skyellipse: Sky Matching of Elliptical Regions
matcher=skyellipse values*='<ra/degrees> <dec/degrees> <primary-radius/arcsec> <secondary-radius/arcsec> <position-angle/degrees>' params='<scale/arcsec>' tuning='<healpix-k>'
ra/degrees: Right ascension of centre
dec/degrees: Declination of centre
primary-radius/arcsec: Length of ellipse semi-major axis
secondary-radius/arcsec: Length of ellipse semi-minor axis
position-angle/degrees: Position angle - measured from north pole to primary axis, in direction of positive RA
scale/arcsec: Rough average of ellipse major radius; just used for tuning to set default pixel size
healpix-k: Controls sky pixel size. Legal range 0 - 29. 0 is 60deg, 20 is 0.2".
skyellipse matcher compares elliptical regions
on the sky for overlap. Each row has to provide five values,
giving the centre, the major and minor radii, and the position angle
of an ellipse.
Rows are considered to match if there is any overlap between the ellipses.
The goodness of match is a normalised generalisation of the symmetrical case
used by the
skyerr matcher, in which the best possible match
is two concentric ellipses, and the worst allowable match is when the
circumferences just touch.
The calculations are approximate since in some cases they rely on projecting the ellipses onto a Cartesian tangent plane before evaluating the match, so for larger ellipses the criterion will be less exact. For objects the size of most observed stars or galaxies, this approximation is not expected to be problematic.
scale parameter must be supplied, and should be
a rough average value of the major radii. it is used only to set
a sensible default for the
healpix-k tuning parameter,
and its value does not affect the result.
If you set
the value of
scale is ignored.