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Quality Control and
Data Processing

CAL | HC | refs | QC
Trending & QC1
   image quality
FORS2 ...
QC links:
FORS2: Image Quality
image quality
QC1 database (advanced users): browse | plot

Starting with May 2000 (P65), 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). 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.

Most of the IQ monitoring was done by processing science frames. With the rescoping of the QC group processes, science processing was terminated on Sep. 30, 2011, and so was the IQ trending. Still, the historical information could be useful.

Image Quality


FITS key QC1 database: table, name definition class* HC_plot** more docu
[calculated by QC procedure] fors2_sci_img..iq FORS image quality (Sextractor FWHM, arcsec) [docuSys coming]
[calculated by QC procedure] fors2_sci_img..iq_rms RMS deviation (arcsec) [docuSys coming]
[calculated by QC procedure] fors2_sci_img..avg_ellipt Average Ellipticity [docuSys coming]
[calculated by QC procedure] fors2_sci_img..dimm_corr average DIMM seeing (arcsec, corrected for airmass and wavelength) [docuSys coming]
*Class: KPI - instrument performance; HC - instrument health; CAL - calibration quality; ENG - engineering parameter
**There might be more than one.


Only IQ measurements obtained with the Standard Resolution (SR) collimator and taken through the broad-band UBVRI filters are trended. 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.
This plot shows the observed image quality for the V_BESS filter (red/green dots: 2 CCDs) vs. the corrected DIMM seeing. The blue line gives the 1:1 relation. Obviously the FORS2 image quality tends to be better than the DIMM seeing for DIMM seeing > 1.2".

Scoring&thresholds Image Quality

No scoring nor thresholds are applied.


No events.

Algorithm Image Quality

The SExtractor (Source Extractor) software (Bertin & Arnouts, 1996, A&A 117, 393) is run on the reduced images to detect the sources. 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. The mode of the FWHM distribution is defined as the image quality.

Error of the image quality is the rms error estimated using a symmetric range |mode_seeing-minimum_seeing| on both sides of the mode seeing (so-called core of the distribution).

The average ellipticty is calculated for those sources that have an image quality within the core of the seeing distribution (see above).

Corrected DIMM seeing is the average seeing observed by the DIMM, corrected to the airmass of the FORS2 observations. In addition the DIMM seeing is corrected to the wavelength of the FORS2 observations and also the different telescope diameters are taken into account, following the prescriptions by Marc Sarazin.

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