# public documents.sextractor_doc

## [/] [crossid.tex] - Diff between revs 29 and 32

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\chapter{Cross-identification within {\sc SExtractor}}
\chapter{Cross-identification within {\sc SExtractor}}
{\sc SExtractor} allows one to perform on-the-fly \index{cross-identification} cross-identification of
{\sc SExtractor} allows one to perform on-the-fly \index{cross-identification} cross-identification of
detections with an {\tt ASCII} list provided by the user. \index{cross-identification} cross-identification
detections with an {\tt ASCII} list provided by the user. \index{cross-identification} Cross-identification
can be done both in pixel and world coordinates. Configuration parameters related
can be done both in pixel or world coordinates. Configuration parameters related
to \index{cross-identification} cross-identification are prefixed with {\tt ASSOC}.
to \index{cross-identification} cross-identification are prefixed with

{\tt ASSOC\_}.
 
 
\section{The {\tt ASSOC} list}
\section{The {\tt ASSOC} list}
The {\tt ASSOC} process is initiated by requesting in the parameter
The {\tt ASSOC} process is initiated by requesting in the parameter
file at least one of the {\tt ASSOC} catalogue parameters: {\tt
file at least one of the {\tt ASSOC} catalogue parameters: {\tt
VECTOR\_ASSOC} and {\tt NUMBER\_ASSOC}. Then {\sc SExtractor} looks
VECTOR\_ASSOC} or {\tt NUMBER\_ASSOC}. Then {\sc SExtractor} looks
for an {\tt ASCII} file (let's call it the {\tt ASSOC} list) whose
for an {\tt ASCII} file (the so-called{\tt ASSOC} list), which must be
file name has to be specified by the {\tt ASSOC\_NAME} configuration
specified using the {\tt ASSOC\_NAME} configuration
keyword. The {\tt ASSOC} list must contain columns of numbers
keyword. The {\tt ASSOC} list should contain columns of numbers
separated by spaces or tabs. Each line describes a source that will
separated by spaces or tabs. Each line describes a source that will
enter the \index{cross-identification} cross-identification process. Lines with zero characters, or
enter the \index{cross-identification} cross-identification process. Lines with
beginning with {\tt \#}'' (for comments) are ignored. This \index{mean} means you
zero characters, or beginning with {\tt \#}'' (for comments) are ignored.
may use any {\tt ASCII} catalogue generated by a previous {\sc
You may therefore use any {\tt ASCII} catalogue generated by a previous
SExtractor} run as an {\tt ASSOC} list.
{\sc SExtractor} run as an {\tt ASSOC} list.
 
 
To perform the \index{cross-identification} cross-identification, {\sc SExtractor} needs to know
In order to perform the \index{cross-identification} cross-identification,
which are the columns that contain the $x$ and $y$
{\sc SExtractor} needs to know which are the columns that contain the
coordinates\footnote{The $x$ and $y$ coordinates must comply with the
coordinates in the {\tt ASSOC} list. These can be designated using the {\tt
FITS (and {\sc SExtractor}) convention: by definition, the center of

the first pixel in the \index{image} image array has pixel-coordinates (1.0,1.0).}

in the {\tt ASSOC} list. These shall be specified using the {\tt

ASSOC\_PARAMS} configuration parameter. The \index{syntax} syntax is: {\tt
ASSOC\_PARAMS} configuration parameter. The \index{syntax} syntax is: {\tt
ASSOC\_PARAMS}~$c_x$,$c_y[$,$c_Z]$'', where $c_x$ and $c_y$ are the
ASSOC\_PARAMS}~$c_x$,$c_y[$,$c_Z]$'', where $c_x$ and $c_y$ are the
positions of the columns containing the $x$ and $y$ coordinates (the
positions of the columns containing the $x$ and $y$ coordinates, or world
first column has position 1). $c_Z$ (optional) specifies an extra
coordinates such as $\alpha$ and $\delta$. By definition, the position of the

first column is 1. $c_Z$ (optional) specifies an extra
column containing some $Z$'' parameter that may be used for
column containing some $Z$'' parameter that may be used for
controlling or weighting the {\tt ASSOC} process. $Z$ will typically
controlling or weighting the {\tt ASSOC} process. $Z$ will typically
be a flux estimate. $c_Z$ is required if {\tt ASSOC\_TYPE} is {\tt
be a flux estimate. $c_Z$ is required if {\tt ASSOC\_TYPE} is {\tt
MIN}, {\tt MAX}, {\tt MEAN} or {\tt MAG\_MEAN} (see \S\ref{assoc} below).
MIN}, {\tt MAX}, {\tt MEAN} or {\tt MAG\_MEAN} (see \S\ref{assoc} below).
 
 
\section{Controlling the {\tt ASSOC} process}
\section{Controlling the {\tt ASSOC} process}
\label{assoc}
\label{assoc}
Two configuration parameters control the {\tt ASSOC} process. The
Three configuration parameters control the {\tt ASSOC} process. The first
first one, {\tt ASSOC\_RADIUS}, accepts a decimal number which
one, {\tt ASSOCCOORD\_TYPE}, should be set to {\tt PIXEL} or {\tt WORLD},

depending on what type of coordinates is in the {\tt ASSOC} list. If

{\tt ASSOCCOORD\_TYPE} is set to {\tt PIXEL}, {\sc SExtractor} expects the $x$

and $y$ coordinates to comply with the FITS convention: by definition, the

center of the first pixel in the \index{image} image array has pixel-coordinates

(1.0,1.0). If {\tt ASSOCCOORD\_TYPE} is set to {\tt WORLD}, {\sc SExtractor}

uses the WCS information found in the FITS image header to convert to

pixel coordinates the coordinates read in the {\tt ASSOC} list prior to

making the cross-identification.

 

{\tt ASSOC\_RADIUS}, accepts a decimal number which
represents the maximum distance (in pixels) one should have between
represents the maximum distance (in pixels) one should have between
the \index{barycenter} barycenter \gam{{\tt X}, {\tt Y} or {\tt XWIN}, {\tt YWIN}?}
the \index{barycenter} barycenter \gam{{\tt X}, {\tt Y} or {\tt XWIN}, {\tt YWIN}?}
of the current {\sc SExtractor} detection and an {\tt
of the current {\sc SExtractor} detection and an {\tt
ASSOC}-list member to consider a match. This number must of course
ASSOC}-list member to consider a match. This number must of course
account for positional uncertainties in both catalogues. In most cases,
account for positional uncertainties in both catalogues. In most cases,
a value of a few pixels will do just fine. The second configuration
a value of a few pixels will do just fine.

 

The last configuration
parameter, {\tt ASSOC\_TYPE}, accepts a keyword as argument and
parameter, {\tt ASSOC\_TYPE}, accepts a keyword as argument and
selects the kind of identification procedure one wants to operate:
selects the kind of identification procedure one wants to operate:
\begin{itemize}
\begin{itemize}
\item {\tt FIRST}: this is the simplest way of performing a
\item {\tt FIRST}: this is the simplest way of performing a
\index{cross-identification} cross-identification. It does not require the $c_Z$ column in {\tt
\index{cross-identification} cross-identification. It does not require the $c_Z$ column in {\tt