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