FORS2 IMAGE QUALITY

QC1 PLOTS: IMAGE QUALITY

QC1 ASCII DATA

CURRENT PERIOD
	gif	ps
imagequality:
imagequality-DIMM:

COMPLETE SET
*Year:**
Item:	image quality
*Chip:**
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(updated regularly)

QC1 database (expert mode)

General

Starting with Period 69 (April 2002), 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 alghoritm (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. The estimated IQ is then logged into the QC1 database together with other parameters, like alt-az field coordinates, DIMM seeing (hereafter DS),wind speed, wind direction and so on. The data is undergoing an extensive analysis in order to asses whether there are any relations between the measured IQ and other physical parameters. Only IQ measurements obtained with the Standard Resolution (SR) collimator and 2x2 binning and taken through the broad-band UBVRI filters are trended here.

IQ vs DIMM

For each frame, the measured IQ is compared with the reference DS value (when available) averaged over the exposure time. In order to allow for a meaningful comparison, the measured FORS2 IQ has been corrected to zenith and to the DIMM central wavelength (0.5 mu). An additional correction due to the different telescope diameters has also been included, following the prescriptions by Marc Sarazin. A quick description of the adopted correction is given here.

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