The zero-point energy of a conducting spherical shell is evaluated by impos
ing boundary conditions on the potential A(mu), and on the ghost fields. Th
e scheme requires that temporal and tangential components of A(mu) perturba
tions should vanish at the boundary, jointly with the gauge-averaging funct
ional, first chosen to be of the Lorentz type. Gauge invariance of such bou
ndary conditions is then obtained provided that the ghost fields vanish at
the boundary. Normal and longitudinal modes of the potential obey an entang
led system of eigenvalue equations, whose solution is a linear combination
of Bessel functions under the above assumptions, and with the help of the F
eynman choice for a dimensionless gauge parameter. Interestingly, ghost mod
es cancel exactly the contribution to the Casimir energy resulting from tra
nsverse and temporal modes of A(mu), jointly with the decoupled normal mode
of A(mu). Moreover, normal and longitudinal components of A(mu) for the in
terior and the exterior problem give a result in complete agreement with th
e one first found by Boyer, who studied instead boundary conditions involvi
ng TE and TM modes of the electromagnetic field. The coupled eigenvalue equ
ations for perturbative modes of the potential are also analyzed in the axi
al gauge, and for arbitrary values of the gauge parameter. The set of modes
which contribute to the Casimir energy is then drastically changed, and co
mparison with the case of a flat boundary sheds some light on the key featu
res of the Casimir energy in noncovariant gauges.