Phase retrieval radio holography in the Fresnel region: tests on the 30 m telescope at 86 GHz

The application of phase retrieval radio holography in the Fresnel region has been studied using a 86 GHz transmitter as signal source. Maps of the aperture-plane field distribution were made with a surface resolution of about 100 independent pixels across the telescope diameter. Over the 2.7 km propagation path used (0.005 of the far-field distance), the measurement accuracy was found to be determined by atmospheric scintillation. The root mean square measurement errors in the derived aperture-plane phase distribution were about 65 microns when expressed as axially resolved surface errors. The signal to noise ratio and dynamic range requirements are easy to fulfil in the Fresnel zone, while the necessary near field corrections can be applied with high accuracy. There are thus definite advantages to phase retrieval radio holography in this near field zone.