A modelling tool for quadruple gravitational lenses
written by Robert Schmidt when working at the
Institute of Astronomy, Cambridge, UK
This applet calculates the position angle of the major or minor
axis (phi) of a singular elliptical potential, given
the three relative separations of the four images of a
gravitational lens.
Instructions:
Enter the three coordinate pairs (e.g., in arcseconds)
relative to a reference image
(xi = xcoord - xcoord_ref, yi = ycoord - ycoord_ref)).
Then click Update for the computation and
graphical representation.
Optionally, a galaxy position xg, yg can be specified.
This position does not enter the calculations.
It will only be marked in the plot, and may then be used to estimate
the amount of external shear necessary to model the system.
If you know the major axis of the light distribution
of a possible lensing galaxy, you can compare if this
fits with the predicted phi from lensing!
The program uses the method by
Witt 1996, ApJ 472, L1. The hyperbolic
curves (solid lines) denote the lines where all four images, a possible
fifth image, and
the galaxy centre lie in the case of the singular elliptical potential.
The curves are described by the equation at the bottom.
The dashed line denotes a line parallel to the major or minor axis of the lens.
If the optional coordinates for the galaxy centre are entered,
this line will be offset to this point.
Note this page does not seem to work with modern browsers anymore, but
possibly your appletviewer application can do it on the command line.
created: 05/07/00