Shedding Light on the Matter of Abell 781
  The galaxy cluster Abell 781 West has been viewed as a challenge to weak
gravitational lensing mass calibration, as Cook and dell'Antonio (2012) found
that the weak lensing signal-to-noise in three independent sets of observations
was consistently lower than expected from mass models based on X-ray and
dynamical measurements. We correct some errors in statistical inference in Cook
and dell'Antonio (2012) and show that their own results agree well with the
dynamical mass and exhibit at most 2.2--2.9$\sigma$ low compared to the X-ray
mass, similar to the tension between the dynamical and X-ray masses. Replacing
their simple magnitude cut with weights based on source photometric redshifts
eliminates the tension between lensing and X-ray masses; in this case the weak
lensing mass estimate is actually higher than, but still in agreement with, the
dynamical estimate. A comparison of lensing analyses with and without
photometric redshifts shows that a 1--2$\sigma$ chance alignment of
low-redshift sources lowers the signal-to-noise observed by all previous
studies which used magnitude cuts rather than photometric redshifts. The
fluctuation is unexceptional, but appeared to be highly significant in Cook and
dell'Antonio (2012) due to the errors in statistical interpretation.
