Asymmetric Figuring
To reliably produce high precision optics (better than l/3 p-v), the final figuring process must be able to remove the asymmetries (surface irregularity) that were introduced in the previous manufacturing steps. One of the principle advantages of the MRF process is its ability to correct non-rotationally symmetric errors (see examples) using the advanced control features built into the Q22 software.

This extremely precise optical surface was produced in a total polishing time of six minutes using a two iteration MRF run (38mm diameter fused silica window, 90% of the aperature).

Material removal in MRF is achieved by dwell time. Simply stated, the longer any area of the lens is polished, the more material is removed. This dwell time variation is accomplished by varying the sweep rate of the lens through the MR fluid. Dwell times in axial zones are easily varied, and therefore rotationally symmetric features can be removed with great precision. For addressing asymmetric errors like astigmatism, our new concept is to vary the rotation rate of the spindle effectively to dwell longer on high zones than on low zones. If an optical surface has a saddle shape, in a single revolution, the lens spindle will rotate faster over the two low parts of the saddle to lower the dwell time, and slower over the two high parts of the saddle to increase the dwell time. Over many revolutions, the effect is the preferential polishing of the highs over the lows in the saddle, while simultaneously correcting the rotationally symmetric errors. The Q22 was specially designed to execute this demanding task, enabling even high order asymmetric figuring.

Example of a single iteration MRF asymmetric figuring run on a 127 mm diameter fused silica window.

The Q22 is installed and available for process demonstrations or part prototypes at QED's Rochester, NY facility. For more information about the Q22 production system, please contact Don Golini at 716-256-6540, fax 716-256-6541.