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<H2><A NAME="SECTION003033000000000000000">
Pre-processing the spectra</A>
</H2>
<P>
Observation pre-processing consists of standard bias, dark and flat-field
corrections. The package provides different commands to average images,
extract sub-images or normalize flat-fields. General information
related to preprocessing commands is available in the
section Dark, Bias and Flat-Field of <TT>SHOW/LONG</TT> and can be
obtained by:
<BR> <BR>
Midas...> <TT>SHOW/LONG d</TT> <BR>
<P>
The command <TT>PREPARE/LONG</TT> extracts sub-images. The pixel positions
<!-- MATH: $(x_{low},y_{low})$ -->
(<I>x</I><SUB><I>low</I></SUB>,<I>y</I><SUB><I>low</I></SUB>) and
<!-- MATH: $(x_{upp},y_{upp})$ -->
(<I>x</I><SUB><I>upp</I></SUB>,<I>y</I><SUB><I>upp</I></SUB>) of the lower-left and
upper-right corners of the extracted sub-image can be specified with the
keyword
<!-- MATH: $\fbox{TRIM}$ -->
<IMG
WIDTH="74" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1017.gif"
ALT="\fbox{TRIM}">.
The command processes a catalog by default or
an image if the name is provided with the extension <B>.bdf</B>.
The sequence:
<BR> <BR>
Midas...> <TT>CREATE/ICAT flatcat ff*.bdf</TT> <BR>
Midas...> <TT>SET/LONG TRIM=10,20,500,550</TT> <BR>
Midas...> <TT>PREPARE/LONG flatcat exf</TT> <BR>
<BR>
generates a catalog <B>flatcat.cat</B> referencing the files <B>ff*.bdf</B>
and extracts the sub-window <TT>[@10,@20:@500,@550]</TT> (see help file
of <TT>EXTRACT/IMAGE</TT>). The output files will
be called <B>exf<EM>nnnn</EM>.bdf</B> where <EM>nnnn</EM> is a four-digit number
starting at 0001.
<P>
Images can be combined using an average or median method with the
command <TT>COMBINE/LONG</TT>. This command processes a catalog of images
and generates a combined image. The two commands:
<BR> <BR>
Midas...> <TT>CREATE/ICAT flatcat exf*.bdf</TT> <BR>
Midas...> <TT>COMBINE/LONG flatcat avflat AVERAGE</TT> <BR>
<BR>
generate a catalog <B>flatcat.cat</B> referencing the files <B>exf*.bdf</B>
and average them to produce the image <B>avflat.bdf</B>. The option
<TT>MEDIAN</TT> can be used to eliminate particle hits when combining
dark exposures. Flat-field normalisation can be achieved by the command:
<BR> <BR>
Midas...> <TT>SET/LONG BIAS=190. FDEG=2</TT> <BR>
Midas...> <TT>NORMALIZE/FLAT avflat normff</TT> <BR>
<BR>
which performs a bias correction of value
<!-- MATH: $\fbox{BIAS}$ -->
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on the image
<B>avflat.bdf</B> and interpolates the flat-field continuum by a polynome
of degree
<!-- MATH: $\fbox{FDEG}$ -->
<IMG
WIDTH="77" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1019.gif"
ALT="\fbox{FDEG}">
to produce the image <B>normff.bdf</B>. It can be
noted that flat-field normalisation can require a different approach than
the one implemented in the command <TT>NORMALIZE/FLAT</TT>. The Section 1.2.4
(Flat-Fielding) of Chapter 6 in Vol. B presents alternative methods.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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