> KSB95 claim somehow that dshift/dtheta ~ r0 theta^2 / 6 (eqn. C4)

Yeah, I don't get that either.  Seems like a pretty straightforward derivative, so I'm not sure how he got it wrong.  Unless there is something we're both missing.

Anyway, I think the shear from the atmospheric refraction is really part of the WCS, not the PSF.  So I'm glad it turned out to be negligible, since I don't particularly want to have to make the WCS stuff color-dependent as well...