The conditions under which the GTD (geometrical theory of diffraction) and Gaussian beam solutions are able to specify the phase and magnitude of the fields around the caustics of offset ellipsoidal reflectors are examined. This is done through the comparison of these methods with the near-field physical optics solution (PO). It is shown that the Gaussian beam results are relatively accurate down to a reflector diameter of the order of 30 lambda , provided that the feed edge taper is at least of the order of 25 dB. As the size of the reflector gets smaller, the maximum field strength predicted by PO becomes lower than that predicted by the Gaussian beam solution. Further, the location of the maximum field strength moves closer to the reflector. The phase at the focal point remains accurate even though the magnitude at the focal point is in error. The GTD solution remains accurate for the magnitude of the fields for distances greater than about 10 lambda from the focus. However, the phase of the fields is in error until a distance of about 30 lambda from the focal point. The results are discussed in connection with high-power microwave applications. >