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<H2><A NAME="SECTION003631000000000000000">
Detection of Arc lines</A>
</H2>
At first you have to find the calibration lines in the wavelength calibration
frame belonging to your object. This is achieved by <TT>SEARCH/MOS</TT>
which detects all lines separately for each slitlet and centers them either
with center of gravity or with a gaussian fit. The search threshold and
window are stored in
<!-- MATH: $\fbox{{\small \tt SEAPAR}}$ -->
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(<B>200,5</B>). The positions of the lines,
the CCD rows and the slitlets where the lines have been detected
are stored in the table
<!-- MATH: $\fbox{{\small \tt LINPOS}}$ -->
<IMG
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ALT="\fbox{{\small \tt LINPOS}}">.tbl (<B>linpos</B>).
First the median of (
<!-- MATH: $\fbox{{\small \tt YBIN(2)}}$ -->
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(<B>3</B>)) rows in the
middle of each slitlet is calculated to search in this average spectrum
for lines. Then in all rows the detected lines are centered using the line
list obtained by the search algorithm. In this
way we want to avoid unstable dispersion solutions which might arise
from different line lists for different rows of the slitlet.
In order to speed up the search you may decide not to take every row, but
only some (
<!-- MATH: $\fbox{{\small \tt YBIN(1)}}$ -->
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(<B>3</B>)) and for the detection of weak lines
you may bin several row together (
<!-- MATH: $\fbox{{\small \tt YBIN(2)}}$ -->
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(<B>3</B>)).
In the case of the Gaussian centering method, blends could prevent the
proper convergence of the non-linear fitting algorithm. The accuracy of
the centering is significantly improved, if the peak flux of the Gaussian is
hold fixed to be the meassured maximum value in the central pixel.
Free parameters are <IMG
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and the center of the line.
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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