Dear ALMA newsletter recipients, Once again, it is a pleasure for me to introduce the new edition of our newsletter. First of all, I would like to wish all of you an happy New Year, which I sincerely hope will bring you success, happiness and interesting new opportunities. For us in ALMA, the end of 2009 and the beginning of 2010 have been very exciting and this is once more a special moment in the development of our observatory. After transporting our third antenna to the high altitude Chajnantor plateau, at 5000 meters above sea level, our team successfully combined the outputs of these antennas using “phase closure”, a standard method in interferometry. This achievement marks one more milestone along the way to the beginning of Commissioning and Science Verification, CSV, which, once completed, will mark the beginning of Early Science for ALMA. There was an official announcement about this milestone at the AAS meeting early January and we also wanted to share this good news with you through this newsletter, which contains the content of the announcement. In another area, this newsletter contains the progress on site and a presentation of the Atacama Compact Array (ACA). This is the second part of a two part series on antennas, a continuation of the article in the last newsletter. The ACA plays a crucial part in the imaging of extended sources with ALMA. Without the ACA, the ability to produce accurate images would be very restricted. Finally, as you know, we like to show the human face of this great endeavour we are building and this time, we decided to highlight the Department of Technical Services, another fundamental piece working actively to make ALMA the most powerful radio observatory ever built. Enjoy ALMA’s universe ! Thijs de Graauw, ALMA Director The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Organization for Astronomical Research in the Southern Hemisphere (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan. ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.
Closing the loop for ALMA The Atacama Large Millimeter/submillimeter Array (ALMA) has passed a key milestone crucial for the high-quality images that will be the trademark of this revolutionary new tool for astronomy. Astronomers and engineers have, for the first time, successfully linked three of the observatory’s antennas at the 5000-meter elevation observing site in northern Chile. Having three antennas observing in unison provides the missing link to correct errors that arise when only two antennas are used, thus paving the way for precise images of the cool Universe at unprecedented resolution. On 20 November 2009 the third antenna for the ALMA observatory was successfully installed at the Array Operations Site, the observatory’s “high site” on the Chajnantor plateau, at an altitude of 5000 meters in the Chilean Andes. Later, after a series of technical tests, astronomers and engineers observed the first signals from an astronomical source making use of all three 12-meter diameter antennas linked together, and are now working around the clock to establish the stability and readiness of the system.
“This is a moment that we have been looking forward to for a very long time”, says Richard Hills, the ALMA Project Scientist. “Now we can finally say that we have put together a full set of all the components of ALMA as a complete system and what’s more, it works! It is particularly pleasing that this was achieved within a few days of the target date, set over a year ago, for reaching this point. We will be looking forward eagerly to more excitement over the next few months as we add more antennas and receivers to the array and start to make our first images with ALMA.” The successful linking of the antenna trio was a key test of the full electronic and software system now being installed at ALMA, and its success anticipates the future capabilities of the observatory. When complete, ALMA will have at least 66 high-tech antennas operating together as an “interferometer”, working as a single, huge telescope probing the sky in the millimeter and submillimeter wavelengths of light. The combination of the signals received at the individual antennas is crucial to achieve images of astronomical sources of unprecedented quality at its designed observing wavelengths. The three-antenna linkup is a critical step towards the observatory’s operations as an interferometer. Although the first, successful measurements employing just two antennas were obtained at the ALMA high site in October 2009 and demonstrated the excellent performance of the instruments, the addition of the third antenna is a leap of vital importance into the future of the observatory. This major milestone for the project is known as “phase closure” and provides an important independent check on the quality of the interferometry. Indeed, the use of a network of three (or more) antennas in an interferometer gives astronomers control over possible features which degrade the quality of the image, arising due to the instrument or to atmospheric turbulence. By comparing the signals received simultaneously by the three individual antennas, these unwanted effects can be cancelled out, which is completely impossible using only two antennas.
To achieve this crucial goal, astronomers observed the light coming from a distant extragalactic source, a quasar with the prosaic name 1924-292, which is well known to astronomers for its bright emission in the millimeter/submillimeter wavelength range probed by ALMA. The stability of the signal measured from this object shows that the antennas are working impressively well. “This result, combining the individual signals received by the first three antennas installed at the Chajnantor site, is a milestone for the observatory”, comments Thijs de Graauw, ALMA Director. “We will have many more antennas installed over the coming months, allowing astronomers to start producing early scientific results with the ALMA system around 2011. The interferometer will then steadily grow over the following years to reach its full scientific potential, with at least 66 antennas.” ALMA, an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile.
