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The performance of telescope systems working at microwave or visible/IRwavelengths is typically described in terms of different parameters accordingto the wavelength range. Most commercial ray tracing packages have beenspecifically designed for use with visible/IR systems and thus, though veryflexible and sophisticated, do not provide the appropriate parameters to fullydescribe microwave antennas, and thus to compare with specifications. In thiswork we demonstrate that the Strehl ratio is equal to the phase efficiency whenthe apodization factor is taken into account. The phase efficiency is the mostcritical contribution to the aperture efficiency of an antenna, and the mostdifficult parameter to optimize during the telescope design. The equivalencebetween the Strehl ratio and the phase efficiency gives the designer/user ofthe telescope the opportunity to use the faster commercial ray-tracing softwareto optimize the design. We also discuss the results of several tests performedto check the validity of this relationship that we carried out using aray-tracing software, ZEMAX and a full Physical Optics software, GRASP9.3,applied to three different telescope designs that span a factor of $\simeq 10in terms of D/lambda. The maximum measured discrepancy between phase efficiencyand 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 than0.5 where the Strehl ratio is >0.95.

Ray-tracing and physical-optics analysis of the aperture efficiency in a radio telescope