In the world of telescopes, light gathering power is dictated by the physical size of the first mirror or lens exposed to the sky. The larger the “aperture” the more light gathering power the instrument has. With ground-based scopes, this is not much of an issue—just build a bigger objective lens or mirror. For space-based telescopes, however, this presents a huge problem, as the size of the scope package is constrained by the physical dimensions and lifting capacity of the launch vehicle. The new field of “Freeform Optics,” while not changing the rules of telescope design, is certainly changing the playing field, enabling telescopes to be shoehorned into smaller or more complex spacecraft volumes.
Traditionally, telescopes have been made using rotationally symmetrical geometries, where the mirrors and lenses are symmetrical about a central optical axis (they are round) and are oriented orthogonal to the incoming light path. Thus, the light gathering power is limited by how “wide” the telescope tube can be made. On the other hand, the physical length of telescopes, which is determined by their focal lengths, can be made shorter by folding the light back upon itself by reflecting it from mirrors and using clever schemes to fit a long light cone into a short instrument.
An example of this is the “Cassegrain” design, whereby light is reflected from a large primary mirror to a smaller secondary mirror and back through a central hole in the primary to a focal plane behind it. The Hubble Space Telescope is of this design. However, the diameter of Hubble’s primary mirror was limited by how wide a telescope tube could be fit into the payload bay of the space shuttle carrying it to orbit. The mirrors in such a design are either of a parabolic or spherical shape and matched to each other so that from any point in the field of view the distance a beam of light has to travel to the focal point is the same, maintaining focus across the field of view and minimizing optical aberrations. These symmetrical designs, while requiring precise control of the mirror’s shape and surface accuracy, are relatively easy to manufacture because of the symmetry of the shapes used.
With the desire for higher light gathering power from existing launch vehicle size constraints, or high performance small telescopes in small CubeSat packages, comes the need to be able to use all available space. This means that the luxury of using a round mirror has to be set aside in favor of using a possibly warped or odd-shaped reflecting surface in its place to maximize the aperture.
For instance, what if the Hubble telescope designers had been able to turn its main mirror at an acute angle, fold it like a potato chip, and utilize most of the length of a shuttle payload bay to fit one dimension of its main mirror rather than just the width of the bay? Even though the complexity of the light path would be increased, so would the total aperture and therefore the light gathering power. Freeform optics, which uses computer modeling and manufacturing of odd-shaped reflective surfaces, makes this possible. As long as the total length of any light ray traveling through the system to the detector is the same, the image will be in focus.
Non-symmetrical reflecting antennas have been used for decades in communications satellites to create spot beam patterns to cover specific geographical areas on Earth and to provide increased performance within size limits. Radio waves behave exactly like light waves, with the only difference being their lower frequency and longer wavelength, therefore it is possible to use the same techniques in optical systems if the surfaces could be made accurately enough. The accuracy requirements of a reflective surface are specified in fractions of a wavelength, so for microwave radio antennas the surface accuracy requirements are stated in millimeters, while in optical reflectors are stated in nanometers. This is why freeform techniques are just now becoming practical for manufacturing telescopes and other optical instruments.
NASA optical engineers at the Goddard Space Flight Center in Greenbelt, Md., are anxious to develop this new technology. To that end, they have established the Freeform Optics Research Group (FORGE), led by engineers Garrett West and Joseph Howard. The group oversees freeform optical research carried out by private firms under NASA’s Small Business Innovative Research program, as well as by Goddard scientists and engineers. The group has already developed and implemented freeform deign practices in Goddard’s own Optical Design Laboratory (the ODL), which provides design and engineering assistance for instrument proposal efforts.
“The use of freeform optics can significantly reduce the package size as well as improve the image quality,” said Joseph Howard. He and his colleague Garrett West are working to design, build, and test a two-mirror optical telescope for imaging and spectroscopic applications. They believe that the technology holds great promise for the development of compact telescopes and optical instruments to be used in CubeSat and other small satellite platforms, thus providing a cost-effective alternative to larger packages that are more expensive to build and launch.
“If you want to put these telescopes into a smaller box, you need to let the mirrors bend like a potato chip,” Howard explained. The team was recently able to replace nine traditional symmetrical mirrors in a coastal measurement instrument with only six computer-optimized freeform mirrors, shrinking the size of the telescope more than tenfold. With a smaller number of reflecting surfaces, the image quality is also improved.
Future plans involve continuing to test and improve their two-mirror telescope design to include 3-D printed optical parts. In this process, called additive manufacturing, a computer-controlled laser melts material and constructs a mirror layer by layer, thus making it possible to construct a mirror of virtually any shape.
Howard and West predict that the new technology will open a new door in space-based telescopes, possibly being key to imaging exoplanets. “NASA will benefit. Freeform optics will be critical. They will enable larger fields of view and fit in size-limited packages, such as those found in CubeSats and small satellites, or on larger missions where space allocations are tight,” Howard said.