  Coherent photoproduction of vector mesons is sensitive to the shape of the
target nucleus, as probed at four-momentum scale $Q^2 \propto (M_V/2)^2$.
Previously STAR presented a high-statistics measurement of $d\sigma_c/dt$ for
coherent $\pi^+\pi^-$ photoproduction in ultra-peripheral gold-gold collisions,
and made a two-dimensional Fourier-Bessel (Hanckel) transformation to give the
transverse distribution of interactions in the nucleus. Here, we study how
$d\sigma_c/dt$ evolves with $Q^2$. We divide the $\pi^+\pi^-$ signal into three
different mass ($Q^2$) bins to measure how $d\sigma_c/dt$ evolves with dipion
mass. Furthermore, we find that the depth of the first diffractive minimum
varies with pair mass. We perform a two-dimensional Fourier-Bessel transform to
see how the effective transverse distribution of the interactions changes with
decreasing pair mass. In the lowest mass bin, the nuclear profile is broader,
which is consistent with expectations from saturation models.
