  Fabry-P\'erot etalons illuminated with collimated beams have been
characterized analytically in detail since their invention. Meanwhile, most of
the features of etalons located in telecentric planes have been studied only
numerically, despite the wide use of this configuration in astrophysical
instrumentation over decades. In this work we present analytical expressions
for the transmitted electric field and its derivatives that are valid for
etalons placed in slow telecentric beams, like the ones commonly employed in
solar instruments. We use the derivatives to infer the sensitivity of the
electric field to variations in the optical thickness for different
reflectivities and apertures of the incident beam and we compare them to the
collimated case. This allows us to estimate the wavefront degradation produced
by roughness errors on the surfaces of the Fabry-P\'erot and to establish the
maximum allowed RMS value of the cavity irregularities across the footprint of
the incident beam on the etalon that ensures diffraction-limited performance.
We also evaluate the wavefront degradation intrinsic to these mounts, which is
produced only by the finite aperture of the beam and that must be added to the
one produced by defects. Finally, we discuss the differences in performance of
telecentric and collimated etalon-based instruments and we generalize our
formulation to anisotropic etalons.
