@@ -29,8 +29,8 @@ For many catalog parameters, especially those related to flux,
position, or shape, several variants of the same measurement are
available:
Fluxes
~~~~~~
Fluxes and magnitudes
~~~~~~~~~~~~~~~~~~~~~
Fluxes may be expressed in linear (ADU) units or as Pogson :cite:`1856MNRAS_17_12P` magnitudes. Flux measurements in ADUs are prefixed with :param:`FLUX_`, for example: :param:`FLUX_AUTO`, :param:`FLUX_ISO`, etc. Magnitudes are prefixed with :param:`MAG_` e.g., :param:`MAG_AUTO`, :param:`MAG_ISO`, ... In
|SExtractor| the magnitude :math:`m` of a source is derived from the flux
...
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@@ -46,8 +46,8 @@ Fluxes may be expressed in linear (ADU) units or as Pogson :cite:`1856MNRAS_17_1
where :math:`m_{ZP}` is the magnitude zero-point set with the
``MAG_ZEROPOINT`` configuration parameter.
Flux uncertainties
~~~~~~~~~~~~~~~~~~
Flux and magnitude uncertainties
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Flux uncertainties follow a scheme similar to that of fluxes. Flux uncertainties are prefixed with :param:`FLUXERR_`, as in :param:`FLUXERR_AUTO` or :param:`FLUXERR_ISO`. Magnitude uncertainties start with :param:`MAGERR_`, for instance: :param:`MAGERR_AUTO`, :param:`MAGERR_ISO`,... Magnitude uncertainties :math:`\sigma_m` are derived from the estimated 1-\ :math:`\sigma` flux error :math:`\sigma_f`:
...
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@@ -65,15 +65,29 @@ Positions and shapes
Positions, distances and position angles are computed in pixel coordinates. They may be expressed in image pixels, world coordinates, or in celestial coordinates, depending on the suffix:
.. _image_coords:
:param:`_IMAGE`
Measurements are given in pixel coordinates, in units of pixels. For example: :param:`Y_IMAGE`, :param:`ERRAWIN_IMAGE`, :param:`THETA_IMAGE` etc. Following the FITS convention, in |SExtractor| the center of the first image pixel has coordinates (1.0,1.0). Position angles are counted from the *x* axis (axis 1), positive towards the *y* axis (axis 2)
.. _world_coords:
:param:`_WORLD`
Measurements are given in so-called “world coordinates”, converted from pixel coordinates using the local Jacobian of the transformation between both systems. This requires World Coordinate System (|WCS|_) metadata :cite:`2002AA_395_1061G` to be present in the FITS image header(s). Position angles are counted from the first world axis, positive towards the second world axis.
.. _sky_coords:
:param:`_SKY`, :param:`_J2000`, :param:`_B1950`
Measurements are given in celestial (equatorial) coordinates, converted from pixel coordinates using the local Jacobian of the transformation between both systems. Positions and distances are in units of degrees. This requires celestial |WCS| metadata :cite:`2002AA_395_1077C` to be present in the FITS image header(s). :param:`_SKY` measurements are given in the native world coordinate system. :param:`_J2000` and :param:`_B1950` measurements are automatically converted from the native |WCS|, taking into account the change of reference frame. In all cases, positions angles are counted East-of-North.
.. _focal_coords:
:param:`_FOCAL`
Measurements are given in “focal plane coordinates”, which are actually projected coordinates, in degrees. This requires World Coordinate System (|WCS|_) metadata :cite:`2002AA_395_1061G` to be present in the FITS image header(s). The computation of focal plane coordinates from pixel coordinates is similar to that of :param:`_SKY` coordinates except that they are not de-projected and remain Cartesian. The main purpose of focal plane coordinates is to provide a common system for all the chips in a mosaic camera.
.. note::
Conversion to :param:`_FOCAL` coordinates is available only for a limited subset of measurements.
