A real-time technique for optical alignment of telescopes with segmented optics

While telescopes with segmented optics (currently Keck and HET and in the future GTC, CELT, GSMT, NGST, etc) present extra challenges in terms of optical alignment, they also present the opportunity for using an alignment technique not available to telescopes with monolithic optics. We present a technique for aligning telescope secondary mirrors utilizing the segmented nature of the primary. The data required is gathered in direct image mode and can be collected from science instrument detectors (as compared to a wavefront sensor). From this data aberrations (focus and coma) are calculated from which secondary piston and tip/tilt (or decenter) corrections are determined. In addition, tip/tilt corrections for each of the primary mirror segments can also be calculated. Furthermore, other aberrations are available to determine other alignment or support issues including differentiating secondary tip/tilt from decenter, focal surface tilt, and instrument aberrations. This technique has been used nightly on the Keck I and II telescopes over the last 8 years and has made a significant improvement in image quality.