Data Reduction Frequently Asked Questions

The sky subtraction in my NODDING data is really bad - what could be the reason?


The NODDING observing technique relies on a constant sky background along the slit, i.e. no spatial variations are present in the sky background. If this is not true (for instance in crowded fields, extended objects, star-forming regions) the sky will be different at different positions and therefore not cancel out.

There is no way to subtract such sky background properly within the X-shooter pipeline. The best way is to process the data as STARE (see without sky subtraction and then use independent software tools to fit and subtract the sky from the rectified, wavelength calibrated 2-dimensional frame (MERGE2D).