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<H1><A NAME="SECTION003640000000000000000">
Definition of objects and sky subtraction</A>
</H1>
<P>
<TT>DEFINE/MOS</TT> helps you to localize your objects and sky regions
and by default works automatically.
It averages
<!-- MATH: $\fbox{{\small \tt XBIN}}$ -->
<IMG
WIDTH="56" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1221.gif"
ALT="\fbox{{\small \tt XBIN}}">
(<B>20</B>) columns around the position
<!-- MATH: $\fbox{{\small \tt SCAN\_POS(1)}}$ -->
<IMG
WIDTH="126" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1222.gif"
ALT="\fbox{{\small \tt SCAN\_POS(1)}}">
(<B>0 = center of frame</B>)
(in world-coordinates!). In the target frame the program will detect objects
above the threshold (
<!-- MATH: $\fbox{{\small \tt THRESH}}$ -->
<IMG
WIDTH="77" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1223.gif"
ALT="\fbox{{\small \tt THRESH}}">,
<B>-0.04</B>, see below)
relative to the local background
within the search window
<!-- MATH: $\fbox{{\small \tt WIND}}$ -->
<IMG
WIDTH="56" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1224.gif"
ALT="\fbox{{\small \tt WIND}}">
(<B>5</B>)
and fits a gaussian to the spatial profile of any detected object.
The threshold may be given in absolute (> 0.0) or relative (< 0.0) numbers.
It may be advisable to do at least a rough sky subtraction ahead of this
command to facilitate the detection of the objects.
In this case you have to use an absolute threshold for the detection of
the object spectra afterwards.
One may also think about rebinning the object frame to constant wavelength
steps because then the search could be done in the same wavelength region
for all slitlets.
The limits of the objects are defined at the position where the gaussian fit
has reached the detection limit
<!-- MATH: $\fbox{{\small \tt INT\_LIM}}$ -->
<IMG
WIDTH="84" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1225.gif"
ALT="\fbox{{\small \tt INT\_LIM}}">
(<B>0.001</B>).
A safety margin of 3 pixels
is taken on both sides of each object where no sky is automatically defined
(can be overridden manually later) and the remaining part of the slitlets is
taken as sky region. The results are stored in
<!-- MATH: $\fbox{{\small \tt WINDOWS}}$ -->
<IMG
WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1226.gif"
ALT="\fbox{{\small \tt WINDOWS}}">.tbl
(<B>window</B>) and can be displayed in the overlay channel display and/or the
graphics window. If you are not satisfied with the results you can change
the windows interactively. You may also choose the
interactive mode from the very beginning with <TT>DEFINE/WIND</TT>.
Then no automatic search is performed; instead you enter the the objects and
sky regions for each slitlet by keyboard input. By default the
sky region is defined as the complete slitlet.
<P>
The sky fit methods (
<!-- MATH: $\fbox{{\small \tt SKYMET}}$ -->
<IMG
WIDTH="77" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1227.gif"
ALT="\fbox{{\small \tt SKYMET}}">)
available for <TT>SKYFIT/MOS</TT>
are a simple median along CCD columns within each slitlet
(<TT>skymet=median</TT>) and a more appropriate polynomial fit along the columns
(<TT>skymet=polynomial</TT>), respectively.
These two methods use only rows marked as sky regions
in the table
<!-- MATH: $\fbox{{\small \tt WINDOWS}}$ -->
<IMG
WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1228.gif"
ALT="\fbox{{\small \tt WINDOWS}}">.tbl (<B>window</B>)
to fit the sky background. With <TT>skymet=nowindows</TT>,
however, the table
<!-- MATH: $\fbox{{\small \tt WINDOWS}}$ -->
<IMG
WIDTH="87" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1229.gif"
ALT="\fbox{{\small \tt WINDOWS}}">.tbl is ignored and the sky is
determined as a simple median over the full slitlet. The limits of the
slitlets are taken in this case from
<!-- MATH: $\fbox{{\small \tt MOS}}$ -->
<IMG
WIDTH="45" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1230.gif"
ALT="\fbox{{\small \tt MOS}}">.tbl. This mode
may be useful for a preliminary sky determination, before the object positions
are known. If no sky regions are marked in some slitlet,
the input frame is just copied to the sky frame for this slitlet. In this way,
after sky subtraction, the slitlet contains only zeros thereby marking that
the sky background is unknown for this slitlet.
The keyword
<!-- MATH: $\fbox{{\small \tt SKYMET}}$ -->
<IMG
WIDTH="77" HEIGHT="26" ALIGN="BOTTOM" BORDER="0"
SRC="img1231.gif"
ALT="\fbox{{\small \tt SKYMET}}">
contains the order of the polynom fit or the
width of the median filtering, respectively. If a polynom fit is performed
the cosmics must be rejected. <TT>SKYFIT/MOS</TT> rejects (but not replaces)
pixels that exceed a given limit before the fit is performed.
Read out noise, gain and the detection limit (in units of <IMG
WIDTH="19" HEIGHT="21" ALIGN="BOTTOM" BORDER="0"
SRC="img1232.gif"
ALT="$\sigma$">)
must be
given by keywords
<!-- MATH: $\fbox{{\small \tt SKYMET(1),SKYMET(2),REJTHRES}}$ -->
<IMG
WIDTH="307" HEIGHT="30" ALIGN="BOTTOM" BORDER="0"
SRC="img1233.gif"
ALT="\fbox{{\small \tt SKYMET(1),SKYMET(2),REJTHRES}}">.
<P>
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<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
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