Added further details on Position suffixes and the three types of measurements.

doc/src/Measurements.rst

 Measurements
 ============
 
-Once sources have been detected and deblended, they enter the
-measurement phase. |SExtractor| performs two categories of measurements.
-Measurements from the first category are made on the isophotal object
-profiles. Only pixels above the detection threshold are considered. Many
-of these isophotal measurements (like ``X_IMAGE``, ``Y_IMAGE``, etc.) are
-necessary for the internal operations of |SExtractor| and are therefore
-executed even if they are not requested. Measurements from the second
-category have access to all pixels of the image. These measurements are
-generally more sophisticated and are done at a later stage of the
-processing (after CLEANing and MASKing).
+Once sources have been detected and deblended, they enter the measurement phase. |SExtractor| performs three categories of measurements: isophotal, full, and model-fitting.
+
+Isophotal
+  Measurements are made on the isophotal object footprints. 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
+  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.
+
+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.
 
 .. toctree::
    :numbered:
@@ -20,5 +20,8 @@ processing (after CLEANing and MASKing).
   PositionWin
   Photom
 
+.. [#thresh] Pixel values also have to exceed the local detection threshold set with ``DETECT_THRESH``
+.. [#psf_models] PSF models be computed using the |PSFEx|_ package.
+
 .. include:: keys.rst
 

doc/src/Param.rst

@@ -36,7 +36,7 @@ available:
 Fluxes
 ~~~~~~
 
-may be expressed in linear (ADU) units or as Pogson
+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
@@ -56,7 +56,7 @@ where :math:`m_{ZP}` is the magnitude zero-point set with the
 Flux uncertainties
 ~~~~~~~~~~~~~~~~~~
 
-They follow a scheme similar to that of fluxes. Flux uncertainties are
+Flux uncertainties 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
@@ -72,14 +72,16 @@ from the estimated 1-\ :math:`\sigma` flux error :math:`\sigma_f`:
 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.
+Positions, distances and position angles can be expressed in image pixels, world coordinates, or in celestial coordinates, depending on the suffix:
+
+_IMAGE
+  Measurements are given in pixel coordinates, in units of pixels. For example: ``Y_IMAGE``, ``ERRAWIN_IMAGE``, ``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
+  Measurements are given in so-called “world coordinates”. This requires World Coordinate System (|WCS|_) metadata :cite:`2002A&A...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, _J2000, _B1950
+  Measurements are given in celestial (equatorial) coordinates, in units of degrees. This requires celestial |WCS| metadata :cite:`2002A&A...395.1077C` to be present in the FITS image header(s). _SKY measurements are given in the native world coordinate system. _J2000 and _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.
 
 .. include:: keys.rst