  The Cherenkov Telescope Array (CTA) is the next generation gamma-ray
observatory. CTA will incorporate about 100 imaging atmospheric Cherenkov
telescopes (IACTs) at a southern site, and about 20 in the north. Previous IACT
experiments have used up to five telescopes. Subsequently, the design of a
graphical user interface (GUI) for the operator of CTA poses an interesting
challenge. In order to create an effective interface, the CTA team is
collaborating with experts from the field of Human-Computer Interaction. We
present here our GUI prototype. The back-end of the prototype is a Python Web
server. It is integrated with the observation execution system of CTA, which is
based on the Alma Common Software (ACS). The back-end incorporates a redis
database, which facilitates synchronization of GUI panels. redis is also used
to buffer information collected from various software components and databases.
The front-end of the prototype is based on Web technology. Communication
between Web server and clients is performed using Web Sockets, where graphics
are generated with the d3.js Javascript library.
