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}