As on-machine metrology for freeform optics becomes increasingly popular, error analysis and compensation techniques and probe path optimization must evolve in response. For this work, a four axis Moore Nanotech Universal Measuring Machine (based on a diamond turning machine design) is used with a Precitec chromatic confocal probe. The system is set up in a thermally controlled enclosure, and a LabVIEW data acquisition program is used to record probe data, time, and machine positioning data directly from the machine scales. Issues addressed include probe testing and calibration, thermal errors, data age, compensation methods, and probe path optimization for error reduction. The combined error reduction/compensation approach improves on-machine metrology for freeform optics.
The latest generation of optical sensing hardware comprises of optical surfaces without rotational symmetry, commonly known as freeform optics. Whether the goal is obtaining more information by using a larger aperture and/or field of view or to decrease the overall footprint of the system, freeform optics have demonstrated unique capabilities to enable these advancements. However, it is critical to use concurrent engineering practices between optical design, mechanical design, manufacture, and metrology to ensure that the final system meets the required optical design specs after applying manufacturing tolerances. Metrology of freeform optics and performance testing of the three-mirror telescope will incorporate multiple measurement technologies and methods. Measurement errors are reduced through machine error corrections using a mathematical machine model and tilt induced error correction in mid-spatial frequency measurements using a scanning white light interferometer (SWLI). The form measurement uncertainty will be evaluated using the machine model and the Monte Carlo method for simulating possible measurement results taking into consideration the uncertainty in the machine error measurements and probe measurement uncertainty. Form measurements include contact and non-contact instruments with different sampling strategies. SWLI and stylus measurements for surface roughness are utilized for both areal and profile measurements. Design for metrology" aims to inform the design and manufacturing process definition as part of the concurrent engineering goal. By considering metrology in the initial design and manufacturing through ease of mounting and appropriate fiducials it is possible to reduce wasted effort and manufacturing time.