The DIMM Seeing and UT Image Quality

The comparison of Image Quality of FORS Science Verification images (X axis: zenith median FWHM in arcsec at 500nm) with the corresponding DIMM estimate of external seeing (Y axis), averaged over the exposure duration and corrected for the same airmass. DIMM assumes an infinite outer scale for atmospheric turbulence and thus overestimates image wander on very large telescopes.

DIMM vs. FORS before correction for Finite Outer Scale

The comparison of Image Quality of FORS Science Verification images (X axis: zenith median FWHM in arcsec at 500nm) with the corresponding DIMM estimate of external seeing, averaged over the exposure duration and corrected for finite outer scale of the turbulence (Y axis). 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. The DIMM data can be corrected for finite outer scale by removing quadratically the excess variance. The correction is non linear with seeing

DIMM vs. FORS after correction for Finite Outer Scale

The comparison of Image Quality of UT1 Science Verification images (X axis: zenith median FWHM in arcsec at 500nm) with the corresponding DIMM estimate of external seeing, averaged over the exposure duration and corrected for finite outer scale of the turbulence (Y axis). Unusually bad seeing conditions occurred during UT1 Science Verification. The few UT images with Image Quality worse than 1.5" seem to indicate that the DIMM overestimates the bad seeing.

DIMM vs. UT Test Camera after correction for Finite Outer Scale