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image quality
old CCD
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FORS1 trending system: History Plots
Last update: 2009-04-14T12:26:51 (UT)
| now: 2017-08-23T00:40:54 (UT)  
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2007-10-20 last image quality data with B_BESS
Average ? Thresholds ? N_
method value unit method value
1 QC1DB no MEDIAN 0.087 arcsec none   4 iq_dimm this | last_yr | all difference between Image Quality and DIMM seeing (arcsec, Chip 1)
2 QC1DB no none   arcsec none   4 iq this | last_yr | all Image Quality vs DIMM seeing with 1:1 line overplotted (arcsec, Chip 1)
*Data sources: QC1DB: QC1 database; LOCAL: local text file | **OPS: to indicate that OPSLOG data are included
This plot

Starting with Period 65 (April 2000), all reduced FORS1 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.  

General information

Click on any of the plots to see a close-up version. The latest date is indicated on top of the plot, data points belonging to that date are specially marked.

If configured,

  • statistical averages are indicated by a solid line, and thresholds by broken lines
  • outliers are marked by a red asterisk. They are defined as data points outside the threshold lines
  • "aliens" (= data points outside the plot Y limits) are marked by a red arrow ( or )
  • you can download the data for each parameter set if the 'Data downloads' link shows up