In article ,
"Phillip Helbig (undress to reply)" writes:
At this level of precision, it's probably not enough to simply
parameterize this, but rather one needs some model of the mass
distribution near the beams.

That's exactly right (at least to the extent I understood Shajib's
talk). In particular, one has to take into account the statistical
distribution of mass all along and near the light path and also (as
others wrote) the mass distribution of the lensing galaxy
itself. It's even worse than that in systems that have multiple
galaxies contributing to the lensing. Not only do their individual
mass distributions matter, their relative distances along the line of
sight are uncertain and must be modeled.

Presumably all that can be automated -- at the cost of many extra cpu
cycles -- but it hasn't been done yet.

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