Qualification of a null lens using image-based phase retrieval
2012
In measuring the figure error of an aspheric optic using a null lens, the wavefront contribution from the null lens must be
independently and accurately characterized in order to isolate the optical performance of the aspheric optic alone.
Various techniques can be used to characterize such a null lens, including interferometry, profilometry and image-based
methods. Only image-based methods, such as phase retrieval, can measure the null-lens wavefront in situ – in single-pass,
and at the same conjugates and in the same alignment state in which the null lens will ultimately be used – with no
additional optical components. Due to the intended purpose of a null lens (e.g., to null a large aspheric wavefront with a
near-equal-but-opposite spherical wavefront), characterizing a null-lens wavefront presents several challenges to image-based
phase retrieval: Large wavefront slopes and high-dynamic-range data decrease the capture range of phase-retrieval
algorithms, increase the requirements on the fidelity of the forward model of the optical system, and make it difficult to
extract diagnostic information (e.g., the system F/#) from the image data. In this paper, we present a study of these
effects on phase-retrieval algorithms in the context of a null lens used in component development for the Climate
Absolute Radiance and Refractivity Observatory (CLARREO) mission. Approaches for mitigation are also discussed.
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