  Vacuum birefringence from Lorentz and CPT violation in the Standard-Model
Extension can be constrained using ground-based optical polarimetry of
extragalactic sources. We describe results from a pilot program with an
automated system that can perform simultaneous optical polarimetry in multiple
passbands on different telescopes with an effective 0.45 m aperture. Despite
the limited collecting area, our polarization measurements of AGN using a wider
effective optical passband than previous studies yielded individual
line-of-sight constraints for Standard-Model Extension mass dimension $d = 5$
operators within a factor of about one to ten of comparable broadband
polarimetric bounds obtained using data from a 3.6 m telescope with roughly 64
times the collecting area. Constraining more general anisotropic Standard-Model
Extension coefficients at higher $d$ would require more AGN along different
lines of sight. This motivates a future dedicated ground-based, multi-band,
optical polarimetry AGN survey with $\gtrsim 1$ m-class telescopes, to obtain
state-of-the-art anisotropic Standard-Model Extension $d = 4, 5, 6$
constraints, while also using complementary archival polarimetry. This could
happen more quickly and cost-effectively than via spectropolarimetry and long
before more competitive constraints from space- or balloon-based
x-ray/$\gamma$-ray polarization measurements.
