM Principle• The variance of the image motion through a circular aperture

of diameter D depends on the seeing as:

350

312222 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


DIMM Principle

The final estimate of the seeing is the average of both parallel and perpendicular motions

FWHMFWHMFWHM 2

1


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


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


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


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


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


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, UzbekistanUzbekistan

The telescope stands in free air circulation to prevent build-up of local thermal pockets


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%)



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



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



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


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


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.


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.


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.


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:


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).


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).


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 .


Conclusion

Intercalibration of the site monitoring instruments is

recommended

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

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