Background estimate by filtering

The command BACKGROUND/SMOOTH enables to define the background without previous order definition. This feature is useful for example to correct for the background the image required for the order definition. This command involves an iterative algorithm which performs the following operations:

• the input frame is heavily smoothed in the direction perpendicular to the orders.
• the original frame is divided by the smoothed one and all pixels which ratio is greater than one are replaced by the smoothed value.
• the corrected frame becomes the new input frame and the two previous steps are repeated until a satisfying solution is obtained.

A more elaborated scheme is required if the contribution to the background from adjacent orders is important, this occurs when the distance between orders is small. All above methods are implemented in command BACKGROUND/ECHELLE and selected by the echelle keyword BKGMTD.