Measurements are made on the isophotal object footprints, which are defined on the filtered detection image. Only pixels with values above the threshold set with ``ANALYSIS_THRESH`` are considered [#thresh]_, which makes the analysis extremely fast, but obviously strongly dependent on the threshold itself. This is an issue particularly when the amplitude of the bakground noise varies over the image. Many of the isophotal measurements (e.g., ``X_IMAGE``, ``Y_IMAGE``, ``FLUX_ISO``) are necessary for the internal operations of |SExtractor| and are therefore executed even if they are not requested.
Measurements are made on the isophotal object footprints, which are defined on the filtered detection image. Only pixels with values above the threshold set with ``ANALYSIS_THRESH`` are considered [#thresh]_, which makes the analysis extremely fast, but obviously strongly dependent on the threshold itself. This is an issue particularly when the amplitude of the bakground noise varies over the image. Many of the isophotal measurements (e.g., ``X_IMAGE``, ``Y_IMAGE``, ``FLUX_ISO``) are necessary for the internal operations of |SExtractor| and are therefore executed even if they are not requested.
Full
Full
Measurements have access to all pixels of the image. These measurements are generally more sophisticated, less affected by the detection threshold, and still reasonably fast. They are done at a later stage of the processing, after CLEANing and MASKing.
Measurements have access to all pixels of the image. These measurements are generally more sophisticated, less affected by variable biases induced by the detection threshold, and still reasonably fast. They are done at a later stage of the processing, after CLEANing and MASKing.
Model-fitting
Model-fitting
Measurements require PSF models [#psf_models]_. They are often the most accurate and can recover the flux of saturated objects. They are also much slower, allowing typically only a few tens of objects to be processed every second.
Measurements require PSF models [#psf_models]_. They are often the most accurate and can recover the flux of saturated objects. They are also much slower, allowing typically only a few tens of objects to be processed every second.
Parameters measured within an object’s isophotal limit are sensitive to
Measurements performed through a *window* function (an *envelope*) do not have many of the drawbacks of isophotal measurements. |SExtractor| implements “windowed” versions for most
two main factors: 1) changes in the detection threshold, which create a
of the measurements described in the :ref:`previous section<position_iso>`:
variable bias and 2) irregularities in the object’s isophotal
boundaries, which act as additional “noise” in the measurements.
.. note::
Unless otherwise noted, all parameter names given below are only prefixes. They must be followed by _IMAGE if the results shall be expressed in pixel coordinates or _WORLD, _SKY, _J2000 or _B1950 for |WCS|_ coordinates (see :ref:`coord_suffix`).
Measurements performed through a *window* function (an *envelope*) do
not have such drawbacks. |SExtractor| implements “windowed” versions for most
