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IQ vs. seeing
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Image quality is calculated from each science frame taken in imaging mode. SExtractor (Bertin and Arnouts 1996, A&A 117, 393) is run on the reduced images to detect objects, compute their FWHM and ellipticity, and select stars. If the number of stars (i.e. objects with stellarity index > 0.5) found is at least 10, the image quality is assumed to be the median of the FWHM of the stars. Outliers (3 sigma) are iteratively excluded for the final value (up to 4 iterations).

The expected image quality is derived from the DIMM seeing measurements that are stored in the header of the science frames when the DIMM seeing monitor is operational. The keywords "TEL AMBI FWHM START" and "TEL AMBI FWHM END" store DIMM seeing measurements at start and at the end of the exposure, respectively. The average is computed and assumed as the DIMM seeing value of the exposure. Because DIMM measurements are done at zenith and at 0.5 micron, the DIMM values are reduced to the zenith angles and wavelengths of the science observation, in addition a first order outer scale correction appropriate for a 8m telescope is applied. The formula used to obtain the expected image quality is given by M. Sarazin.

 

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Trending shows the image quality of each science frame vs. the expected image quality.