Asphericity is a useful value when manufacturing an asphere or a freeform surface. It is typically defined as the distance between the smallest best-fit sphere (BFS) that touches the surface without intersecting and the ideal surface shape, as sketched below. (The BFS usually contacts the surface at one radial location; in the part sketched below, the BFS contacts the part at the outer edge.)

OpticStudio can return both the BFS and the asphericity using the Surface Sag map and/or the DSAG operand in the Merit Function. Choose *Analyze/Surface/Sag* to open the Sag map of the surface and then set “Remove” to “Best Fit Sphere” with the “Minimum Volume” option, as shown below. The peak-to-valley of the resulting plot is the asphericity. The radius of curvature of the minimum volume BFS is also listed.

Both values can also be retrieved using the DSAG operand with Remove = 2 and BFS = 0. Data = 2 retrieves the P-V of the sag map, equal to the asphericity. Data = 9 retrieves the radius of curvature of the Minimum Volume best-fit sphere.