# public documents.sextractor_doc

## [/] [measure_astrom.tex] - Diff between revs 25 and 29

Rev 25 Rev 29
Line 77... Line 77...
\label{chap:abtheta}
\label{chap:abtheta}
These parameters are intended to describe the detected object as an elliptical
These parameters are intended to describe the detected object as an elliptical
shape. {\tt A} and {\tt B} are its \index{semi-major} semi-major and \index{semi-minor axis} semi-minor axis lengths,
shape. {\tt A} and {\tt B} are its \index{semi-major} semi-major and \index{semi-minor axis} semi-minor axis lengths,
respectively. More precisely, they represent the maximum and minimum spatial
respectively. More precisely, they represent the maximum and minimum spatial
\rms dispersion of the object profile along any direction. {\tt THETA} is the
\rms dispersion of the object profile along any direction. {\tt THETA} is the
position-angle between of the {\tt A} axis relative to the {\tt NAXIS1} \index{image} image axis. It is
position-angle of the {\tt A} axis relative to the first image axis. It is counted positive in the direction of the second axis. Here is how they are computed:
counted counter-clockwise. Here is how they are computed:

 
 
2nd-order \index{moments} moments can easily be expressed in a referential rotated from the
2nd-order \index{moments} moments can easily be expressed in a referential rotated from the
$x,y$ \index{image} image coordinate system
$x,y$ \index{image} image coordinate system
by an angle +$\theta$:
by an angle +$\theta$: