  Realization of efficient spin-orbit torque switching of the N\'eel vector in
insulating antiferromagnets is a challenge, often complicated by spurious
effects. Quantifying the spin-orbit torques in antiferromagnet/heavy metal
heterostructures is an important first step towards this goal. Here, we employ
magneto-optic techniques to study damping-like spin-orbit torque (DL-SOT) in
a-plane $\alpha$-Fe$_2$O$_3$ (hematite) with a Pt spin-orbit overlayer. We find
that the DL-SOT efficiency is two orders of magnitude larger than reported in
c- and r-plane hematite/Pt using harmonic Hall techniques. The large magnitude
of DL-SOT is supported by direct imaging of current-induced motion of
antiferromagnetic domains that happens at moderate current densities. Our study
introduces a new method for quantifying spin-orbit torque in antiferromagnets
with a small canted moment and identifies a-plane $\alpha$-Fe$_2$O$_3$ as a
promising candidate to realize efficient SOT switching.
