public documents.sextractor_doc

\chapter{Cross-identification within {\sc SExtractor}}

{\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

can be done both in pixel and world coordinates. Configuration parameters related

to \index{cross-identification} cross-identification are prefixed with {\tt ASSOC}.

\section{The {\tt ASSOC} list}

The {\tt ASSOC} process is initiated by requesting in the parameter

file at least one of the {\tt ASSOC} catalogue parameters: {\tt

VECTOR\_ASSOC} and {\tt NUMBER\_ASSOC}. Then {\sc SExtractor} looks

for an {\tt ASCII} file (let's call it the {\tt ASSOC} list) whose

file name has to be specified by the {\tt ASSOC\_NAME} configuration

keyword. The {\tt ASSOC} list must contain columns of numbers

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

beginning with ``{\tt \#}'' (for comments) are ignored. This \index{mean} means you

may use any {\tt ASCII} catalogue generated by a previous {\sc

SExtractor} run as an {\tt ASSOC} list.

To perform the \index{cross-identification} cross-identification, {\sc SExtractor} needs to know

which are the columns that contain the $x$ and $y$

coordinates\footnote{The $x$ and $y$ coordinates must comply with the

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}~$c_x$,$c_y[$,$c_Z]$'', where $c_x$ and $c_y$ are the

positions of the columns containing the $x$ and $y$ coordinates (the

first column has position 1). $c_Z$ (optional) specifies an extra

column containing some ``$Z$'' parameter that may be used for

controlling or weighting the {\tt ASSOC} process. $Z$ will typically

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).

\section{Controlling the {\tt ASSOC} process}

\label{assoc}

Two configuration parameters control the {\tt ASSOC} process. The

first one, {\tt ASSOC\_RADIUS}, accepts a decimal number which

represents the maximum distance (in pixels) one should have between

the \index{barycenter} barycenter \gam{{\tt X}, {\tt Y} or {\tt XWIN}, {\tt YWIN}?}

of the current {\sc SExtractor} detection and an {\tt

ASSOC}-list member to consider a match. This number must of course

account for positional uncertainties in both catalogues. In most cases,

a value of a few pixels will do just fine. The second configuration

parameter, {\tt ASSOC\_TYPE}, accepts a keyword as argument and

selects the kind of identification procedure one wants to operate:

\begin{itemize}

\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

ASSOC\_PARAMS}. The first geometrical match encountered while scanning

the {\tt ASSOC} list is retained as the actual match. This can used

for catalogues with low spatial density.

\item {\tt NEAREST}: this option does not require the $c_Z$ column in

{\tt ASSOC\_PARAMS}. The match is performed with the {\tt ASSOC}-list

member the closest (in position) to the current detection, provided

that it lies within the {\tt ASSOC\_RADIUS}.

\item {\tt SUM}: all parameters issued from {\tt ASSOC}-list members

which geometrically match the current detection are summed. $c_Z$ is

not required.

\item {\tt MAG\_SUM}: all parameters $c_i$ issued from {\tt

ASSOC}-list members which geometrically match the current detection

are combined using the following law: $-2.5\log(\sum_i 10^{-0.4c_i})$.

This option allows one to sum flux contributions from magnitude data.

$c_Z$ is not required.

\item {\tt MIN}: among all geometrical matches, retains the {\tt

ASSOC}-list member which has the smallest $Z$ parameter.

\item {\tt MAX}: among all geometrical matches, retains the {\tt

ASSOC}-list member which has the largest $Z$ parameter.

\item {\tt MEAN}: all parameters issued from {\tt ASSOC}-list members

which geometrically match the current detection are weighted-averaged,

using the $Z$ parameter as the weight.

\item {\tt MAG\_MEAN}: all parameters issued from {\tt ASSOC}-list

members which geometrically match the current detection are

weighted-averaged, using $10^{-0.4Z}$ as the weight. This option is

useful for weighting catalogue sources with magnitudes.

\end{itemize}

\section{Output from {\tt ASSOC}}

Now that we have described the \index{cross-identification} cross-identification process, let's see

how informations coming from the matching with the {\tt ASSOC} list

are propagated to the output {\sc SExtractor} catalogue.

The output of {\tt ASSOC} data in {\sc SExtractor} catalogue is done

through the {\tt VECTOR\_ASSOC()} catalogue parameter. {\tt

VECTOR\_ASSOC()} is a vector, each element of which refers to a column

from the input {\tt ASSOC} list. {\tt VECTOR\_ASSOC()} contains either

{\tt ASSOC}-list member data from the best match (if {\tt ASSOC\_TYPE}

is {\tt FIRST}, {\tt NEAREST}, {\tt MIN} or {\tt MAX}), or a

combination of {\tt ASSOC}-list member data (if {\tt ASSOC\_TYPE} is

{\tt MEAN}, {\tt MAG\_MEAN}, {\tt SUM} or {\tt MAG\_SUM}). If no match

has been found, it just contains zeros. The {\tt NUMBER\_ASSOC}

contains the number of {\tt ASSOC}-list members that geometrically

match the current {\sc SExtractor} detection, and obviously, if

different from zero, indicates that {\tt VECTOR\_ASSOC()} has a

\index{mean} meaningful content.

The {\tt ASSOC\_DATA} configuration parameter is used to tell {\sc

SExtractor} to which column refers each element of {\tt

VECTOR\_ASSOC()}. The \index{syntax} syntax of {\tt ASSOC\_DATA} is similar to that

of {\tt ASSOC\_PARAMS}: ``{\tt ASSOC\_DATA} $c_1$,$c_2$,$c_3$,...''

where the $c_i$ are the column positions in the {\tt ASSOC} list. The

special case ``{\tt ASSOC\_DATA} 0'' tells {\sc SExtractor} to

propagate all columns from the {\tt ASSOC} file to the output catalogue.

There are situations where it might be desirable to keep in the output

{\sc SExtractor} catalogue only those detections that were matched with

some {\tt ASSOC}-list member. Such a feature is controlled by the {\tt

ASSOCSELEC\_TYPE} configuration parameter, which accepts one of the

three following keywords:

\begin{itemize}

\item {\tt ALL}: keep all {\sc SExtractor} detections, regardless of

matching. This is the default.

\item {\tt MATCHED}: keep only {\sc SExtractor} detections that were

matched with at least one {\tt ASSOC}-list member.

\item {\tt -MATCHED}: keep only {\sc SExtractor} detections that were

not matched with any {\tt ASSOC}-list member.

\end{itemize}