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