Grid Phase

This surface is nearly identical to the Grid Sag surface (see "Grid Sag"). The key differences are:

• The units of sag are radians of phase instead of units of length.
• The unitflag data is only used to scale the delx, dely, and derivative values.
• The surface shape is a plane.
• A diffraction order is supported. The diffraction order is a multiplier on the phase values. A value of zero will turn "off" the phase effects. Setting the order to -1 will reverse the sign of all defined phase values.
• A "shear distance" is supported. See "Using the shear distance" below.

The Interpolate column determines the interpolation method used between the data points. Use 0 for bicubic spline, 1 for linear, and 2 for pixelated. For interpolation methods 0 and 1 the phase and phase slope vary continuously over the surface. When using interpolation method 2, the Grid Phase surface will have distinct pixels. The phase change is determined based on the individual pixel. The phase slope will be determined based on the derivatives (dz/dx, dz/dy, and d2z/dxdy) present in the .dat file. If all derivative values are zero for every point in the file, then OpticStudio will automatically estimate the derivatives based on the pixel and the neighboring values.

The file format and general information provided in the grid sag description is otherwise valid.

Using the shear distance

One method of modeling atmospheric turbulence is to use a Grid Phase surface with data generated by an external atmospheric modeling program. Accurate modeling may require multiple surfaces with different phase data modeling atmospheric layers separated by long distances. Because of the separation between these layers, different field angles pass through different parts of the atmosphere, depending upon the distance between the layers and the ray angle. The separation causes a field dependent lateral shearing between the surfaces.

To model this shearing, one could in principle place multiple Grid Phase surfaces separated by large distances in front of the optical system. In practice, this works poorly, because the bending of the rays as they pass through a Grid Phase surface may cause the rays to miss the entrance pupil entirely. To avoid this problem, the Grid Phase surface may be placed at the entrance pupil of the system, and then the "shear distance" corresponding to the actual position of the phase layer may be defined. When ray tracing to the Grid Phase surface, for phase purposes only, OpticStudio will adjust the ray coordinates to correspond to the coordinates the incoming ray would have had if the Grid Phase surface had been placed at the shear distance. Normally, shear distances are negative as the distance is measured from the surface to the effective position of the phase layer. If the shear distance is zero, no lateral shifting occurs.

Phase coefficients sign conventions

See "Binary Optic 1" for a discussion of sign conventions.

Parameter #	Definition
0	Diffraction order
1	Shear Distance
2	Interpolation method. Use 0 for bicubic spline, 1 for linear and 2 for pixelated. See "Bicubic spline vs. linear interpolation"

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• Grid Sag