Lessons Learned Deploying a Second Generation Observation Control System for Sabaru Telescope

Subaru Telescope is deploying and commissioning a second-generation Observation Control System (OCS), building upon a 10 hear history of using the first generation OCS, and seeking to improve several key aspects of managing and using it. Replacing an extensive, functional, mission-critical software at the core of the telescope is an ambitious undertaking. In this paper we present some important and sometimes surprising lessons learned during the buildout and commissioning phase of the Generation 2 OCS at Subaru Telescope. We present our experience with the rewrite vs. refactor decision, aspects of testing including unit and functional tests, compatibility decisions regarding legacy systems, and managing telescope priorities vs. developer priorities.