On-orbit adjustment concepts for the Generation-X Observatory

Generation-X is required to be an X-ray observatory with 50 m2 effective collecting area and 0.1 arcsec half-power diameter (HPD) angular resolution at 1 keV. It is conceived that a launch vehicle such as that studied for the Ares V will carry a monolithic 16-m-diameter mirror to the earth-sun L2 point. Even with such a vehicle, the reflectors comprising the ~ 250 nested shells must be extremely light-weight. Therefore their figure and alignment cannot be achieved on the ground, and likely could not be maintained through the launch environment. We will present a conceptual solution to those constraints: adjustable X-ray optics, as a case of "adaptive" optics where the stability once in orbit should require adjustments no more frequently than yearly. The figure would be adjusted via thin-film actuators deposited directly to the back (non-reflecting) side of each element. This bi-morph configuration would impart in-plane strains via the piezoelectric or electrostrictive effect. Requirements of the adjustment are to the order of a few nanometer precision. Each shell, and each module, must also be aligned, to tolerances of about 0.1 micrometer. We conceive that on-orbit data would be acquired by a built-in Hartmann system for the alignment adjustments and low-order figure, and by ring profile measurements of a very bright celestial X-ray source to correct figure errors up to the mid-frequency range of several hundredths cycles mm^-1.