  The nonrelativistic ionization energy levels of a helium atom are calculated
for $S$, $P$, $D$ and $F$ states. The calculations are based on the variational
method of "exponential" expansion. The convergence of the calculated energy
levels is studied as a function of the number of basis functions $N$. This
allows us to claim that the obtained energy values (including the values for
the states with a nonzero angular momentum) are accurate up to 28-35
significant digits. Calculations of the nonrelativistic ionization energy of
the negative hydrogen ion H$^-$ are also presented.
