PHASING THE MIRROR SEGMENTS OF THE KECK TELESCOPES - THE BROAD-BAND PHASING ALGORITHM

To achieve its full diffraction limit in the infrared, the primary mir ror of the Keck telescope (now telescopes) must be properly phased: Th e steps or piston errors between the individual mirror segments must b e reduced to less than 100 nm. We accomplish this with a wave optics v ariation of the Shack-Hartmann test, in which the signal is not the ce ntroid but rather the degree of coherence of the individual subimages, Using filters with a variety of coherence lengths, we can capture seg ments with initial piston errors as large as +/- 30 mu m and reduce th ese to 30 nm-a dynamic range of 3 orders of magnitude. Segment aberrat ions contribute substantially to the residual errors of similar to 75 nm. (C) 1998 Optical Society of America.