Advanced Concepts Studies

A 16-20 m Telescope to
Search for Extrasolar Life

In a series of papers published in 1986 and 1987 by Roger Angel of the Stewart Observatory a case is made for the potential detection of extrasolar life through the use of a 16-20 m aperture space telescope operating at 10 micron wavelength. The large mirror needed for the infrared was proposed to be made of segments. These telescope concepts for earthlike planet detection were further analyzed by Angel and an optimal configuration was proposed. Instead of a 16 m circular aperture Angel derived a diamond configuration with four 8-m circular subapertures to eliminate diffraction spikes and provide excellent star rejection down to planet separations of 0.1 arc sec at 10 mm. The analyses show that the major advance in technology required to build the telescope is in mirror fabrication. Off-axis segments need to be polished to very high accuracy. The tolerance on phase errors is very severe. According to Angel simply to overcome the photon noise of scattered starlight requires a gain of _ 10E7. For example the surface amplitude (rms) would have to be in the order of 0.6 nm on a scale of 40 cm. For a comparison the HST primary mirror, probably the most perfect mirror made to date, has an rms amplitude of ~ 5 nm on a scale of 40 cm for visible light.