  The performance of telescope systems working at microwave or visible/IR
wavelengths is typically described in terms of different parameters according
to the wavelength range. Most commercial ray tracing packages have been
specifically designed for use with visible/IR systems and thus, though very
flexible and sophisticated, do not provide the appropriate parameters to fully
describe microwave antennas, and thus to compare with specifications. In this
work we demonstrate that the Strehl ratio is equal to the phase efficiency when
the apodization factor is taken into account. The phase efficiency is the most
critical contribution to the aperture efficiency of an antenna, and the most
difficult parameter to optimize during the telescope design. The equivalence
between the Strehl ratio and the phase efficiency gives the designer/user of
the telescope the opportunity to use the faster commercial ray-tracing software
to optimize the design. We also discuss the results of several tests performed
to check the validity of this relationship that we carried out using a
ray-tracing software, ZEMAX and a full Physical Optics software, GRASP9.3,
applied to three different telescope designs that span a factor of $\simeq 10
in terms of D/lambda. The maximum measured discrepancy between phase efficiency
and Strehl ratio varies between $\simeq 0.4 and 1.9 up to an offset angle of
>40 beams, depending on the optical configuration, but it is always less than
0.5 where the Strehl ratio is >0.95.
