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The Atacama Large Millimeter Array (ALMA) is an international telescope project currently under construction in the Atacama desert of Chile. It has a provision for 64 antennas of 12m each, arranged over a geographical area of a few square kilometers. Antenna control and correlated data acquisition is implemented by means of a distributed set of realtime Linux computers, each one hosting ALMA Common Software (ACS) based applications and connected to a common time base distributed by the ALMA Master Clock as a 48ms electronic pulse signal (time event). All these computers require to be time synchronized for achieving coordination between commands and data acquisition. For this purpose, the ArrayTime system presented here implements a real-time software facility that makes possible to unambiguously time-stamp each time event arriving at each computer node (distributed clock), relative to an external time source of 1Hz and in phase to the TAI second. Array time is the absolute time of each time event, and synchronization of distributed clocks is resolved by communicating the array time, via ACS services, for the next time event interrupt at least once during the operational cycle of the distributed clock. Thereafter, it is possible to schedule application tasks within a latency range of 100us by extrapolating from the last interrupt and based on the current CPU Time Stamp Counter (TSC) and the estimated frequency of the CPU clock. In the following, we present a description of the elements that constitute the ArrayTime facility.

The Atacama Large Millimeter Array (ALMA) will, when it is completed in 2012, be the world's largest millimeter & sub-millimeter radio telescope. It will consist of 64 antennas, each one 12 meters in diameter, connected as an interferometer. The ALMA Test Interferometer Control System (TICS) was developed as a prototype for the ALMA control system. Its initial task was to provide sufficient functionality for the evaluation of the prototype antennas. The main antenna evaluation tasks include surface measurements via holography and pointing accuracy, measured at both optical and millimeter wavelengths. In this paper we will present the design of TICS, which is a distributed computing environment. In the test facility there are four computers: three real-time computers running VxWorks (one on each antenna and a central one) and a master computer running Linux. These computers communicate via Ethernet, and each of the real-time computers is connected to the hardware devices via an extension of the CAN bus. We will also discuss our experience with this system and outline changes we are making in light of our experiences.

The Atacama Large Millimeter Array (ALMA) is a joint project between astronomical organizations in Europe and North America. ALMA will consist of at least 64 12-meter antennas operating in the millimeter and sub-millimeter range, with baselines up to 14 km. It will be located at an altitude above 5000m in the Chilean Atacama desert. The ALMA Common Software (ACS) provides a software infrastructure common to all partners and consists of a documented collection of common patterns and of components that implement those patterns. The heart of ACS is an object model based on Distributed Objects (DOs), implemented as CORBA objects. The teams responsible for the control system development use DOs as the basis for components and devices such as an antenna mount control. ACS provides common CORBA-based services such as logging, error and alarm management, configuration database and lifecycle management. A code generator creates a Java Bean for each DO. Programmers can write Java client applications by connecting those Beans with data-manipulation and visualization Beans. ACS is based on the experience gained in the astronomical and particle accelerator domains, and reuses and extends proven concepts and components. Although designed for ALMA, ACS can be used in other new control systems, since it implements proven design patterns using state of the art, stable and reliable technology. This paper presents the architecture of ACS and its status, detailing the object model and major services.

The ESO VLT control system is based on a distributed computer architecture, where a number of workstations are interconnected. They provide user interfaces and high level functions, while real-time control is handled by some 120 - 150 microprocessors, embedded in Local Control Units.

The New Technology Telescope (NTT) installed at the Observatory of ESO, La Silla, Chile, employs a 3.5m active primary mirror and foresees that two instruments are mounted all the time at the Nasmyth foci. Remote observations from Germany will also be possible. The NTT is in many ways a prototype for ESO's Very Large Telescope (VLT), an array of four 8m telescopes now in the construction phase. This applies also to the control/acquisition system software, developed for the NTT, i.e. the software environment where specific control programs for telescope, adapters and instruments are running. Characteristic aspects of this system, which allows simultaneous multi instrument and multi-user operation, are described in this paper. The system is intrinsecally distributed both in hardware (several microprocessors on Ethernet) and in software (CPU independent program communication). System-wide information and complete decoupling of control programs from the user-end are obtained via a central parameters database (Pool) which supports data both on disc and memory, for time critical operations. This allows local and remote users to access in real-time all information on telescope and instruments without directly interfering with control programs. It permits also to have a truly open system with coherent expansion as soon as new modules are added. The distribution of the Pool on many CPUs and remote access methods will allow to develop a portable user-end for the remote use of the NTT and the VLT. This will be implemented on a workstation accomodating the user-end both for image processing and control and will be software configurable to act as a local control console of one or several telescopes and instruments or a remote observing tool useable from several astronomical Institutes in Europe.