Out-Of-Focus (OOF) Holography

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). More precisely, OOF holography measures the total on-axis aberration present in the telescope including the complete optical path to the receiver being used.

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. These can not be inverted in a linear way to obtain the aperture-plane phases distributions but must processed using an iterative inverse-problem algorithm, and hence the name phase-retrieval holography. In order to make this phase-retrieval unique and robust, it is necessary to make a number (usually three) of beam maps at different focus setting 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.

There are two recent papers describing OOF holography; see the publications page. These are some other sources:

• There is info on Zernike polynomials on this page.
• An example of what OOF data and results look like is shown here.
• The OOF wiki pages
• The OOF software download page
• The URSI write up from 2002 (pdf ). Note that this of course is not a reflection of current status of GBT surface (these data were taken before the active surface was even commissioned).
• My old OOF holography pages at the MRAO.
• Experimental mustang data.

Typographical and other errors in published papers

• In "Measurement of antenna surfaces from in- and out-of-focus beam maps using astronomical sources", Astronomy and Astrophysics 2007: Equation 4 has a typographical error, there should be a factor of "i" (the imaginary unit) in the exponent multiplying both the phi and the delta values. Thanks to Fujun Du for pointing this out to me.