Chanan Gary, Troy Mitchell
Appl Opt. 2023 Aug 1;62(22):5982-5990. doi: 10.1364/AO.496259.
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.
目前正在建造的超大型分段镜望远镜（ELT），其分段时钟控制（面内旋转）会导致相邻分段间产生活塞误差，这一点已经得到充分证实。相比之下，凯克望远镜采用了一种截然不同的边缘传感器几何结构，原则上这种结构应使其对这种效应不敏感。然而，我们发现，由于凯克边缘传感器存在系统性的内部失调，实际上它们也受到与分段时钟相关的效应影响，这些效应与分段镜ELT预期的效应非常相似。因此，凯克望远镜为研究分段时钟控制及其相关效应提供了一个便利的试验台。对凯克望远镜相位数据的分析表明，分段时钟控制效应并非随机的，而是系统性的，并且会导致一种具有显著对称性的全局分段失调模式，在这种模式下，主镜呈现出梯田状结构。这种梯田模式在凯克望远镜上已存在一段时间，但直到最近才被理解为分段时钟控制的直接结果。对梯田模式的定量测量可用于诊断和校准凯克望远镜以及未来ELT的分段时钟控制效应。
