Starting with Period 71 (April 2003), all reduced FORS2 science images
taken in Service Mode are analyzed in order to estimate their Image
Quality (hereafter IQ). This is done in a fully automatic way using a
stand-alone procedure based on the Sextractor object detection
algorithm (Bertin and Arnouts 1996 A&A 117,393). Stars are identified
based on Sextractor parameters (Stellarity Index > 0.5, ellipticity
<0.5 and quality FLAG=0) and their FWHM is measured by the
program. Then the mode of the IQ distribution is computed and the RMS
deviation is estimated using the left wing of the distribution
only. Only IQ measurements obtained with the Standard Resolution (SR)
collimator and taken through the broad-band UBVRI filters are trended
here. To reduce the uncertainty, only image quality values are
plotted, which result from <b>more than 10 stars</b> and have at <b>at
least one DIMM seeing measurement</b> during their exposure time. To
avoid a bias from deep exposures, where galaxies dominate the field,
we require in addition that the <b>average ellipticity is less than
0.3</b> and the <b>rms of the image quality is below 0.2</b>.

For comparison we use the image quality as reported by the Active
Optics, corrected to the airmass of the respective science images.

More information can be found on
the <a href="http://www.eso.org/observing/dfo/quality/FORS2/qc/imagequality/imagequality.html">plot tutorial page</a>