public documents.sextractor_doc

\section{Astrometry and {\tt WORLD} coordinates}
\label{astrom}
All {\sc SExtractor} measurements related to positions, distances and
areas in the image, like those described above can also be expressed
in {\tt WORLD} coordinates in the output catalogue. These parameters
simply have the {\tt \_WORLD} suffix instead of the {\tt \_IMAGE}
appended to them. The conversion from {\tt IMAGE} to {\tt WORLD}
coordinates is presently performed by using \gam{WCS?} information found in the
FITS header of the {\em measurement} image, even if the parameter is
originally computed from the {\em detection} image (like the basic
shape parameters for instance).
 
To understand how this is done in practice, let's have a general look
at the way the mapping from {\tt IMAGE} to {\tt WORLD} coordinates is
currently described in a FITS image header. First, a linear
transformation (involving most of the time only scaling and possibly
rotation, and more rarely shear) allows one to convert integer pixel
positions (1,2,...) for each axis to some ``projected'' coordinate
system. This is where you might want to stop if your {\tt WORLD}
system is just some kind of simple focal-plane coordinate-system (in
meters for instance), or for a calibrated wavelength axis (spectrum).
Now, the FITS WCS (World Coordinate System) convention allows you to
apply to these ``projected coordinates'' a non-linear transformation,
which is in fact a de-projection back to ``local'' spherical
(celestial) coordinates. Many types of projections are allowed by the
WCS convention, but the traditional tangential (gnomonic) projection
is the most commonly used. The last step of the transformation is to
convert these local coordinates, still relative to a projection
reference point, to an absolute position in celestial longitude and
latitude, for instance right-ascension and declination. For this one
needs to know the reference frame of the coordinate system, which
often requires some information about the equinox or the observation
date. At this level, all transformations are matters of spherical
trigonometry.
 
\subsection{Celestial coordinates}
We will not describe here the transformations $(\alpha,\delta) =
f(x,y)$ themselves. {\sc SExtractor} de-projections rely on the WCSlib
2.4 written by Mark Calabretta, and all the details concerning those
can be found in Greisen \& Calabretta (1995). In addition to the {\tt
\_WORLD} parameters, 3 purely angular ``world'' coordinates are
available in {\sc SExtractor}, expressed in decimal degrees:
\begin{enumerate}
\item{}{\tt \_SKY} coordinates: strictly identical to {\tt \_WORLD} coordinates, except that
the units are explicitly degrees. They correspond to sky coordinates in the
``native'' system without any precession correction, conversion, etc.
\item{}{\tt \_J2000} coordinates: precession corrections are applied in the FK5 system to
convert to J2000 coordinates if necessary.
\item{}{\tt \_B1950} coordinates: precession corrections are computed in the FK5 system and
transformation to B1950 is applied.
\end{enumerate}
 
Transformation to J2000 or B1950 is done without taking into account
proper motions, which are obviously unknown for the detected objects.
In both cases, epoch 2000.0 is assumed.
\gam{Why not use the date from the FITS header instead of 2000.0? --- or did
  I misunderstand what EPOCH means?}
 
Here is a list of catalogue parameters currently supporting angular coordinates:
 
{
%\tiny
\scriptsize
\tabcolsep 3pt
\begin{tabular}{lll}
Image  parameters & World parameters & Angular parameters \\
\hline
{\tt X\_IMAGE}, {\tt Y\_IMAGE} & {\tt X\_WORLD}, {\tt Y\_WORLD} & {\tt
ALPHA\_SKY}, {\tt DELTA\_SKY} \\
& & {\tt ALPHA\_J2000}, {\tt DELTA\_J2000} \\
& & {\tt ALPHA\_B1950}, {\tt DELTA\_B1950} \\
{\tt XWIN\_IMAGE}, {\tt YWIN\_IMAGE} &
{\tt XWIN\_WORLD}, {\tt YWIN\_WORLD} &
{\tt ALPHAWIN\_SKY}, {\tt DELTAWIN\_SKY} \\
& & {\tt ALPHAWIN\_J2000}, {\tt DELTAWIN\_J2000} \\
& & {\tt ALPHAWIN\_B1950}, {\tt DELTAWIN\_B1950} \\
{\tt XPEAK\_IMAGE}, {\tt YPEAK\_IMAGE} & {\tt XPEAK\_WORLD}, {\tt
YPEAK\_WORLD} & {\tt ALPHAPEAK\_SKY}, {\tt DELTAPEAK\_SKY} \\
 & & {\tt ALPHAPEAK\_J2000}, {\tt DELTAPEAK\_J2000} \\
 & & {\tt ALPHAPEAK\_B1950}, {\tt DELTAPEAK\_B1950} \\
{\tt X2\_IMAGE}, {\tt Y2\_IMAGE}, {\tt XY\_IMAGE} &
{\tt X2\_WORLD}, {\tt Y2\_WORLD}, {\tt XY\_WORLD} &\\
{\tt X2WIN\_IMAGE}, {\tt Y2WIN\_IMAGE}, {\tt XYWIN\_IMAGE} &
{\tt X2WIN\_WORLD}, {\tt Y2WIN\_WORLD}, {\tt XYWIN\_WORLD} &\\
{\tt CXX\_IMAGE}, {\tt CYY\_IMAGE}, {\tt CXY\_IMAGE} &
{\tt CXX\_WORLD}, {\tt CYY\_WORLD}, {\tt CXY\_WORLD} &\\
{\tt CXXWIN\_IMAGE},{\tt CYYWIN\_IMAGE},{\tt CXYWIN\_IMAGE} &
\multicolumn{2}{l}{{\tt CXXWIN\_WORLD}, {\tt CYYWIN\_WORLD}, {\tt CXYWIN\_WORLD}} \\
\hline
\end{tabular}
}
 
{\bf TO BE WRITTEN}
 
\subsection{Use of the FITS keywords for astrometry}
{\bf TO BE WRITTEN}