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<H1><A NAME="SECTION003620000000000000000">
Location of slitlets and flat-field correction</A>
</H1>
<P>
The very first step after correcting bias, dark, and overscan is to find the
edges of the slitlets. This is done by the command <TT>LOCATE/MOS</TT>.
This command locates the slitlets in an MOS flat-field frame
by searching for the maximum (normalized) gradient in a trace
perpendicular to the direction of dispersion. Position and width of
the trace are given by
<!-- MATH: $\fbox{{\small \tt SCAN\_POS}}$ -->
<IMG
WIDTH="95" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1172.gif"
ALT="\fbox{{\small \tt SCAN\_POS}}">
(<B>0</B>).
<!-- MATH: $\fbox{{\small \tt FLATLIM(1)}}$ -->
<IMG
WIDTH="119" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1173.gif"
ALT="\fbox{{\small \tt FLATLIM(1)}}">
(<B>0</B>) gives the minimum normalized gradient that must
be exceeded, after median filtering the scan with a median of
width
<!-- MATH: $\fbox{{\small \tt FLATLIM(2)}}$ -->
<IMG
WIDTH="119" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1174.gif"
ALT="\fbox{{\small \tt FLATLIM(2)}}">
(<B>0</B>) and discarding scan values below
<!-- MATH: $\fbox{{\small \tt FLATLIM(3)}}$ -->
<IMG
WIDTH="119" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1175.gif"
ALT="\fbox{{\small \tt FLATLIM(3)}}">
(<B>0</B>).
The result is written
to the output table
<!-- MATH: $\fbox{{\small \tt MOS}}$ -->
<IMG
WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1176.gif"
ALT="\fbox{{\small \tt MOS}}">.tbl (which is used by most MOS commands)
and the number of detected slitlets is written to
<!-- MATH: $\fbox{{\small \tt NSLIT}}$ -->
<IMG
WIDTH="66" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1177.gif"
ALT="\fbox{{\small \tt NSLIT}}">
(<B>0</B>).
The programs allow at most 100 slitlets.
If the algorithm does not find any slitlets the chosen threshold
(
<!-- MATH: $\fbox{{\small \tt FLATLIM(1)}}$ -->
<IMG
WIDTH="119" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1178.gif"
ALT="\fbox{{\small \tt FLATLIM(1)}}">
(<B>0</B>))
may either be too high (above the intensity of the flats in the
center of the frame) or too low (below bias value). Also the width
(
<!-- MATH: $\fbox{{\small \tt FLATLIM(2)}}$ -->
<IMG
WIDTH="119" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1179.gif"
ALT="\fbox{{\small \tt FLATLIM(2)}}">
(<B>0</B>)) may be chosen to high or too small. Typical values
are between 0.1 and 0.2 and 3 and 5, respectively.
<P>
It is also possible to define the slitlets interactively with
<TT>DEFINE/SLIT</TT>. Here you first initialize the table
<!-- MATH: $\fbox{{\small \tt MOS}}$ -->
<IMG
WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1180.gif"
ALT="\fbox{{\small \tt MOS}}">.tbl (<B>mos</B>)
and then enter the limits with the cursor on the displayed flat field
frame. This comand also allows an easy definition of the
<!-- MATH: $\fbox{{\small \tt MOS}}$ -->
<IMG
WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1181.gif"
ALT="\fbox{{\small \tt MOS}}">
table
for long-slit data.
<P>
With <TT>LOCATE/MOS</TT> the offsets in dispersion direction between
the slitlets will be read from
the header of the flat-field frame for FORS data and stored in the
table
<!-- MATH: $\fbox{{\small \tt MOS}}$ -->
<IMG
WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1182.gif"
ALT="\fbox{{\small \tt MOS}}">.tbl (<B>mos</B>) in column <TT>:xoffset</TT>.
For other data or <TT>DEFINE/SLIT</TT>
you will have to determine the offsets yourself using the
command <TT>OFFSET/MOS</TT> on a wavelength calibration frame (see below).
<P>
As spectroscopic flat-fields normally exhibit the spectral characteristic
of the lamp that was used to produce them you have to take out this
characteristic in order to correct the CCD sensitivity variation and keep
the original flux distribution. This is done with the command <TT>NORM/MOS</TT>.
It takes an averaged flat frame and the slit limits stored in the
table
<!-- MATH: $\fbox{{\small \tt MOS}}$ -->
<IMG
WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1183.gif"
ALT="\fbox{{\small \tt MOS}}">.tbl.
There are two methods provided for the normalization (
<!-- MATH: $\fbox{{\small \tt NORMMET}}$ -->
<IMG
WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1184.gif"
ALT="\fbox{{\small \tt NORMMET}}">
(<B>poly</B>)): In case <TT>NORMMET=poly</TT>
it averages separately for each slitlet the rows, fits a polynomial
of chosen degree (
<!-- MATH: $\fbox{{\small \tt FFORD}}$ -->
<IMG
WIDTH="66" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1185.gif"
ALT="\fbox{{\small \tt FFORD}}">
(<B>3</B>)) to the flux distribution obtained
this way
and divides each row in the slitlet by this polynomial.
In case <TT>NORMMET=median</TT>
it averages separately for each slitlet the rows, smooths with a median
filter of
<!-- MATH: $\fbox{{\small \tt FFORD}}$ -->
<IMG
WIDTH="67" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1186.gif"
ALT="\fbox{{\small \tt FFORD}}">
pixels width
and divides each row in the slitlet by the filtered average.
You may also perform the flat correction at the same step using the
command <TT>FLAT/MOS</TT>.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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