Quality control of VLT ISAAC data

For 3 years the Infrared Spectrometer And Array Camera (ISAAC) has been operating at the 8m Antu (UT1) telescope of the European Southern Observatory Very Large Telescope (ESO VLT). As part of ESO data flow operations ISAAC data are processed and quality control checked by the Data Flow Operations group (often known as QC Garching). at ESO headquarters in Garching. The status of the instrument is checked in terms of QC parameters, which are derived from raw and processed data and compared against reference values. Low level parameters include detector temperature and zero level offset, other parameters include image quality and spectrum curvature. Complicated instrumental behaviors like the odd-even column effect and the appearance of pupil ghosts require more sophisticated QC tools. Instrumental interventions of cryogenic instruments like ISAAC include a defrost and re-freeze sequence which can be traced in trending plots of the QC1 parameters, which are published regularly (see http://www.eso.org/qc). We present recent highlights of the ISAAC QC process and their role as feedback to the observatory to retain the performance of the instrument.