GeminiFocus October 2017 - Page 21

Figure 1 (top left). A drive chain being extracted from the top of the arch girders. At this point, the tip of the crane is about 200 feet above ground level. Figure 2 (top right). Handling drive chains at the very top of the dome. Figure 3 (below left). Extracting a drive box from the dome. At this point the shutter structure, to the left in this photo, has been immobilized so that it will not move once disconnected from the drive chain. Gemini North Shutdown Ends On August 25th, Gemini North returned to regular night-time operations after an ex- tensive seven-week shutdown. As quoted, the project’s definition was to “place the shutter mechanism in an as-new state or better, and ensure that no major failure oc- curs in the coming 15 years or longer.” You may recall the failure of shutter drive boxes, which cost significant observing time in late 2013/early 2014, and again in in mid- 2014. Although we fixed those failures by in- stalling spares, a deep analysis of root causes revealed two fundamental issues: (1) the drive chains suffered differential stretching over time, and (2) the drive assemblies were not mechanically free enough within their drive boxes. These issues contributed to the further failure of a lower-shutter drive box in August 2016. At that time, we responded by locking the lower shutter in place until the work required to fix it could be planned and budgeted. As we said in the October 2016 is- sue of GeminiFocus, only complete replace- ment of the chains and refurbishment of the drive boxes would fully mitigate the chance of future failures. The drive chains are essentially stationary relative to the dome arch girders; the drive boxes are attached to the dome shutters, and crawl along these chains carrying the shutters with them. Extracting and replacing the drive boxes require pinning the dome shutters at various locations; the chains can only be replaced by extracting them from the very top of the dome and inserting new chains in the same way. The top of the Gem- ini dome is the highest point in the Pacific, thus we needed the largest crane on the is- land of Hawai’i to do the chain replacement. Figure 4 (right). The extracted drive being lowered to ground level for refurbishment. 19 GeminiFocus October 2017