Null Lens Assembly for X-Ray Mirror Segments

Goddard Space Flight Center, Greenbelt, Maryland

A document discusses a null lens assembly that allows laser interferometry of 60° slumped glass mirror segments used in x-ray mirrors. The assembly consists of four lenses in precise alignment to each other, with incorporated piezoelectric nanometer stepping actuators to position the lenses in six degrees of freedom for positioning relative to each other.

Each lens is first installed and epoxied into a 410 stainless steel “cell.” The outer housing is designed to allow five degrees of freedom of the lens. The cell is placed onto a 3/8-in. (≈ 9-mm) ball bearing in the base of the housing, which provides a pivot point for the rotations, and allows slight x and y translations (microns) by allowing the cell to slide against it. Rotations are accomplished by 5 commercial picomotors that push on the cell in 30-nanometer increments. Spring plungers on the opposite side of the cell from the picomotors secure the cell in the housing.

The 410 stainless steel is used for the cell, baseplate, and rails because it has a low CTE (coefficient of thermal expansion) relative to most other metals. It is used exclusively in the “growth path” of the optical assembly so that when bulk temperature changes occur in the lab, the lenses will move a consistent amount apart from each other (which is a less sensitive factor in alignment), but will not tilt or rotate (alignment is very sensitive to rotations).

This work was done by David W. Robinson of Goddard Space Flight Center. For more information, download the Technical Support Package (free white paper) at www.techbriefs.com/tsp under the Physical Sciences category. GSC-15790-1