Help file for the gravitational lens simulator

Galaxy Einstein radius

This parameter controls the overall mass of the galaxy, in the form of the Einstein radius (given in pixels). The larger the parameter, the larger the galaxy mass and the further out from the centre of the galaxy sources will be lensed. A formula is derived in the first part of the course for the Einstein radius in terms of the mass.

Galaxy orientation angle

This parameter controls the orientation angle of the major axis of the galaxy.

Ellipticity

This parameter controls the ellipticity of the galaxy mass distribution. For example, 0.0 is completely circular; 0.5 represents a galaxy whose minor axis is half its major axis; and 1.0 represents a completely linear galaxy. Most galaxies have modest ellipticities (0.0-0.5).

Source X position

Source Y position

These control the position of the source on the 128x128 modelling grid. The galaxy is always at the centre, (64,64). Notice what happens as the source is brought closer to the galaxy; it becomes successively more distorted, and is multiply imaged if it is close enough to the galaxy centre.

Source major axis

This parameter controls the size of the source. Most sources found in the CLASS survey are pointlike quasars. However, in some cases we also see the host galaxy of the background source in the optical, and the resulting large size of the source means that some of it lies directly behind the lensing galaxy, resulting in arcs and Einstein rings. (In the radio, a similar effect can be produced if the source has extended radio structure). Try increasing the size of the source and noting the increased tendency to produce arcs and Einstein rings.

Model type

This parameter controls the type of model to be used for the galaxy. Type "SIE" is a singular isothermal ellipsoid, which has a constant bend angle for all impact parameters. In practice it is found to be a reasonably good representation of the mass distribution of the elliptical galaxies which form the majority of the lensing population.

Plot caustics?

This parameter controls whether the critical curve and caustic of the lensing galaxy are plotted. The number of images (1, 3 or 5) depends on the position of the source with respect to these curves, and in particular 5 images are produced if the source lies within the diamond-shaped caustic. Try plotting these curves for a range of galaxy ellipticity; for less elliptical galaxies the caustic shrinks until it becomes very small and five images are then less likely to be produced.

Plot Fermat surfaces?

This parameter controls whether Fermat surfaces are plotted. These are contours which represent equal times of flight from rays passing through any part of each individual contour. By Fermat's principle, images will form in local maxima or minima; note how the surfaces become steadily more complicated as the source approaches the lensing galaxy and multiple images form.

Not used

There are a few parameters which are reserved for further development of the program.