Long-term performance of the VISIR/VLT instrument before the upgrade

The ESO's VLT Spectrometer and Imager for the Mid-Infrared (VISIR) has been in operation at the Paranal Observatory since 2005. It is equipped with two DRS (formerly Boeing) 256 × 256 BIB arrays. The project to replace detectors into new Raytheon, 1k × 1k AQUARIUS devices as well as to modify observing modes, software, etc. is underway. The VISIR upgrade creates a well defined break point in the instruments' characteristics. For nearly 7 years of the VISIR operations we have been collecting and processing calibration data, in particular observations of the imaging and spectroscopic standard stars, within a regular data flow operation scheme. The derived quality control parameters have been systematically written into a database, which allows the analysis of their temporal behavior. We present an overview of the long term variations of the VISIR quality control parameters: sensitivity, conversion factor and mean background level estimations. The results will be later used to compare performance of VISIR before and after the upgrade.