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<H2><A NAME="SECTION003632000000000000000">
Offsets between slitlets</A>
</H2>
<P>
If you have used <TT>DEFINE/SLIT</TT> there are no
offsets copied to
<!-- MATH: $\fbox{{\small \tt MOS}}$ -->
<IMG
WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1193.gif"
ALT="\fbox{{\small \tt MOS}}">.tbl automatically and you have
to determine the offsets yourself using the
command <TT>OFFSET/MOS</TT> on a wavelength calibration frame.
This command will take the calibration spectra of the different slitlets
and search for arc lines with the parameters given by
<!-- MATH: $\fbox{{\small \tt SEAPAR}}$ -->
<IMG
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ALT="\fbox{{\small \tt SEAPAR}}">
(<B>200,5</B>).
Correlating these line lists with that of
the first slitlet gives the offsets relative to the first slitlet.
The resulting offsets may be wrong if there are not
enough or too many lines to get unambiguous correlation results.
The safest way is to use this command after you are satisfied with the
results of <TT>SEARCH/MOS</TT> <B>and</B> you have stored the parameters for
threshold and width to
<!-- MATH: $\fbox{{\small \tt SEAPAR}}$ -->
<IMG
WIDTH="77" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1195.gif"
ALT="\fbox{{\small \tt SEAPAR}}">.
In this case you will <B>NOT</B> have <B>the offsets from the
center of the CCD but relative to the position of the first slitlet!</B>
The changing image scale over the detector can make the method unappropiate.
Therefore it is strongly recommended to copy the offsets from descriptor data
to the
<!-- MATH: $\fbox{{\small \tt MOS}}$ -->
<IMG
WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1196.gif"
ALT="\fbox{{\small \tt MOS}}">.tbl table, but not from <TT>OFFSET/MOS</TT> - if possible.
<P>
<BLOCKQUOTE>
<DIV ALIGN="CENTER">
<B>Note</DIV>
<I>Before starting the calibration, a carefully selected line catalog must
be prepared. Reject any line that is below the detection limit and any line
which is blended!</I></B></BLOCKQUOTE><HR>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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