The sky intensity at each increment along the order may be modelled using a polynomial or spline curve. The degree of the curve fit is adjustable. By default it is set to zero to obtain the mean sky behavior.
Higher-degree polynomials are useful when there is a gradient to the sky intensity along the slit, as they are used to predict the sky intensity at each object pixel in the order independently.
Note that the meaning of 'increment' differs between regular and 2-D distortion corrected extractions.
For a simple extraction an increment is a single pixel column.
For a 2-D extraction each increment is a scrunched wavelength scale unit, thus ensuring the accurate modelling of distorted bright emission lines in the sky.
The selection of sky pixels is done using the masks determined by the profiling task/option. These masks can be freely edited to cope with any special requirements as to which regions of the slit are to be used for the sky. This facility is of use when processing frames where 'periscopes' have been used to add in sky regions when observing an extended source. In such cases ECHOMOP currently provides no special treatment and the periscope sky pixels' positions will have to be edited into the sky mask using the ech_spatial task/option.
Martin Clayton