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\chapter{Cross-identification within {\sc SExtractor}}
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\chapter{Cross-identification within {\sc SExtractor}}
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{\sc SExtractor} allows one to perform on-the-fly \index{cross-identification} cross-identification of
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{\sc SExtractor} allows one to perform on-the-fly \index{cross-identification} cross-identification of
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detections with an {\tt ASCII} list provided by the user. \index{cross-identification} cross-identification
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detections with an {\tt ASCII} list provided by the user. \index{cross-identification} Cross-identification
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can be done both in pixel and world coordinates. Configuration parameters related
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can be done both in pixel or world coordinates. Configuration parameters related
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to \index{cross-identification} cross-identification are prefixed with {\tt ASSOC}.
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to \index{cross-identification} cross-identification are prefixed with
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{\tt ASSOC\_}.
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\section{The {\tt ASSOC} list}
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\section{The {\tt ASSOC} list}
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The {\tt ASSOC} process is initiated by requesting in the parameter
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The {\tt ASSOC} process is initiated by requesting in the parameter
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file at least one of the {\tt ASSOC} catalogue parameters: {\tt
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file at least one of the {\tt ASSOC} catalogue parameters: {\tt
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VECTOR\_ASSOC} and {\tt NUMBER\_ASSOC}. Then {\sc SExtractor} looks
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VECTOR\_ASSOC} or {\tt NUMBER\_ASSOC}. Then {\sc SExtractor} looks
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for an {\tt ASCII} file (let's call it the {\tt ASSOC} list) whose
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for an {\tt ASCII} file (the so-called{\tt ASSOC} list), which must be
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file name has to be specified by the {\tt ASSOC\_NAME} configuration
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specified using the {\tt ASSOC\_NAME} configuration
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keyword. The {\tt ASSOC} list must contain columns of numbers
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keyword. The {\tt ASSOC} list should contain columns of numbers
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separated by spaces or tabs. Each line describes a source that will
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separated by spaces or tabs. Each line describes a source that will
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enter the \index{cross-identification} cross-identification process. Lines with zero characters, or
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enter the \index{cross-identification} cross-identification process. Lines with
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beginning with ``{\tt \#}'' (for comments) are ignored. This \index{mean} means you
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zero characters, or beginning with ``{\tt \#}'' (for comments) are ignored.
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may use any {\tt ASCII} catalogue generated by a previous {\sc
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You may therefore use any {\tt ASCII} catalogue generated by a previous
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SExtractor} run as an {\tt ASSOC} list.
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{\sc SExtractor} run as an {\tt ASSOC} list.
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To perform the \index{cross-identification} cross-identification, {\sc SExtractor} needs to know
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In order to perform the \index{cross-identification} cross-identification,
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which are the columns that contain the $x$ and $y$
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{\sc SExtractor} needs to know which are the columns that contain the
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coordinates\footnote{The $x$ and $y$ coordinates must comply with the
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coordinates in the {\tt ASSOC} list. These can be designated using the {\tt
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FITS (and {\sc SExtractor}) convention: by definition, the center of
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the first pixel in the \index{image} image array has pixel-coordinates (1.0,1.0).}
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in the {\tt ASSOC} list. These shall be specified using the {\tt
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ASSOC\_PARAMS} configuration parameter. The \index{syntax} syntax is: ``{\tt
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ASSOC\_PARAMS} configuration parameter. The \index{syntax} syntax is: ``{\tt
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ASSOC\_PARAMS}~$c_x$,$c_y[$,$c_Z]$'', where $c_x$ and $c_y$ are the
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ASSOC\_PARAMS}~$c_x$,$c_y[$,$c_Z]$'', where $c_x$ and $c_y$ are the
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positions of the columns containing the $x$ and $y$ coordinates (the
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positions of the columns containing the $x$ and $y$ coordinates, or world
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first column has position 1). $c_Z$ (optional) specifies an extra
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coordinates such as $\alpha$ and $\delta$. By definition, the position of the
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first column is 1. $c_Z$ (optional) specifies an extra
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column containing some ``$Z$'' parameter that may be used for
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column containing some ``$Z$'' parameter that may be used for
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controlling or weighting the {\tt ASSOC} process. $Z$ will typically
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controlling or weighting the {\tt ASSOC} process. $Z$ will typically
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be a flux estimate. $c_Z$ is required if {\tt ASSOC\_TYPE} is {\tt
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be a flux estimate. $c_Z$ is required if {\tt ASSOC\_TYPE} is {\tt
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MIN}, {\tt MAX}, {\tt MEAN} or {\tt MAG\_MEAN} (see \S\ref{assoc} below).
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MIN}, {\tt MAX}, {\tt MEAN} or {\tt MAG\_MEAN} (see \S\ref{assoc} below).
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\section{Controlling the {\tt ASSOC} process}
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\section{Controlling the {\tt ASSOC} process}
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\label{assoc}
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\label{assoc}
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Two configuration parameters control the {\tt ASSOC} process. The
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Three configuration parameters control the {\tt ASSOC} process. The first
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first one, {\tt ASSOC\_RADIUS}, accepts a decimal number which
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one, {\tt ASSOCCOORD\_TYPE}, should be set to {\tt PIXEL} or {\tt WORLD},
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depending on what type of coordinates is in the {\tt ASSOC} list. If
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{\tt ASSOCCOORD\_TYPE} is set to {\tt PIXEL}, {\sc SExtractor} expects the $x$
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and $y$ coordinates to comply with the FITS convention: by definition, the
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center of the first pixel in the \index{image} image array has pixel-coordinates
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(1.0,1.0). If {\tt ASSOCCOORD\_TYPE} is set to {\tt WORLD}, {\sc SExtractor}
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uses the WCS information found in the FITS image header to convert to
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pixel coordinates the coordinates read in the {\tt ASSOC} list prior to
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making the cross-identification.
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{\tt ASSOC\_RADIUS}, accepts a decimal number which
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represents the maximum distance (in pixels) one should have between
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represents the maximum distance (in pixels) one should have between
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the \index{barycenter} barycenter \gam{{\tt X}, {\tt Y} or {\tt XWIN}, {\tt YWIN}?}
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the \index{barycenter} barycenter \gam{{\tt X}, {\tt Y} or {\tt XWIN}, {\tt YWIN}?}
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of the current {\sc SExtractor} detection and an {\tt
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of the current {\sc SExtractor} detection and an {\tt
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ASSOC}-list member to consider a match. This number must of course
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ASSOC}-list member to consider a match. This number must of course
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account for positional uncertainties in both catalogues. In most cases,
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account for positional uncertainties in both catalogues. In most cases,
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a value of a few pixels will do just fine. The second configuration
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a value of a few pixels will do just fine.
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The last configuration
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parameter, {\tt ASSOC\_TYPE}, accepts a keyword as argument and
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parameter, {\tt ASSOC\_TYPE}, accepts a keyword as argument and
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selects the kind of identification procedure one wants to operate:
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selects the kind of identification procedure one wants to operate:
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\begin{itemize}
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\begin{itemize}
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\item {\tt FIRST}: this is the simplest way of performing a
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\item {\tt FIRST}: this is the simplest way of performing a
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\index{cross-identification} cross-identification. It does not require the $c_Z$ column in {\tt
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\index{cross-identification} cross-identification. It does not require the $c_Z$ column in {\tt
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