Manufacturing of a freeform phase plate for suppression of diffraction in an astronomical telescope

The addition of a high resolution encoder to the spindle of a standard diamond turning lathe has allowed for precision control of angular rotation. With three precision controlled axes, (rotational C, linear X and linear Z), tool path programs can be defined in cylindrical coordinates, which enables the production of freeform geometries. Optical designers are now exploring complex shapes that were previously unachievable. These shapes range from long radius toroids to freeform wavefront corrector plates. From a manufacturing point of view, interfacing between optical design programs, fabrication equipment, and metrology equipment often proves to be the most difficult part of the production process. The optical design must be translated into a tool path for the diamond turning lathe, and in some cases the design must be imported into the metrology software for surface comparison. The purpose of this report is to inform the reader about some of these manufacturing challenges using one specific example: a freeform phase plate that suppresses diffraction in an astronomical image and enhances searches for extrasolar planets around nearby stars. Designed by Johnan Codona and Roger Angel from the University of Arizona, this ZnSe lens has many ridges and valleys that have been optimized to reduce the 4 micron wavelength light observed from a nearby star to a level that makes planet detection possible. The phase plate had an aperture of 4.44mm and was placed on a 12.7mm diameter 4mm thick substrate. Surface feature size was approximately 2.5 micron peak-to-valley. In on-sky testing, the optic attenuated diffracted light from the star approximately 100 fold.