Terrace mode of the Keck telescope primary mirror.

It has been well-established that for the extremely large segmented-mirror telescopes (ELTs) currently under construction, segment clocking (in-plane rotation) will result in piston errors between neighboring segments. By contrast, the Keck telescopes utilize a fundamentally different edge sensor geometry, which should in principle render them insensitive to this effect. However, we show that as the result of a systematic internal misalignment of the Keck edge sensors, they in fact suffer from clocking-dependent effects that are remarkably similar to those anticipated for segmented-mirror ELTs. The Keck telescopes thus provide a convenient testbed for studying segment clocking and its associated effects. Analysis of Keck phasing data shows that the segment clocking effects are not random, but systematic and lead to a global segment misalignment mode of striking symmetry in which the primary mirror assumes a terraced structure. This terrace mode has been known for some time at Keck but has only recently been understood to be a direct consequence of segment clocking. A quantitative measurement of terrace mode can be used to diagnose and calibrate segment clocking effects at Keck and for future ELTs.