public documents.sextractor_doc

\section{Thresholding}
Thresholding is applied to the background-subtracted, filtered \index{image} image
to isolate connected groups of pixels. Each group defines the
approximate position and shape of a basic {\sc SExtractor} detection
that will be processed further in the pipeline. Groups are made of
pixels whose values exceed the local \index{threshold} threshold and which touch each
other at their sides or angles (``8-connectivity'').
 
\subsection{Configuration parameters.}
Thresholding is mostly controlled through the {\tt DETECT\_THRESH},
{\tt DETECT\_MINAREA} and {\tt DETECT\_MAXAREA} keywords.
 
{\tt DETECT\_THRESH} sets the \index{threshold} threshold value. If one single value is
given, it is interpreted as a \index{threshold} threshold in units of the (unfiltered) background's
\index{standard deviation} standard deviation. For example:
\begin{verbatim}
DETECT_THRESH 1.5
\end{verbatim}
will set the detection \index{threshold} threshold at 1.5$\sigma$ above the local
background. It is important to note that {\em the \index{standard deviation} standard deviation
quoted here is that of the un{\tt FILTER}ed \index{image} image, at the pixel
scale}. Hence, on \index{image} images with white Gaussian background noise for
instance, a {\tt DETECT\_THRESH} of $3.0$ will be close to optimum if
low-pass {\tt FILTER}ing is turned off, but sub-optimum (too high) if
it is on. On the contrary, if the background noise of the \index{image} image is
intrinsically correlated from pixel-to-pixel, a {\tt DETECT\_THRESH}
of $3.0$ (with no {\tt FILTER}ing) will be too low and will result in a
poor reliability of the extracted catalogue.
\gam{Umm, my past \index{image} image simulations convinced me that $3\,\sigma$ \index{threshold} thresholds were
  optimal on smoothed \index{image} images, even if the noise is correlated in these.}
 
Two numbers can be given as arguments to {\tt DETECT\_THRESH}, in
which case the first one is interpreted as an absolute \index{threshold} threshold in
units of ``magnitudes per square-arcsecond'', and the second as a
\index{zero-point} zero-point in the same units.
\begin{verbatim}
DETECT_THRESH 27.2,30.0
\end{verbatim}
will for example set the \index{threshold} threshold at $10^{-0.4 (27.2-30)} = 13.18$
ADUs above the \index{local background} local background.
 
{\tt DETECT\_MINAREA} sets the minimum number of pixels a group should
have to trigger a detection. Obviously this parameter can be used just
like {\tt DETECT\_THRESH} to detect only bright and ``big'' sources,
or to increase detection reliability. It is however more tricky to
manipulate at low detection \index{threshold} thresholds because of the complex
interplay of object topology, noise correlations (including those
induced by filtering), and sampling. In most cases it is therefore
recommended to keep {\tt DETECT\_MINAREA} at a small value, typically
1 to 5 pixels, and let {\tt DETECT\_THRESH} and the filter define {\sc
SExtractor}'s sensitivity.
 
{\tt DETECT\_MAXAREA}, on the other hand, sets the maximum number of pixels
a group must have in order to trigger a detection. Thus, this parameter may be
used in conjunction with {\tt DETECT\_MINAREA} in order to detect only objects
whose size is within a certain range. Note that, although large objects may
be removed from the catalogue by filtering out those with {\tt ISOAREAF\_IMAGE}
larger than some \index{threshold} threshold, these detections would still appear in the
\index{check-image} check-image. If it is required that large objects be not present in it,
{\tt DETECT\_MAXAREA} should be used in order to effectively exclude them from
the \index{check-image} check-image. See fig. \ref{fig:detect_maxarea_example} for an example. 
 
%---------------------------------- Fig. segmentmeth --------------------------------
\begin{figure}[htbp]
   \centerline{\includegraphics[width=14cm]{ps/detect_maxarea_example.ps}}
    \caption{Example of how the {\tt DETECT\_MAXAREA} parameter can be used in order
             not to detect objects larger than a determined number of pixels.
             {\em Left}: close-up of the original \index{image} image.
             {\em Center}: {\tt OBJECTS} \index{check-image} check-image generated without {\tt DETECT\_MAXAREA}.
             {\em Right}: the same {\tt OBJECTS} \index{check-image} check-image, when generated with {\tt DETECT\_MAXAREA} = 100.
        }
    \label{fig:detect_maxarea_example}
\end{figure}