Researchers are currently working on an "antihole" photon sieve that would measure nearly 20 meters across and eventually consist of trillions of holes. This would give space telescopes the ability to provide an extremely sharp focus.
This foreseen leap in space-telescope expertise has its own interesting history. Traditionally, space exploration has been the forte of ground-based telescopes, which require precise optics for their primary optical and focusing elements. Due to advancements in space exploration and humanity's increased curiosity about outer space, the focus turned towards large ground-based telescopes. Some ground-based telescopes use large primary mirrors, some of which are nearly 10 meters across, to gather even the faintest light coming from distant celestial objects.
However, the turbulence in the earth's atmosphere presents a variety of problems, such as extinction, seeing, and blurring, which limit the role of ground-based telescopes in space exploration. Furthermore, the earth's atmosphere absorbs a large portion of the electromagnetic spectrum, thereby blocking faint light entering the atmosphere from deep space.
As scientists explored various options, the idea of mounting a telescope in space gained prominence. Because there is no atmosphere in space, the images produced from a space telescope are better in image quality than those produced from ground-based telescopes. The Hubble Space Telescope (HST), launched in 1990, represented a quantum improvement in the quality and depth of space-exploration images.
The HST uses a mirror that is only 2.4 meters in diameter. A minor polishing defect in this mirror caused a spherical aberration that severely limited the operations of the telescope until astronauts on several space missions installed corrective optics aboard the HST to rectify the problem.
Developing better imaging technology is a challenge for engineers, as next-generation space technology will require the use of apertures that are more than 20 meters in diameter. Currently, payload technology restricts deployment of conventional telescope primaries of such dimensions. Recently, researchers have concentrated on utilizing curved "gossamer" membranes inflatable under zero-gravity conditions in space. However, the optical quality is not up to the levels required for space-based observations.
Researchers say that a photon sieve approximately 20 meters in diameter will be relatively easier and simpler to deploy. They propose that such a sheet could take the form of a flat membrane. Compared to deploying large mirrors or curved membranes in space, it is easier to roll out this device on a flat plane in space. Further, as in the case of other primaries, the photon sieve membrane does not require any supporting structures.
Besides its use in space telescopes, this technology has many other uses. One such application is the location of distant planetary systems using large space telescopes. Photon sieves also have tremendous potential applications in a variety of industries, including photolithography, electron-beam imaging, and others. Photon sieves can also be used in the construction of inexpensive "null correctors" for determining the curvature of parabolic surfaces.
While the future looks promising for space telescopes, NASA is currently burdened with its fleet of aging shuttles. NASA has already lost two shuttles in mission-related accidents. Other orbiters constantly face mission-related safety concerns. At a time when developments in space technology look promising, it seems NASA needs to overhaul its current orbiters and consider building a new orbiter to provide the necessary momentum for a space-telescope program.
