FORS2 IMAGE QUALITY

QC1 PLOTS: IMAGE QUALITY

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CURRENT PERIOD
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imagequality:
imagequality-DIMM:

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Item:	image quality
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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 on reduced science images according to an analysis of the Sextractor parameters (Stellarity Index > 0.5). Then, the mode of the IQ distribution is found and the RMS deviation is estimated using the left wing of the distribution only. The estimated IQ is then logged into a 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.

IQ vs DIMM

In this section we present the results of the comparison between the nominal DS and the measured IQ. This section is usually updated at the end of the Period ( Last Update: Period 71).

In order to allow for a meaningful comparison, the measured IQ has been corrected to the 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. To reduce the uncertainty on the measured IQ, only values obtained from more than 10 stars have been considered. The DIMM seeing values are the average of the measurements recorded during an exposure (at a rate of one/minute). In the following plots, only images for which at least one DIMM measurement is available during the exposure have been included. No selection has been performed on the basis of the exposure times, which range from 1 minute up to 30 minutes.

In the following figures, we have plotted the deviations IQ-DS (in arcsec) vs. DS (upper panel) and the UBVRI IQ vs. DS (lower panel). The black dotted line represents the locus where DS and IQ are identical, while the red dotted line is a fit to the data. The scatter is quite large, but as a general rule, IQ is on average better than DS when DS >1.0", while it tends to be larger than DS when DS<1.0". As it is seen, the scatter appears to be quite large, indicating that the measured IQ depends not only on the actual seeing, but also on other parameters, like wind speed and wind direction with respect to the telescope pointing.

CHIP 1

CHIP 2

IMPORTANT NOTE: The Standard Resolution (SR) collimator produces a detector scale of 0".25/pixel (2x2 binning). When the seeing is less than 0".4-0".5 this leads to some significative undersampling of the Point Spread Function (PSF). Even though Sextractor applies a correction for undersampling effects down to a FWHM=1 pixel (assuming an underlying gaussian PSF), a residual effect is expected in the region of excellent seeing. Of course the effect should be much less pronounced when the High Resolution (HR) collimator is used, with a detector scale of 0".125/pixel(2x2 binning). This is in fact visible in the above figure, where the measurements obtained with the HR collimator have been plotted in green. They clearly distribute at the lower edge, even though this is not totally due to the sampling effect. In fact, the HR collimator is used when seeing conditions are very good and this naturally tends to bias the correspondent distribution towards the lower seeing values.

In the next figure we show the distribution of IQ-DS deviations (upper panel) and the distribution of DIMM seeing and corrected FORS2 IQ (lower left and right panels) for both CHIP 1 and CHIP 2. In each panel the dashed line marks the median of the distribution.

CHIP 1

CHIP 2

The lower figures show the cumulative function of teh FORS2 IQ-DIMM deviations for CHIP 1 and CHIP 2. These show that for CHIP 1 in ~64% of the cases (light shaded region) the measured is within 0".1 from the DIMM seeing, while in ~52% of the cases (dark shaded region)it is equal or better than DIMM seeing. For CHIP 2, the measured IQ is within 0".1 from the DIMM seeing in 53% of the cases (light shaded region), while it is equal or better than DIMM seeing in ~45% of the cases (dark shaded region). In both plots, the dashed line marks the median of the distribution.

CHIP 1

CHIP 2

Postscript versions of the above figures can be found here:

DIMM vs. FORS2 IQ: CHIP 1 CHIP 2
IQ-DS distributions: CHIP 1 CHIP 2
Deviations cumulative function: CHIP 1 CHIP 2

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