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<H2><A NAME="SECTION001143000000000000000">
Background estimate by filtering</A>
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<P>
The command <TT>BACKGROUND/SMOOTH</TT> 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:
<UL>
<LI>the input frame is heavily smoothed in the direction perpendicular to the
orders.
<LI>the original frame is divided by the smoothed one and all pixels which
ratio is greater than one are replaced by the smoothed value.
<LI>the corrected frame becomes the new input frame and the two previous
steps are repeated until a satisfying solution is obtained.
</UL>
<P>
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
<TT>BACKGROUND/ECHELLE</TT> and selected by the echelle keyword <TT>BKGMTD</TT>.
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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