Mass sheet

In set_lens.c:set_dynamics(), the impact parameter is computed as such:

b_0 = \frac{1}{2} \frac{\Sigma_0}{\Sigma_{\rm crit}}

with \Sigma_0 in \rm g/cm^2, and \Sigma_{\rm crit} = \frac{c^2}{4 \pi G}\frac{D_{\rm OS}}{D_{\rm OL} D_{\rm LS}} \,{\rm g/cm^2}.

Note that the factor 1/2 makes the estimated density to be half the true value.

In file e_grad2.c, the 2nd derivatives of the potential are computed as such:

\partial_{xx} \phi = \partial_{yy} \phi = b_0

\partial_{xy} \phi = \partial_{yx} \phi = 0

In file e_grad.c, the 1st derivates of the potential are computed as such:

\partial_x \phi = b_0 (x_{\rm img} -x_{\rm lens})

\partial_y \phi = b_0 (y_{\rm img} -y_{\rm lens})

This profile is not well tested.