public documents.sextractor_doc

\section{The catalogue \index{parameter file} parameter file}
In addition to the \index{configuration file} configuration file detailed above, {\sc SExtractor} needs a file containing
the list of parameters that will be listed in the output catalogue for every detection.
This allows the software to compute only catalogue parameters that are needed. The name
of this catalogue-parameter file is traditionally suffixed with {\tt .param}, and must be specified
using the {\tt PARAMETERS\_NAME} config parameter.
The full set of parameters can be queried with the command
 
{\tt \% sex -dp} 
 
\subsection{Format}
The format of the catalogue parameter list is ASCII, and there must be {\em only one keyword
per line}. Presently two kinds of keywords
are recognized by {\sc SExtractor}: scalars and vectors. Scalars, like {\tt X\_IMAGE}, yield
single numbers
in the output catalogue. Vectors, like {\tt MAG\_APER(4)} or {\tt VIGNET(15,15)}, yield arrays
of numbers.
The order in which the parameters will be listed in the catalogue are the
same as that of the keywords in the parameter list. Comments are allowed, they
must begin with a ``\#''.
 
\subsection{Variants}
For many catalogue parameters, especially those related to flux, position, or
shape, several variants of the same measurement are available:
 
\paragraph{Fluxes} may be expressed in linear (ADU) units or as Pogson
\citep{pogson:1856} magnitudes. Flux measurements in ADUs are prefixed with
{\tt FLUX\_}, for example: {\tt FLUX\_AUTO}, {\tt FLUX\_ISO}, etc.. Magnitudes
are prefixed with {\tt MAG\_} (e.g., {\tt MAG\_AUTO}, {\tt MAG\_ISO},...).
In {\sc SExtractor} the magnitude $m$ of a source is derived from its flux $f$:
\begin{equation}
m = \left\{\begin{array}{ll}
m_{ZP} -2.5 \log_{10} f\ &\mbox{if } f > 0\\
99.0 &\mbox{otherwise},
\end{array}\right.
\end{equation}
where $m_{ZP}$ is the magnitude \index{zero-point} zero-point set with the {\tt MAG\_ZEROPOINT}
configuration parameter.
 
\paragraph{Flux uncertainties} follow a scheme similar to that of fluxes.
Flux uncertainties are prefixed with {\tt FLUXERR\_}, as in {\tt FLUXERR\_AUTO}
or {\tt FLUXERR\_ISO}. Magnitude uncertainties start with {\tt MAGERR\_}, for
instance: {\tt MAGERR\_AUTO}, {\tt MAGERR\_ISO},... Magnitude uncertainties
$\sigma_m$ are derived from the estimated 1-$\sigma$ flux error $\sigma_f$:
\begin{equation}
\sigma_m = \left\{\begin{array}{ll}
(2.5/\ln 10) (\sigma_f/f)\ &\mbox{if } f > 0\\
99.0 &\mbox{otherwise}.
\end{array}\right.
\end{equation}
 
\paragraph{Positions} and small distances can be expressed in \index{image} image pixels,
\index{world coordinates} world coordinates, or in \index{celestial coordinates} celestial coordinates. Measurements in units of \index{image} image
pixels are indicated by the suffix {\_IMAGE}, for example: {\tt Y\_IMAGE},
{\tt ERRAWIN\_IMAGE},... Following the FITS convention, in {\sc SExtractor} the
center of the first \index{image} image pixel has coordinates (1.0,1.0).
Positions and small distances may also be expressed in so-called ``world
coordinates'', if World Coordinate System (WCS) metadata
\citep{greisen:calabretta:2002} are present in the \index{image} image \index{FITS header} FITS header.