The execution of a mission as ambitious and challenging as TPF requires careful evaluation of potential technological approaches, often referred to by mission planners as "mission architectures." After an extended period of study and evaluation, NASA in April 2004 decided to move forward with TPF as a suite of two complementary architectures: a visible-light coronagraph and a mid-infrared formation-flying interferometer. The combination of these two space observatories will provide definitive characterization of planets.

The visible-light concept uses a single telescope with an effective diameter of 6.5~8 meters, operating at room temperature, that must achieve a billion to one image contrast. Very precise, stable control of the telescope optical quality is required.

Infrared TPF concepts call for multiple, smaller telescopes (3~4 meter) configured as an interferometer and spread out over a large (<40 meter) boom or operated on separated spacecraft over distances of a few hundred meters. The telescopes must operate at extremely low temperatures, and the observatory would necessarily be larger. However, the image contrast requirement is much easier at infrared wavelengths -- only a million to one -- and thus the system optical quality is easier to achieve.

In its simplest form, a coronagraph blocks the direct and diffracted light of a bright object so that faint nearby objects and structures can be seen. Coronagraphs have been used this way to study the corona surrounding our Sun and to search for sub-stellar companions (failed stars known as brown dwarfs) of nearby stars.

Using this technique to study the area around a nearby star requires dealing with the diffraction of light around the edges of the telescope, which detracts from the potential angular resolution of the image.

The diffraction pattern of a simple round telescope, for example, is a series of concentric rings with a bright central spot. Blocking the light from a star in order to see an orbiting planet requires suppressing the first several bright rings without blocking out the planet. By using masks to simulate a telescope with a different effective shape, the diffraction pattern can be controlled so that the starlight is much dimmer closer to the center in some areas, and brighter in others. The telescope can be rotated about its line-of-sight so that the planet image passes in an out of the regions where the starlight is dim.

Managing this diffraction pattern isn't too difficult -- there are a number of options available to accomplish this.

A more critical issue for TPF is wavefront control, which must be mastered in order for a visible-light TPF to work. This includes correcting for imperfections in the optics, which scatter light and degrade image contrast.

To correct for its own internal imperfections, TPF would use active optics, similar to the technology that the Keck Observatory and other ground-based telescopes use to correct for wavefront distortion in the Earth's atmosphere, though not operating at such a high rate.

TPF designs operating at visible wavelengths offer several advantages. At shorter wavelengths, a smaller telescope can obtain the required resolution. Optical detectors require less thermal control, reducing the need for onboard cooling. A visible-light telescope can operate at room temperature, while a telescope operating in the thermal infrared must be cooled to about 40 degrees Kelvin.

A variety of missions have been proposed that would demonstrate technologies using a 1.8 meter space telescope, a coronagraphic camera and precision active optics for control of scattered light to search for Jupiter-size planets in advance of a larger mission to look for Earths.

Reconstruction of a TPF interferometric observation of a terrestrial planet 33 light-years away.

Using interferometers to study distant planets allows smaller, widely separated mirrors to work together as a giant virtual telescope. The resolution obtained would be the same as one would get from a telescope as big as the separation between the individual telescopes.

To get enough of this information to build up a good picture, the interferometer must rotate around its line of sight to different relative positions and repeat the "exposures." As well as taking a picture, an interferometer can obtain spectra of the targets it is looking at.

Prototype TPF interferometer concepts involve placing the telescope array on a physical structure or flying multiple telescopes in formation along with a central spacecraft that houses the beam-combining apparatus.

As a precursor, the Space Inteferometry Mission will demonstrate spaceborne interferometry and the pathlength control needed for TPF interferometry.

For a detailed introduction to the technology of combining and canceling light through interferometery, see Technology > Interferometry.