Angle deflection from gravitational lenses

[Niles]

Hi all

The expression for the angle deflection on the following page: http://relativity.livingreviews.org/open?pubNo=lrr-1998-12&page=node2.html

Is the impact parameter r in the expression the distance to the surface of the objects that deflects the light or the the center of mass of the object? Unfortunately they do not specify this.

 

[Ignition]

It is the radius.

 

[astrorob]

I'm thinking it's probably the latter..