What is CGH mapping and how does it get corrected?
In a CGH test of surface figure, rays must arrive normal to the test optic surface. Equally spaced rays normal to a non-spherical test optic surface are no longer equally spaced on the CGH.
Typically the more aspheric and the larger the CGH-test optic distance, the worse mapping distortion is.
Mapping distortion can be corrected using a polynomial mapping equation.
Note that when mapping distortion is corrected, the surface deviation map represents equally spaced points conformal to the UUT surface, and surface deviation is in the direction of the UUT surface normal.