Aberration Correction In A Telescope With A Segmented Primary

In a Cassegrain telescope with a segmented primary mirror, wavefront aberrations are introduced by segment figure errors, segment positioning errors, secondary figure errors, and secondary positioning errors. We give here an analytic description of these errors. The various sources contribute different aberrations to the wavefront. These differences suggest an ordering to the aberration measurement and correction, and allow options in alignment procedures and tolerances. The segmentation and control of the primary mirror segments allow the partial correction of figure aberrations in the secondary mirror and aberrations caused by secondary misalignment. We use the W. M. Keck Ten Meter Telescope as an example for these considerations. The ten-meter diameter actively-controlled primary mirror is a mosaic of thirty-six hexagonal segments and is currently under construction.