VI.C Space Interferometer

A thermal-IR interferometer located in space would have obvious advantages in sensitivity to exozodiacal clouds and extrasolar planets over ground-based facilities (Bély et al. 1998). For a space mission where telescope diameter is especially critical to cost, the background power needs to be well known in advance for the success of the mission because noise depends on the ratio of (background intensity) to (4th power of telescope diameter).

Figure 11 a

Figure 11 b

Figure 11: Signal/Noise ratio for a space IR interferometer detecting Earth at 10 µm from a distance of 10 pc in 104 seconds integration. The family of curves shows sensitivies for a range of sizes of 4 unit telescopes along a 75-meter array baseline. The horizontal axis is density of the exozodiacal cloud in which the earth-like planet is embedded, in multiples of our 1-zodi cloud. "Struct = 0.0" means these models assume the exozodiacal cloud is perfectly smooth and structureless. a) Case of an interferometer located at r = 1 AU. b) Interferometer located at 5 AU.

Figures 11a and 11b compare the signal/noise ratios for exozodiacal dust detection by interferometers located at 1 and 5 AU from the Sun, respectively. The set of curves illustrate performance by a range of sizes for the 4 unit telescopes in a baseline Terrestrial Planet Finder (TPF) instrument design (ExNPS 1996).

An important performance criterion for exozodiacal dust and extrasolar planet detection is that telescope noise should be less than local zodiacal noise. By equating the corresponding surface brightnesses and solving for the temperature, an upper limit for the telescope temperature in terms of the telescope emissivity and local zodiacal brightness can be found. For a nominal value of emissivity the telescope temperature must be T_tel 69 K if the spacecraft is at 1 AU or 54 K if at 5 AU. There is no clear need to cool a space interferometer's telescopes much below these values (Traub et al. 1996).