Aspheric Surface Errors from Foucault Test Data

The numerical integration of Foucault test data is simplified when readings are taken for the centers of contiguous zones of equal area. Height errors from the reference surface are shown to be proportional to the simple sump of knife-edge shifts from nominal settings. Change of reference curvature for minimum zonal error is effected by changing each shift by the same amount. Errors are exact to third-order for conic surfaces. The method is especially helpful when many tests must be made in the figuring process. Formulas are derived and an example is given, using both arithmetical and graphical error adjustment.