The ISAAC calibration scheme requires careful consideration of interdependencies between raw and product calibration files. The calibration products have to be processed step by step to follow these interdependencies. This step-by-step scheme is called a calibration cascade.

QC Garching mode: The date reduction cascade in Garching is a quasi-static scheme with dynamically updated calibration products. The closest in time calibration is available for the next processing step of the cascade. The quality control process assures, that only certified calibration products are used for the follow up steps.

Paranal mode: The data reduction cascade on Paranal uses a static database of calibration products, that are updated from time to time. In the near-IR, and in particular for ISAAC, for which the recipes have been designed such, that no calibration product is operationally required, the calibration database is mostly empty. This means that science images are reduced without dark subtraction and flat fielding and bad pixel masking. Science spectra are reduced without flat fielding and optical distortion correction, but wavelength calibrated using the sky OH lines. The justification for this one-step cascade is simply: the infrared data reduction scheme (the double subtraction scheme [ (A minus B) minus (B minus A) ] for a A-B raw frame sequence) removes the instrumental dark and the variable sky background sufficiently for quick-look purpose. The data reduction on Paranal is therefore for most of the raw types a single-step reduction.

The following calibration products are generated by QC Garching but are not (yet) part of the calibration cascade:

• illumination correction
• spectrophotometric calibration

These two calibration types are required for the purpose of enhanced science data reduction.

ISAAC reduction cascade IMAGING The reduction cascade is shown for SW imaging, but is valid for for LW imaging as well. with the minor difference in the way the photometric zeropoints are handled.

ISAAC reduction cascade SPECTROSCOPY The reduction cascade for spectroscopy differs for the SW-arm/LW-arm LR-grating/MR-grating versions mostly in the handling of the arc frames.