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The DIMM Seeing and UT Image Quality


The DIMM Seeing and ISAAC Image Quality

When the outer scale is of the same order of magnitude as the telescope diameter, the contribution of the image motion to the long exposure image quality is negligible. To predict the science Image Quality (IQ), The DIMM data can be corrected for finite outer scale by removing quadratically the excess variance. The correction is non linear with seeing, it is computed for a 25m finite Outer Scale following a formula derived in Tokovinin, A., From differential image motion to seeing; PASP, 2002, V. 114, P. 1156. The actual improvement depends on the UT observing wavelength (much larger at large wavelengths) and on zenith distance. Note that for good seeing at 10 mu, the atmospheric spatial coherence scale (r0) is no more small compared to the outer scale and we reach the limit of validity of the analytical formulation.
A database of ISAAC observations taken on 9-12 February 2003 is used. In what follows, it is suggested that the disagreement with the finite L0 model could be due to IQ limitations of ISAAC in good seeing conditions. Note however that a later analysis by M. Casali concludes that the finite Lo model is not representative of science IQ at the UT foci.

ISAAC (horizontal) vs. DIMM (vertical) at ISAAC observing airmass and wavelength before and after correction of DIMM seeing for finite Outer Scale. The original DIMM seeing (zenith and 0.5mu) is given to show the amplitude of the correction (green crosses). In good seeing conditions, DIMM (L0=25m) predicts better IQ than measured by ISAAC.
The comparison is now made at zenith and 0.5mu: DIMM seeing (horizontal) vs. ISAAC IQ (vertical) before and after inverse correction of ISAAC IQ for infinite L0. The raw ISAAC IQ is given to show the amplitude of the correction (green crosses). In good seeing conditions, ISAAC IQ (infinite L0) is worse than DIMM seeing.

DIMM predicted seeing at ISAAC observing wavelength before and after correction for finite outer scale, for all the observations of the database. At ISAAC wavelength and with a large telescope, the images have 30% better quality than the site seeing.
DIMM indeed characterises the site seeing, as a proof the the good agreement of DIMM and UT4 active optics hartmann FWHM, both using apertures much smaller than L0.

The quality of the images obtained with large telescopes is better than the site seeing, the correction varies from a few % to a few hundred % depending on wavelength and seeing conditions.

Credits: Science images were processed by Wolfgang Hummel

See also: Study of the Outer Scale at Paranal

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