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 http://www.eso.org/sci/data-processing/faq/can-i-process-offset-or-nodding-mode-data-as-stare-mode-data.html) without sky subtraction and then use independent software tools to fit and subtract the sky from the rectified, wavelength calibrated 2-dimensional frame (MERGE2D).