Here is a short synopsis regarding the recent progress of the site construction work: Array Operations Site (AOS)
At the ALMA Operations Site (AOS), the pouring of concrete for the antenna foundations for the ACA (Atacama Compact Array) is now complete. All of the ACA foundations are the smaller, triangular-shaped structures. These must be anchored to bedrock for stability. In addition to the concrete work, this anchoring has been completed, and the lightning protection grounding of the foundations is finished. During the curing process, the concrete shrinks slightly. However the accuracy needed for the placement of the antenna inserts is so high that the positioning of these inserts must wait until the concrete stabilizes. During this waiting period, work at the ACA goes on: connections for the utilities (i.e. the signals for steering the antennas and the signals from the receivers) and the power for the antennas are being completed. The work is moving ahead with the highest speed. Within 1 year, the ACA and the Central Cluster with 66 foundations will be completed. By 31 December 2009, 166 (of 192) antenna foundations at the AOS have been provisionally accepted. The remaining 26 foundations have been finished. The actual coordinates of all foundations have been communicated to the Science IPT (Integrated Product Team) for verification. Next will be the installation of the inserts in the antenna foundations. In 18 months, all of the foundations to 5 km will be completed and within 2 years, the entire set of antenna foundations, with roads, signal and power connections will be completed. An additional project is the permanent power supply. At present, there are temporary power connections to allow interferometry at the AOS with three antennas. These antennas were placed on foundations that were separated from the rest of the construction and also spaced by less than 200 meters. The next step is the installation of a temporary substation at AOS. This allows a better organization of the power distribution, and also an increase in the voltage from 400 volts to 23 KV, as a preparation for the final stage of the “island mode” power generation at the Operations Support Facility (OSF). Operations Support Facility (OSF)
The provisional implementation of the new Computer Room occurred in September 2009. All the Heating, Ventilating and Air Conditioning equipment for the OSF has arrived to the ALMA site. Additional antenna foundations at AEM and MELCO areas are finished, documented and delivered to the corresponding teams. The Culverts and Drainage structures for the AOS and OSF construction is complete and the Road Pavement construction is ongoing. As far as the communications AOS-OSF is concerned, the temporary microwave link will be sufficient to support operations until end of 2010. From then on the permanent high speed glass fiber link installed, as part of the 23 kV power distribution system, will become available. Antennas
News from East Asia: News from North America: News from Europe: Santiago Central Office
This picture shows the progress in the construction of the future ALMA Santiago Central Office in Vitacura. It is expected that the ALMA Headquarters will be ready for occupancy toward the end of 2010. Nov 11/13, 2009 | ALMA Board Meeting
The ALMA Board is the observatory’s highest governance body. Its members convene eight to ten times per year, two or three of which are face-to-face meetings. Their most recent meeting took place at the ALMA site in November 2009. At the meeting, the cost-to-completion and status of the most critical items to complete the observatory were presented by Joint ALMA Observatory (JAO) and Executive staff. These presentations discussed progress of the antennas and front-ends, and actions aimed at meeting the delivery schedule. Other topics discussed were the update of the ALMA Operations Plan, the ASAC report and reviews of the Science Operations Implementation Plan and Commissioning and Science Verification. The Board expressed their gratitude and pleasure at the progress made and their excitement about the cutting-edge instrument ALMA is building. The next ALMA Board face-to-face meeting will take place in April 2010 in Santiago, Chile Nov 17/19, 2009 | ALMA Annual External Review (AAER)
The Atacama Compact Array: An Overview When completed, ALMA will comprise a 12-meter diameter antennas array (12-m Array) of a minimum of fifty antennas, and the ACA (Atacama Compact Array), composed of four 12-meter diameter antennas and twelve 7-meter diameter antennas. Out of the fifty antennas of the 12-m Array, one-half are provided by the North American partners of ALMA, the other half by the European partners. The sixteen antennas that will comprise the ACA are provided by the East Asian Partners of ALMA. In the last issue of the ALMA Science Newsletter, we outlined the testing of the prototype ALMA 12-meter diameter antennas and the procurement process for these antennas. In that article, only a short account was given of the antennas for the Atacama Compact Array (ACA). In the following we give an overview of the ACA, starting with an introduction to imaging using interferometers. Imaging and Interferometry When waves pass through an aperture, there is a fundamental limit to details that can be imaged with any instrument. At 150 meters, the human eye can recognize features in the face of a person, while with a single ALMA 12 meter antenna at a wavelength of 4 mm, one can distinguish features that are separated by 3% of the diameter of the Moon. For an interferometer, finer details can be imaged, so the situation is better. However, there is a drawback. First, the advantage. For the ALMA 12-m Array, the largest spacing between antennas is 18.5 km. This spacing determines the angular resolution of ALMA from the relation θ (arcsec)= 0.14 λ (mm) /D (km). At the shortest wavelength λ, of 0.3 mm, when D is 18.5 kilometers, one can distinguish details to an angular scale of θ=0.002 arc seconds or 2 milli arc seconds. This fineness of detail exceeds that of the Hubble Space Telescope; it corresponds to the size of an auto at the distance of the Moon. The drawback is that the details that can be recorded are limited by the minimum possible spacing for the ALMA 12 meter interferometer array. This is determined by the minimum spacing between two antennas, 15 meters. Thus at 4 mm, measurements of details larger than 50 arc seconds are not recorded by the 12-m Array. If this lack of response is not compensated for, the science that can be done with ALMA becomes seriously restricted. This lack of response to extended source structures is illustrated in Fig. 1. For unresolved sources, this lack of response is not important, since the image of an unresolved source is just the instrumental beamsize. However for extended sources, features are not recorded. Typical examples of extended sources are galactic molecular clouds, or CO in nearby galaxies. For these, the lack of short antenna spacings causes a loss of image fidelity.
