NASA Exploring Freeform Optics for Compact Space Telescopes

Freeform optics technology allows telescope mirrors to take almost any shape, potentially improving image quality over a larger field of view.

Prompted by advances in computer-controlled fabrication and testing, NASA engineers are now using freeform optics to explore cost-effective alternatives to more traditional space telescope missions, such as CubeSats and other small satellites.

"If you want to put these telescopes into a smaller box, you need to let the mirrors bend like a potato chip," said Joseph Howard, an optical engineer at NASA's Goddard Space Flight Center.

Telescopes have traditionally used rotationally symmetric optics (top). Freeform optics (like the one at bottom) take asymmetrical shapes and are being investigated for use in space-based instruments. Courtesy of NASA.

Traditional two-mirror telescopes consist of a primary light-gathering mirror and smaller, secondary mirrors, which relay the incoming light and direct it onto a detector. The rotationally symmetric (i.e., round) mirrors need to be aligned along the axis of the system to reduce optical aberrations that produce blurry images.

Asymmetric mirrors produced using freeform optics can better correct for these aberrations to provide a larger usable field of view, as well as dramatically reduce the light path, or package size.

As part of their research effort, Howard and Goddard engineer Garrett West evaluated the optical system of a coastal measurement instrument originally equipped with nine symmetrical mirrors. By replacing the mirrors with freeform optics, they reduced the size the mirrors and decreased their number to six, shrinking the telescope's overall packaging more than tenfold.

Next year, the team plans to continue testing a two-mirror instrument, which includes a 3D-printed freeform mirror. With this additive manufacturing technique, a computer-controlled laser melts material in precise locations as indicated by computer-aided design. Because the mirror will be constructed layer by layer, it will be possible to construct a mirror with any shape.

The technology could prove to be game-changing for a number of future missions, including instruments for imaging exoplanets. Howard and West have established the Freeform Optics Research Group Endeavor to oversee freeform optics research carried out by private industry under NASA's Small Business Innovative Research program and Goddard scientists and engineers.