.. File Param.rst

The catalog parameter file
============================

In addition to the configuration file detailed above, |SExtractor| requires a
file containing the list of parameters that will be listed in the output
catalog for every detection. This allows the software to compute only
catalog parameters that are needed. The name of this
catalog-parameter file is traditionally suffixed with ``.param``, and must
be specified using the ``PARAMETERS_NAME`` config parameter. The full set
of parameters can be queried with the command

.. code-block:: console

 $ sex -dp

Format
------

The format of the catalog parameter list is ASCII, and there must be
*a single keyword per line*. Presently two kinds of keywords are
recognized by |SExtractor|: scalars and vectors. Scalars, like ``X_IMAGE``,
produce single numbers in the output catalog. Vectors, like ``MAG_APER(4)``
or ``VIGNET(15,15)``, produce arrays of numbers. The ordering of measurements
in the output catalog is identical to that of the keywords in the parameter
list. Comments are allowed, they must begin with a ``#``.

Variants
--------

For many catalog parameters, especially those related to flux,
position, or shape, several variants of the same measurement are
available:

Fluxes
~~~~~~

may be expressed in linear (ADU) units or as Pogson
:cite:`1856MNRAS..17...12P` magnitudes. Flux measurements in ADUs
are prefixed with ``FLUX_``, for example: ``FLUX_AUTO``, ``FLUX_ISO``, etc.
Magnitudes are prefixed with ``MAG_`` e.g., ``MAG_AUTO``, ``MAG_ISO``, ... In
|SExtractor| the magnitude :math:`m` of a source is derived from the flux
:math:`f`:

.. math::

   m = \left\{\begin{array}{ll}
   m_{ZP} -2.5 \log_{10} f\ &\mbox{if } f > 0\\
   99.0 &\mbox{otherwise},
   \end{array}\right.

where :math:`m_{ZP}` is the magnitude zero-point set with the
``MAG_ZEROPOINT`` configuration parameter.

Flux uncertainties
~~~~~~~~~~~~~~~~~~

They follow a scheme similar to that of fluxes. Flux uncertainties are
prefixed with ``FLUXERR_``, as in ``FLUXERR_AUTO`` or ``FLUXERR_ISO``. Magnitude
uncertainties start with ``MAGERR_``, for instance: ``MAGERR_AUTO``,
``MAGERR_ISO``,... Magnitude uncertainties :math:`\sigma_m` are derived
from the estimated 1-\ :math:`\sigma` flux error :math:`\sigma_f`:

.. math::

   \sigma_m = \left\{\begin{array}{ll}
   (2.5/\ln 10) (\sigma_f/f)\ &\mbox{if } f > 0\\
   99.0 &\mbox{otherwise}.
   \end{array}\right.

Positions
~~~~~~~~~

Positions and distances can be expressed in image pixels, world coordinates,
or in celestial coordinates. Measurements in units of image pixels are
indicated by the suffix ``_IMAGE``, for example: ``Y_IMAGE``,
``ERRAWIN_IMAGE``, ... Following the FITS convention, in SExtractor the
center of the first 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 :cite:`2002A&A...395.1061G` are
present in the FITS image header.

.. include:: keys.rst