Paper Synopsis: Lens Centering using the Point Source Microscope Robert E. Parks by Wenrui Cai Opti521

The Importance of lens centering Modern lens design, Interferometric testing of optical surfaces, and Computer controlled polishing make it possible to produce excellent optical elements. However, all the potential gain in performance can be lost at the last step of assembly. Thus it is important that there is a method of assuring the lens elements are well centered in their lens cell.

Definition of Centering The optical axis Centering is moving (tilt and decenter) optical surfaces until their axis lies on the mechanical axis defined by the lens barrel. We will use an air bearing rotary table to help with centering as well as the PSM.

Point Source Microscope (PSM) Objective focus at CC. of the surface A return spot from the surface focused on CCD Viewing spot on a monitor. If the alignment is less than ideal, the spot will defocused or decentered. When the cell is rotating about its axis, the return spot will move in a circle on screen.

Viewing both centers of curvature The alignment of one center may disturb the centering of the other. It is more expedient to view both centers of curvature simultaneously With a convex surface we need to add a relay lens to find the center of curvature. Use Zemax to calculate the distances among the lenses and the PSM the location of the upper surface center of curvature and the rear surface center after refraction in the front surface.

Viewing both centers of curvature Separate two centers of curvature Only light from the correct conjugate is well focused and bright on the detector of that PSM. All calculations (locations of beamsplitter and PSMs) are first order. what we are ultimately interested in is whether the relayed return spots from the centers of curvature move or not as the rotary table is turned.

Sensitivity of Centering Spot motion (rotate 180) is 4 μm for a 1 μm decenter from the rotary table axis. there is another factor of 2.5 for 5X objective. image motion at the detector is 10 μm. The detector has 4.65 μm pixels and can centroid to 0.1 pixels. A theoretical sensitivity to surface decenter of about 50 nm.

Conclusion The paper showed how to implement such a method to assure the centering as lenses are assembled using a precise rotary table and a PSM. The method can sense the CC. of both surfaces of an element so that any errors in tilt and decenter can be corrected during assembly to the sub- micron and sub-second level.