  We present an experimental investigation of the Rydberg 23 P$_{1/2}$ state of
laser-cooled $^{40}$Ca$^+$ ions in a radiofrequency ion trap. Using micromotion
sideband spectroscopy on a narrow quadrupole transition, the oscillating
electric field at the ion position was precisely characterised, and the
modulation of the Rydberg transition due to this field was minimised. From a
correlated fit to this P line and previously measured P and F level energies of
Ca II, we have determined the ionization energy of 95 751.916(32) $\rm
{cm}^{-1}$, in agreement with the accepted value, and the quantum defect for
the $n$ P$_{1/2}$ states.
