Nicholas D. Whyborn

ALMA array element astronomical verification

The Atacama Large Millimeter/submillimeter Array (ALMA) will consist of at least 54 twelve-meter antennas and 12 seven-meter antennas operating as an aperture synthesis array in the (sub)millimeter wavelength range. The ALMA System Integration Science Team (SIST) is a group of scientists and data analysts whose primary task is to verify and characterize the astronomical performance of array elements as single dish and interferometric systems. The full set of tasks is required for the initial construction phase verification of every array element, and these can be divided roughly into fundamental antenna performance tests (verification of antenna surface accuracy, basic tracking, switching, and on-the-fly rastering) and astronomical radio verification tasks (radio pointing, focus, basic interferometry, and end-to-end spectroscopic verification). These activities occur both at the Operations Support Facility (just below 3000 m elevation) and at the Array Operations Site at 5000 m.

Assembly, integration, and verification (AIV) in ALMA: series processing of array elements

The Atacama Large Millimeter/submillimeter Array (ALMA) is a joint project between astronomical organizations in Europe, North America, and East Asia, in collaboration with the Republic of Chile. ALMA will consist of at least 54 twelve-meter antennas and 12 seven-meter antennas operating as an aperture synthesis array in the (sub)millimeter wavelength range. It is the responsibility of ALMA AIV to deliver the fully assembled, integrated, and verified antennas (array elements) to the telescope array. After an initial phase of infrastructure setup AIV activities began when the first ALMA antenna and subsystems became available in mid 2008. During the second semester of 2009 a project-wide effort was made to put in operation a first 3- antenna interferometer at the Array Operations Site (AOS). In 2010 the AIV focus was the transition from event-driven activities towards routine series production. Also, due to the ramp-up of operations activities, AIV underwent an organizational change from an autonomous department into a project within a strong matrix management structure. When the subsystem deliveries stabilized in early 2011, steady-state series processing could be achieved in an efficient and reliable manner. The challenge today is to maintain this production pace until completion towards the end of 2013. This paper describes the way ALMA AIV evolved successfully from the initial phase to the present steady-state of array element series processing. It elaborates on the different project phases and their relationships, presents processing statistics, illustrates the lessons learned and relevant best practices, and concludes with an outlook of the path towards completion.

ALMA band 10 (787-950 GHz) first astronomical fringes

The Atacama Large Millimeter/submillimeter Array (ALMA) Band 10 receiver covering 787 to 950 GHz is the highest frequency receiver of the ten bands envisioned for the ALMA Front End system. The Band 10 receivers have been undergoing installation and commissioning since 2012. After the Band 10 receiver tuning scripts (Josephson currents suppression, LO power optimization) and operation procedures had been developed and implemented, astronomical verification procedures (radio pointing, focus, beam squint, and end-to-end spectroscopic verification) were established in single dish mode at the ALMA Operations Support Facility (OSF; 2900 m elevation). Subsequently, the first Band 10 astronomical fringes were achieved at the Array Operations Site in October 2013 (AOS; 5000 m elevation). This is the highest frequency ever achieved by a radio interferometer and opens up a new window into submillimeter astrophysics.

Engineering within the assembly, verification, and integration (AIV) process in ALMA

The Atacama Large Millimeter/submillimeter Array (ALMA) is a joint project between astronomical organizations in Europe, North America, and East Asia, in collaboration with the Republic of Chile. ALMA will consist of at least 54 twelve-meter antennas and 12 seven-meter antennas operating as an interferometer in the millimeter and sub-millimeter wavelength range. It will be located at an altitude above 5000m in the Chilean Atacama desert. As part of the ALMA construction phase the Assembly, Verification and Integration (AIV) team receives antennas and instrumentation from Integrated Product Teams (IPTs), verifies that the sub-systems perform as expected, performs the assembly and integration of the scientific instrumentation and verifies that functional and performance requirements are met. This paper aims to describe those aspects related to the AIV Engineering team, its role within the 4-station AIV process, the different phases the group underwent, lessons learned and potential space for improvement. AIV Engineering initially focused on the preparation of the necessary site infrastructure for AIV activities, on the purchase of tools and equipment and on the first ALMA system installations. With the first antennas arriving on site the team started to gather experience with AIV Station 1 beacon holography measurements for the assessment of the overall antenna surface quality, and with optical pointing to confirm the antenna pointing and tracking capabilities. With the arrival of the first receiver AIV Station 2 was developed which focuses on the installation of electrical and cryogenic systems and incrementally establishes the full connectivity of the antenna as an observing platform. Further antenna deliveries then allowed to refine the related procedures, develop staff expertise and to transition towards a more routine production process. Stations 3 and 4 deal with verification of the antenna with integrated electronics by the AIV Science Team and is not covered directly in this paper. It is believed that both continuous improvement and the clear definition of the AIV 4-station model were key factors in achieving the goal of bringing the antennas into a state that is well enough characterized in order to smoothly start commissioning activities.

