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<H2><A NAME="SECTION003036000000000000000"> </A>
<A NAME="fit-sky"> </A>
<BR>
Estimating the Sky Background
</H2>
<P>
The sky is measured in two regions usually located below and above
the object spectrum and which pixel boundary limits are stored
in the keywords
<!-- MATH: $\fbox{LOWSKY}$ -->
<IMG
WIDTH="112" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1042.gif"
ALT="\fbox{LOWSKY}">
and
<!-- MATH: $\fbox{UPPSKY}$ -->
<IMG
WIDTH="105" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1043.gif"
ALT="\fbox{UPPSKY}">.
The algorithm
removes the cosmic rays from the sky spectrum before interpolation,
using the CCD detector parameters
<!-- MATH: $\fbox{RON}$ -->
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WIDTH="63" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1044.gif"
ALT="\fbox{RON}">
for the read-out-noise,
<!-- MATH: $\fbox{GAIN}$ -->
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WIDTH="72" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1045.gif"
ALT="\fbox{GAIN}">
for the gain,
<!-- MATH: $\fbox{SIGMA}$ -->
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SRC="img1046.gif"
ALT="\fbox{SIGMA}">
as threshold of the
kappa-sigma clipping and 2*
<!-- MATH: $\fbox{RADIUS}$ -->
<IMG
WIDTH="100" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1047.gif"
ALT="\fbox{RADIUS}">+1 as size of the
rejection window. The sky is fitted along the columns of the spectrum by
a polynomial of degree
<!-- MATH: $\fbox{SKYORD}$ -->
<IMG
WIDTH="109" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1048.gif"
ALT="\fbox{SKYORD}">.
Unique or independent spatial profiles
are computed depending of the value of the parameter
<!-- MATH: $\fbox{SKYMOD}$ -->
<IMG
WIDTH="113" HEIGHT="29" ALIGN="BOTTOM" BORDER="0"
SRC="img1049.gif"
ALT="\fbox{SKYMOD}">.
Sky estimation
parameters are displayed together with extraction parameters by:
<BR> <BR>
Midas...> <TT>SHOW/LONG e</TT>.
<BR>
<BR>
The commands:
<BR> <BR>
Midas...> <TT>SET/LONG LOWSKY=50,180 UPPSKY=220,350</TT> <BR>
Midas...> <TT>SKYFIT/LONG ccd0056 sky56</TT> <BR>
<BR>
generate an image <B>sky56.bdf</B> of which size is identical to the input
image <B>ccd0056.bdf</B> and corresponding to the interpolated sky
background. Sky subtraction is performed with:
<BR> <BR>
Midas...> <TT>COMPUTE/IMAGE corr56 = ccd0056 - sky56</TT>
<P>
<BR><HR>
<ADDRESS>
<I>Petra Nass</I>
<BR><I>1999-06-15</I>
</ADDRESS>
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