<title>Effects Of Primary Mirror Segmentation On Telescope Image Quality</title>

Abstract

A number of designs for future astronomical telescopes call for large primary mirrors that are mosaics of smaller mirrors. We describe here a study of some characteristics of the images expected from a telescope with a primary mirror composed of 36 hexagonal segments. Various effects caused by the segmentation and imperfections in the segment fabrication and control have been analyzed using physical optics. The diffraction-limited image distribution from the segmentation geometry of the primary is derived, and the diffraction spikes are shown to be similar to those caused by secondary support struts in existing telescopes. A general relation between surface quality and image quality is given, and the implications for surface quality tolerances are discussed. The optical effects of segment phasing errors are derived, showing that at least for visible light observations the phasing is unimportant. For observations at 10 μm near diffraction-limited perfor-mance can be achieved with a 10 meter aperture requiring that the segments be phased correctly at this wavelength.