This command performs a Zernike decomposition from the computed wavefront data. The user can specify the number of Zernike terms for fitting and optionally subtract out each of the first six Zernike terms. Fit coefficients for each Zernike term as well as the fit statistics are printed to the output window in units of waves.
This command is accessible after making a Coherent Scalar Wave Field or Coherent Vector Wave Field calculation and computing the wavefront. From the resulting wavefront plot, right mouse click in the chart view and select "Decompose Wavefront" from the context menu.
There are two aperture normalizations that need to occur for proper decomposition of the wavefront. First, the X,Y semi-aperture settings need to be correctly specified in the dialog. Second, the size of the beam from which the wavefront is decomposed must be in agreement with the aperture settings.
For example, consider a mirror with a Zernike surface applied onto which a coherent beam is incident and decomposed after propagation. In order for the decomposition to represent the Zernike coefficients applied to the mirror surface both the aperture settings in the decomposition dialog must be sized to the mirror AND the size of the beam on the mirror must be as large as the aperture. If the beam is undersized with respect to the Zernike mirror aperture, then the wavefront being decomposed will NOT represent the coefficients applied to the mirror.
FRED computes the coefficients of the Zernike terms be performing an overlap integral of the E-field with each of the Zernike terms. Note that there will be some small residual error in this calculation because the field is calculated on a rectangular grid and not a spherical coordinate system. The coefficient units are reported in waves.
As the coefficients are calculated by an overlap integral of the E-field with the Zernike terms, the piston term will not be zero unless the phase shift in propagation from the source plane to the detector plane is a multiple of the wavelength. Observe that shifting the analysis plane position directly correlates to a change in the piston term.
Subtraction of the coefficient terms in the decomposition applies ONLY to the chart view. The data reported in the output window corresponds to the original dataset.
Coherent Wave Field - Show Computed Wavefront
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