Method for accurate high-resolution measurements of aspheric surfaces

In a metrology system including a part-positioning means and a wavefront-measuring gauge having a focusing stage and a plurality of constant first-order properties, a method for determining the first-order optical properties of the wavefront-measuring gauge and imaging conjugates as described by the

• equation
• ⁢
• ⁢
• 1
• ob
• +
• ob_ref
• +
• 1
• im
• +
• im_ref
• =
• 1
• f
• ,
• the method comprising the steps of:
• a) determining “ob_ref”, the offset from an object reference point to a first principal plane within said gauge;
• b) determining “ob”, the displacement from a test surface to said object reference point;
• c) determining “im_ref”, the displacement from an F-axis mechanical origin Fo to a second principal plane within said gauge;
• d) determining “im”, the position of the F-axis within the gauge;
• and
• e) determining “f”, the focal length of the gauge's optical system.

In a metrology system including a part-positioning means and a wavefront-measuring gauge, a method for estimating and calibrating the systematic gauge wavefront error when testing a specific surface or wavefront of a part having underlying symmetry in a design prescription, the method comprising the steps of:

• a) identifying said symmetry of said surface or wavefront;
• b) selecting the gauge and test part measurement conditions under which to calibrate, including gauge configuration and position of said part;
• c) acquiring a plurality of measurements at different relative positions of said part and gauge wherein said measurements meet said test conditions and said positions follow said symmetry, such that said gauge measurements at all of said positions would be identical if said part were perfectly matched to said design prescription;
• and
• d) calculating a calibration of the systematic gauge wavefront error from said measurements.

In a metrology system including a part-positioning means and a wavefront-measuring gauge, a method for reducing the effects of systematic gauge wavefront error when testing a specific surface or wavefront from a part, the method comprising the steps of:

• a) selecting a nominal position on said part to measure;
• b) acquiring a plurality of measurements at said nominal position, said measurements having data points in common, wherein each of said measurements differs from said other measurements in at least one of position and orientation;
• and
• c) averaging said measurements to form a common coordinate system, whilst accounting for any relative translations in said different measurements.

A method for synthesizing a full-aperture numerical data map of a test surface of an object from a plurality of overlapping subaperture data maps of said surface, comprising the steps of:

• a) selecting at least one method for reduction of measurement error in said subaperture data maps;
• b) collecting said plurality of subaperture numerical data maps of said surface from a plurality of regions of said surface, at least a portion of each such map overlapping a portion of at least one adjacent map to create a region of overlapping data;
• c) projecting said plurality of overlapping data maps onto a global coordinate system with a nominal distortion map;
• d) selecting a plurality of error compensators from at least one of two groups consisting of free compensators having free amplitude range and interlocked compensators having identical constrained amplitudes to compensate for stitching errors in said overlapping data regions;
• and
• e) simultaneously minimizing mismatch of data from each of said data maps in said overlapping regions through linear combinations of said compensators.