public documents.sextractor_doc

\section{Astrometry and {\tt WORLD} coordinates}
\label{astrom}
All {\sc SExtractor} measurements related to positions, distances and
\index{area} areas in the \index{image} 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
\index{FITS header} FITS header of the {\em measurement} \index{image} image, even if the parameter is
originally computed from the {\em detection} \index{image} 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 \index{image} 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 \index{WCS} 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
\index{WCS} 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 \index{equinox} 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 \index{WCS} 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 \index{precession} precession correction, conversion, etc.
\item{}{\tt \_J2000} coordinates: \index{precession} precession corrections are applied in the FK5 system to
convert to \index{J2000} J2000 coordinates if necessary.
\item{}{\tt \_B1950} coordinates: \index{precession} precession corrections are computed in the FK5 system and
transformation to \index{B1950} B1950 is applied.
\end{enumerate}
 
Transformation to \index{J2000} J2000 or \index{B1950} B1950 is done without taking into account
proper motions, which are obviously unknown for the detected objects.
In both cases, \index{epoch} epoch 2000.0 is assumed.
\gam{Why not use the date from the \index{FITS header} FITS header instead of 2000.0? --- or did
  I misunderstand what EPOCH \index{mean} 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}