Health Check monitor

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 more than 10 stars and have at at least one DIMM seeing measurement during their exposure time. To avoid a bias from deep exposures, where galaxies dominate the field, we require in addition that the average ellipticity is less than 0.3 and the rms of the image quality is below 0.2. For comparison the DIMM seeing values are corrected to the airmass and wavelength of the respective science images. More information can be found on the plot tutorial page