  We have measured and analyzed the hyperfine structure of two lines, one at
583nm and one at 401nm, of the only stable fermionic isotope of atomic erbium
as well as determined its isotope shift relative to the four most-abundant
bosonic isotopes. Our work focuses on the J->J+1 laser cooling transitions from
the [Xe] 4f12 6s2 (3H6) ground state to two levels of the excited [Xe] 4f12
6s6p configuration, which are of major interest for experiments on quantum
degenerate dipolar Fermi gases. From a fit to the observed spectra of the
strong optical transition at 401nm we find that the magnetic dipole and
electric quadrupole hyperfine constants for the excited state are
Ae/h=-100.1(3)MHz and Be/h=-3079(30)MHz, respectively. The hyperfine spectrum
of the narrow transition at 583nm, was previously observed and accurate Ae and
Be coefficients are available. A simulated spectrum based on these coefficients
agrees well with our measurements. We have also determined the hyperfine
constants using relativistic configuration-interaction ab initio calculations.
The agreement between the ab initio and fitted data for the ground state is
better than 0.1%, while for the two excited states the agreement is 1% and 11%
for the Ae and Be constants, respectively.
