- Zernike Polynomials
- How de-focusing makes wavefront errors more apparent
- Example of an OOF analysis for the GBT
- OOF Reduction Software
- History of the OOF data reduction software
- Python/NumPy Version
- C++/Python Version
- Description of the OOF software architecture
- BNMin1
- Building the OOF package: worked solutions
- De-convolution of a Gaussian beam from planetary observations
- Aperture-plane phase change due to defocus of a Gregorian telescope

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 phase distributions but must instead be 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. The following are some other sources of further information:

- 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

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.