Even simpler modeling of quadruply lensed quasars (and random quartets)
  using Witt's hyperbola
  Witt (1996) has shown that for an elliptical potential, the four images of a
quadruply lensed quasar lie on a rectangular hyperbola that passes through the
unlensed quasar position and the center of the potential as well. Wynne and
Schechter (2018) have shown that, for the singular isothermal elliptical
potential (SIEP), the four images also lie on an `amplitude' ellipse centered
on the quasar position with axes parallel to the hyperbola's asymptotes. Witt's
hyperbola arises from equating the directions of both sides of the lens
equation. The amplitude ellipse derives from equating the magnitudes. One can
model any four points as an SIEP in three steps. 1. Find the rectangular
hyperbola that passes through the points. 2. Find the aligned ellipse that also
passes through them. 3. Find the hyperbola with asymptotes parallel to those of
the first that passes through the center of the ellipse and the pair of images
closest to each other. The second hyperbola and the ellipse give an SIEP that
predicts the positions of the two remaining images where the curves intersect.
Pinning the model to the closest pair guarantees a four image model. Such
models permit rapid discrimination between gravitationally lensed quasars and
random quartets of stars.
