MRI: Development of an Improved Keck II Laser Launch Facility

地面上的望远镜必须通过地球大气层的干扰来观测深空中的物体。当来自遥远物体的光穿过大气层时，由于风的切变以及大气层内温度和压力的变化，它被湍流扩散开来，这有效地降低了地面望远镜的分辨率。这一困难在很大程度上可以通过使用激光导星(LGS)和自适应光学(AO)的相对较新的技术来克服，在这种技术中，明亮的激光照射目标对象附近的一片天空。激光被上层大气中的钠原子吸收，然后由钠原子重新发射，位于干扰较低的大气的大部分上方。然后，返回的信号被用于跟踪光的随时间变化的大气失真。自适应光学利用这些信息来快速调整望远镜仪器中的光学元件，以将图像恢复到(几乎)从大气层上方所能看到的状态。就在几年前，W.M.凯克天文台开发并安装了第一批成功的LGS AO系统之一。就科学生产率而言，凯克在天文学方面遥遥领先于世界天文学团体。凯克望远镜的时间是极其宝贵的，其有效性的任何提高都肯定会在科学回报中带来可观的回报。今天正在进行的一些最令人兴奋的一线天文研究来自Keck AO系统。这两个10米长的望远镜拥有当今使用的最大的反射望远镜镜面。由于光学系统的固有分辨率随其物镜的大小而变化，因此有必要使用自适应光学来接近衍射限制的分辨率。现在，Keck LGS系统将进行重大升级，用新设计和改进的系统取代目前位于望远镜支架一侧的激光发射投影仪，该系统将放置在望远镜光轴上。这次升级将减少波前传感系统看到的透视伸长。新系统将显著提高分辨率、对比度和动态范围，并将导致天体测量和光度学性能提高高达两倍。

Telescopes on the ground must observe objects in deep space through the interference of the earth's atmosphere. As light from a distant object passes through the atmosphere it gets spread out by turbulence due to wind shear and changes in temperature and pressure within the atmospheric layers, which effectively diminishes the resolving power of earth-based telescopes. This difficulty can be overcome to a large extent by relatively new techniques employing Laser Guide Stars (LGS) and Adaptive Optics (AO) where a bright laser illuminates a patch of sky near the target object. The laser's light is absorbed and then re-emitted by sodium atoms in the upper atmosphere, above much of the interfering lower atmosphere. The return signal is then used to track the time-dependent atmospheric distortion of the light. Adaptive Optics uses this information to rapidly adjust optics in the telescope's instrumentation to restore the image to (nearly) what would be seen from above the atmosphere. The W.M. Keck Observatory developed and installed one of the first successful LGS AO systems at an astronomical observatory just a few years ago. Keck has led the world-wide astronomical community in AO in terms of scientific productivity by a significant margin. Keck telescope time is extremely valuable and any increase in its effectiveness will certainly pay significant payoffs in science return. Some of the most exciting front-line astronomical research being done today comes from the Keck AO system. The twin 10-meter telescopes house the largest reflecting telescope mirrors in use today. Because the native resolution of an optical system scales with the size of its objective, it is necessary to use adaptive optics to approach diffraction-limited resolution. Now the Keck LGS system will undergo a significant upgrade, replacing a laser launch projector currently on the side of the telescope mount with a newly designed and improved system to be placed on the telescope optical axis. This upgrade will reduce the perspective elongation seen by the wavefront sensing system. The new system will have significantly improved resolution, contrast, and dynamic range and will lead to improved astrometric and photometric performance by up to a factor of two.