... Out-Of-Focus (OOF), or phase-retrieval, holography is a technique to measure the shape of surfaces of radio telescopes (i.e., to do antenna metrology). ... In contrast to traditional holography, which measures both the amplitude and phase of the far-field antenna response pattern, the input to OOF holography are total-power beam-maps of the telescope. ... Typically, one of these beam maps is with the telescope at nominal focus and others out-of-focus, hence the name `Out-Of-Focus' holography. ...