Image quality for large segmented space telescopes

Some proposed architectures for large telescopes in space such as the Next Generation Space Telescope (NGST) utilize a segmented primary mirror with active control. The image quality for those systems can be characterized by the quality of the point spread function (PSF). We will present results from our simulations that demonstrate the quality of the PSF at various wavelengths, as characterized by the metrics of Strehl Ratio and encircled energy fraction. These metrics capture the critical features of the PSF that ensure good spatial resolution of the core at the nominal diffraction limited wavelength without having excessive scattering into the nearby diffraction pattern for observations at shorter wavelengths. The relationship of the image quality to the spatial frequency content of the system wave front error and to the image motion that occurs during the exposure (line-of-sight jitter) are explored.