Background subtraction

The background of the FEROS spectra consists of several components, mainly

• an electronic bias level, which can be determined from the overscan region of the CCD or from bias exposures,
• the CCD dark current, which can be determined from a series of long darks, and
• scattered light, which is smoothly varying over the CCD.

The latter contribution is determined by measuring its level outside the spectra. In the case of FEROS, this is the region between the orders and between the fibers. The region between the fibers is small, but is independent of order number. The distance between the orders is strongly depending on order number and is largest in the blue region. The background level is measured by taking the median in small regions at regular intervals in the y-direction. The measured values are approximated by fitting a 2-D smoothing spline function. Since the 2-D spline requires a rectangular grid, the median values are interpolated in x-direction before the spline fit.

The dark current should be subtracted before scattered light is subtracted, since it may have strong spatial variations which can not be taken into account by the spline fit. Background subtraction is done with the command BACKGR/FEROS.