A modern approach to upgrading the telescope control system of the CTIO Blanco 4-m telescope

In preparation for the arrival of the Dark Energy Camera (DECam) at the CTIO Blanco 4-m telescope, both the hardware and the software of the Telescope Control System (TCS) have been upgraded in order to meet the more stringent requirements on cadence and tracking required for efficient execution of the Dark Energy Survey 1 . This upgrade was also driven by the need to replace obsolete hardware, some of it now over half a century old. In this paper we describe the architecture of the new mount control system, and in particular the method used to develop and implement the servo-driver portion of the new TCS. This portion of the system had to be completely rethought, when an initial approach, based on commercial off the shelf components, lacked the flexibility needed to cope with the complex behavior of the telescope. Central to our design approach was the early implementation of extensive telemetry, which allowed us to fully characterize the real dynamics of the telescope. These results then served as input to extensive simulations of the proposed new servo system allowing us to iteratively refine the control model. This flexibility will be important later when DECam is installed, since this will significantly increase the moving mass and inertia of the telescope. Based on these results, a fully digital solution was chosen and implemented. The core of this new servo hardware is modern cRIO hardware, which combines an embedded processor with a high-performance FPGA, allowing the execution of LabVIEW applications in real time.