The following link GenerateWavefrontFile rev 1 will download software (in a zip file) which corrects the spherical aberration inherent in the design of the SPPDI®.
Spherical aberration arises when a diverging or converging beam passes through a given thickness of flat glass. In the case of the SPPDI, there is 32.5 mm of flat glass (beam splitters and other flat components) in the internal signal beam path, which is not compensated by an equal amount of glass in the internal reference beam path. Measurements obtained with the SPPDI will therefore exhibit a small and predictable amount of spherical aberration that may be subtracted from interferometric measurements. This condition is further discussed in the Technical Information section of this website.
The free spherical aberration compensation software available at the link above generates a DFTFringe-style wavefront file (denoted by file type *.wft) that can be used to remove the small amount of wavefront error that is introduced by the SPPDI.
This console application requests various input values that are needed in order to generate the spherical aberration correction wavefront file. Requested inputs are as follows:
“Enter size in pixels of the output wavefront:” The value entered here will typically be 640.
“Enter Wavelength in nm:” The value entered here is the operating wavelength of the SPPDI, which will typically be around 650 nm. This wavelength value is used by this software to compute the refractive index of the glass used internally in the SPPDI. The wavelength value is also expected by DFTFringe to be present in the wavefront file. Note that if this value is different from the present value in the DFTFringe Configuration -> Mirror dialog box, a warning will be issued by DFTFringe.
“Enter diameter of test article in mm:” This value is not used by this software, but must be included in the output wavefront file for compatibility with DFTFringe. Note that if this value is different from the present value in the DFTFringe Configuration -> Mirror dialog box, a warning will be issued by DFTFringe.
“Enter radius of curvature of test article in mm:” This value is not used by this software, but must be included in the output wavefront file for compatibility with DFTFringe. Note that if this value is different from the present value in the DFTFringe Configuration -> Mirror dialog box, a warning will be issued by DFTFringe.
“Enter focal ratio (f/#) of the test article:” This is the focal ratio of the beam entering the SPPDI, and may be a factor of two larger than the focal ratio of the optic being tested. For example, if a 4″ diameter f/4 concave spherical mirror is being tested at its center of curvature, the proper entry here would be “8,” not “4.”
The generation of the output wavefront file will typically take only a few seconds, and will have a size of around 4 MBytes. The file name of the output wavefront correction file will be “SPPDI Corrector.wft” and will be located in the same directory from which the software was executed.
The generated correction wavefront file is read in to DFTFringe by clicking the “Read Wavefront/s” button near the top left of the DFTFringe main form, and then following the instructions to indicate to DFTFringe where the wavefront file is located.
After the artificial correction wavefront file is read-in to DFTFringe, it can be subtracted from other wavefronts produced by DFTFringe. The list of available wavefronts is found by clicking the “Surfaces” tab on the right-hand side of the DFTFringe main form.
Note that subtraction of the correction wavefront should reduce the magnitude of the Z8 “Spherical” Zernike term of the wavefront to be corrected. If the result of the subtraction causes an increase in the “Spherical” term, then it will be necessary to invert the correction wavefront and perform the subtraction again. This is done by selecting (highlighting) the artificial correction wavefront in the list of wavefronts, and then clicking the “Invert” button. After subtraction from the target wavefront, the resulting corrected wavefront should then exhibit a reduction in the “Spherical” Zernike term, as well as a reduction in the overall RMS wavefront error.