@@ -29,35 +29,58 @@ For many catalog parameters, especially those related to flux,
position, or shape, several variants of the same measurement are
available:
.. _fluxes_and_magnitudes:
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
:math:`f`:
Fluxes may be expressed in counts (|ADU|\ s) or as Pogson :cite:`1856MNRAS_17_12P` magnitudes.
Flux measurements in |ADU|\ s 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`, ...
The ``MAG_ZEROPOINT`` configuration parameter sets the magnitude zero-point of magnitudes:
where :math:`m_{ZP}` is the magnitude zero-point set with the
``MAG_ZEROPOINT`` configuration parameter.
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`:
Flux uncertainties (error estimates) follow a scheme similar to that of fluxes.
They 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`,...
Setting ``PIXEL_SCALE`` to 0 instructs |SExtractor| to compute the pixel scale from the local Jacobian of the astrometric deprojection, based on the celestial |WCS| info in the |FITS| image header, if available.
Besides |PSF| and :ref:`model-fitting <models_def>` flux estimates, |SExtractor| can currently perform four types of flux measurements: :ref:`isophotal <flux_iso_def>`, :ref:`corrected-isophotal <mag_isocor_def>`, :ref:`fixed-aperture <flux_aper_def>` and :ref:`adaptive-aperture <flux_auto_def>`.
For every :param:`FLUX_` measurement, an error estimate :param:`FLUXERR_`, a magnitude :param:`MAG_` and a magnitude error estimate :param:`MAGERR_` are also available.
The ``MAG_ZEROPOINT`` configuration parameter sets the magnitude zero-point
For every :param:`FLUX_` measurement, an error estimate :param:`FLUXERR_`, a magnitude :param:`MAG_` and a magnitude error estimate :param:`MAGERR_` are also available: see :ref:`fluxes_and_magnitudes`.
An estimate of the error is available for each type of flux.
For aperture fluxes, the flux uncertainty is computed using
...
...
@@ -30,7 +16,7 @@ For aperture fluxes, the flux uncertainty is computed using
where :math:`{\cal A}` is the set of pixels defining the photometric aperture, and :math:`\sigma_i`, :math:`p_i`, :math:`g_i` respectively the standard deviation of noise (in ADU) estimated from the local background, :math:`p_i` the measurement image pixel value subtracted from the background, and :math:`g_i` the effective detector gain in :math:`e^- / \mbox{ADU}` at pixel :math:`i`. Note that this error estimate provides a lower limit of the true uncertainty, as it only takes into account photon and detector noise.
...
...
@@ -204,6 +190,8 @@ The Petrosian flux is the sum of pixel values from the measurement image, subtra
{\tt FLUX\_PETRO} = \sum_{i\in\cal P} p_i.
.. [#petro_radius]
Some authors prefer to define the Petrosian radius as :math:`r_{\rm P}` instead of :math:`N_{\rm P}r_{\rm P}`.
Photographic photometry
-----------------------
...
...
@@ -259,6 +247,4 @@ However in practice the ``BACKPHOTO_TYPE LOCAL`` option has not proven as being
In both ``LOCAL`` and ``GLOBAL`` modes, the :param:`BACKGROUND` catalog parameter gives the value of the background estimated at the centroid of the object.
.. [#petro_radius]
Some authors prefer to define the Petrosian radius as :math:`r_{\rm P}` instead of :math:`N_{\rm P}r_{\rm P}`.
