  We present a study of the Rydberg spectrum in \ts{166}Er for series connected
to the $4f^{12} (^3H_6) 6s$, $J_c=13/2 $ and $J_c=11/2 $ ionic core states
using an all-optical detection based on electromagnetically induced
transparency in an effusive atomic beam. Identifying approximately 550
individual states, we find good agreement with a multi-channel quantum defect
theory (MQDT) which allows assignment of most states to $ns$ or $nd$ Rydberg
series. We provide an improved accuracy for the lowest two ionization
thresholds to $E_{\textrm{IP}, J_c = 13/2 } = 49260.750(1)\,$cm$^{-1}$ and
$E_{\textrm{IP}, J_c = 11/2 } = 49701.184(1)\,$cm$^{-1}$ as well as the
corresponding quantum defects for all observed series. We identify Rydberg
states in five different isotopes, and states between the two lowest ionization
thresholds. Our results open the way for future applications of Rydberg states
for quantum simulation using erbium and exploiting its special open-shell
structure.