ALMA Long Baseline Campaigns: Phase Characteristics of Atmosphere at Long Baselines in the Millimeter and Submillimeter Wavelengths

This paper presents the first detailed investigation of the characteristics of mm/submm phase fluctuation and phase correction methods obtained using ALMA with baseline lengths up to ~15 km. Most of the spatial structure functions (SSFs) show that the phase fluctuation increases as a function of baseline length, with a power-law slope of ~0.6. In many cases, we find that the slope becomes shallower (average of ~0.2-0.3) at baseline lengths longer than ~1 km, namely showing a turn-over in SSF. The phase correction method using water vapor radiometers (WVRs) works well, especially for the cases where PWV >1 mm, which reduces the degree of phase fluctuations by a factor of two in many cases. However, phase fluctuations still remain after the WVR phase correction, suggesting the existence of other turbulent constituent that cause the phase fluctuation. This is supported by occasional SSFs that do not exhibit any turn-over; these are only seen when the PWV is low or after WVR phase correction. This means that the phase fluctuation caused by this turbulent constituent is inherently smaller than that caused by water vapor. Since there is no turn-over in the SSF up to the maximum baseline length of ~15 km, this turbulent constituent must have scale height of 10 km or more, and thus cannot be water vapor, whose scale height is around 1 km. This large scale height turbulent constituent is likely to be water ice or a dry component. Excess path length fluctuation after the WVR phase correction at a baseline length of 10 km is large (>200 micron), which is significant for high frequency (>450 GHz or <700 micron) observations. These results suggest the need for an additional phase correction method, such as fast switching, in addition to the WVR phase correction. We simulated the fast switching, and the result suggests that it works well, with shorter cycle times linearly improving the coherence.

ALMA SIS mixer optimization for stable operation

The Atacama Large Millimeter/Submillimeter Array (ALMA), an interferometric radio telescope will have 66 array elements when complete. The ALMA Front End is designed to accommodate up to 10 receiver bands covering most of the wavelength range from 10 to 0.3 mm. Superconductor-insulator-superconductor (SIS) mixers are employed for Bands 3 (~3 mm) through 10 (~0.3 mm). Ordinarily the SIS bias is selected to achieve the lowest receiver noise temperatures. However, in order to obtain the lowest detection threshold, the SIS bias also needs to be optimized with respect to receiver stability. There are also other parameters to be optimized such as the magnetic field strength used to suppress the Josephson currents and avoidance of Shapiro. This paper will summarize the results of work carried out to derive the optimal operating parameters for the large number of mixers in use on the telescope so as to keep the telescope operating reliably and repeatably.

ALMA system verification

The ALMA aperture synthesis radio telescope is under construction in northern Chile. This paper presents the organization and process of ALMA System Verification. The purpose of System Verification is to measure the performance of the integrated instrument with respect to the ALMA System Technical Requirements. The System Technical Requirements flow down from the Science Requirements of the telescope and are intended to guide the design of the array and set the standards for technical performance. The process of System Verification will help determine how well the ALMA telescope meets its science goals. Some verification results are discussed.

Problem reporting and tracking system: a systems engineering challenge

The problem reporting and tracking system (PRTS) is the ALMA system to register operational problems, track unplanned corrective operational maintenance activities and follow the investigations of all problems or possible issues arisen in operation activities. After the PRTS implementation appeared several issues that finally produced a lack in the management of the investigations, problems to produce KPIs, loss of information, among others. In order to improve PRTS, we carried out a process to review the status of system, define a set of modifications and implement a solution; all according to the stakeholder requirements. In this work, we shall present the methodology applied to define a set of concrete actions at the basis of understanding the complexity of the problem, which finally got to improve the interactions between different subsystems and enhance the communication at different levels.

Centralized operations and maintenance planning at the Atacama Large Millimeter/submillimeter Array (ALMA)

The Atacama Large Millimeter/submillimeter Array (ALMA) is a joint project between astronomical organizations in Europe, North America, and East Asia, in collaboration with the Republic of Chile. ALMA consists of 54 twelve-meter antennas and 12 seven-meter antennas operating as an aperture synthesis array in the (sub)millimeter wavelength range. Since the inauguration of the observatory back in March 2013 there has been a continuous effort to establish solid operations processes for effective and efficient management of technical and administrative tasks on site. Here a key aspect had been the centralized maintenance and operations planning: input is collected from science stakeholders, the computerized maintenance management system (CMMS) and from the technical teams spread around the world, then this information is analyzed and consolidated based on the established maintenance strategy, the observatory long-term plan and the short-term priorities definitions. This paper presents the high-level process that has been developed for the planning and scheduling of planned- and unplanned maintenance tasks, and for site operations like the telescope array reconfiguration campaigns. We focus on the centralized planning approach by presenting its genesis, its current implementation for the observatory operations including related planning products, and we explore the necessary next steps in order to fully achieve a comprehensive centralized planning approach for ALMA in steady-state operations.

Artificial calibration source for ALMA radio interferometer

The ALMA (Atacama Large Millimeter/submillimeter Array) radio interferometer has some different types of antennas which have a variation of gain and leakages across the primary beam of an individual antenna. We have been developing an artificial calibration source which is used for compensation of individual difference of antennas. In a high-frequency antenna, using astronomical sources to do calibration measurement would be extremely time consuming, whereas with the artificial calibration source becomes a realistic possibility. Photonic techniques are considered to be superior to conventional techniques based on electronic devices in terms of wide bandwidth and high-frequency signals. Conversion from an optical signal to a millimeter/sub-millimeter wave signal is done by a photo-mixer.