The ACA Design The problem of a lack of response to extended structure is well known; this is referred to as ‘missing short spacings’ problem. To improve images, one must provide additional data. Two possible solutions for this problem were discussed during the planning phase of the ALMA project. One possible solution was a 20 meter diameter single radio telescope. The difficulty consisted in calibrating and combining data from this single dish with data produced by the 12 meter array. The technical demands on the 20 meter antenna would be most severe at 0.3 mm, the shortest wavelength. There, the pointing accuracy would have to be 0.4 arc seconds, which is at the limits of mechanical stability but also at the limits of atmospheric stability. These considerations led the project to consider a set of antennas that would improve the response in Fig. 1. From design studies, the best approach consisted of the Atacama Compact Array (ACA). This is combination of an interferometer consisting of 7 meter diameter antennas (the “ACA 7-m array”) together with single 12 meter antennas for total power (ACA TP Array). Use of this combination of smaller antennas and interferometry would avoid instrumental difficulties but still provide the response to extended emission. Simulations of the response of the ACA allow an estimate of the performance. The result is the response of the combination of the ACA with the 12 meter interferometer array as shown in Fig. 2.
From Fig. 2, the response of the ACA four 12 meter Total Power antennas (in the ACA TP Array) is about a factor of two below the response of other parts of the system. In part this is caused by the smaller number of antennas in the ACA TP Array.
As shown in Fig. 3, for the ACA the four 12 meter Total Power antennas will be located at the corners of the area containing the twelve 7 meter interferometer antennas of the ACA Interferometer array. The dashed circles marked “NS-7” through “NS-12” are those antenna positions for measuring sources in the northern sky. These sources reach lower elevations, so to avoid one antenna blocking the view of another, the spacings must be larger. ACA Antennas As reported in the previous article on the ALMA 12 meter antennas, a Mitsubishi Electric Corporation (MELCO) 12-meter prototype antenna was constructed and brought to the Antenna Test Facility at Socorro New Mexico for testing. This antenna fulfilled the ALMA specifications, but was returned to Japan for refitting and upgrading. This antenna and three other newly constructed MELCO antennas are now in Chile. These will form the ACA TP Array (see Fig. 3). The MELCO PM03 was conditionally accepted by ALMA in December 2008. This was the first antenna brought to the Array Operations Site (AOS) on September 17, 2009. The second MELCO 12-meter antenna, PM02 is now undergoing tests at the OSF. The first of the 7-meter antennas for the ACA Interferometer Array arrived at the Operations Support Facility (OSF) in October 2009 and was assembled in November. In Fig. 4 are cross sectional drawings of the 12-m and 7-m antennas. The MELCO 12-m antennas are functionally compatible with the 12 meter antennas in the 12-m array. The 7-m antennas are also compatible, in that these are to be placed on any of the ALMA antenna foundations, will be moved with the ALMA transporter and must accommodate the same receiver equipment. Thus to meet these conditions, the design must be radically different from the 12-m antennas. Some of the changes are obvious from the drawings.
The ACA TP Array will be used to take broadband continuum data by scanning the instrument in one or two directions. Since the earth’s atmosphere also contributes broad band continuum radiation, the 12-meter antennas will be equipped with a nutating subreflector or “wobbler”. The wobbler allows rapid comparison of emission toward the source with a position nearby. For spectral line measurements, an off-source reference is needed, but under normal circumstances, the wobbler will not be used. Data Taking and Correlator for ACA Since the ACA must spend four times as much time on a source as the 12-meter Array, it must have a separate data taking system. The data transmission is the standard ALMA system, but a specific correlator, housed in the AOS Technical Building, has been designed for the ACA. In ACA Correlator, the outputs of each ACA 12-meter antenna and each ACA 7-meter antenna are converted from a series of time samples to a set of frequencies, and are then processed so the data can be then placed in the (u,v) plane. We can obtain high-fidelity images by combining the data from the 12-meter Array and ACA.
References
Preparing for the Future
At present, the high-profile group at the JAO is AIV (Assembly, Integration and Verification). As described in the #2 issue of this Newsletter, this group is responsible for receiving equipment from the Construction IPTs (Integrated Product Teams) – everything from antennas to water vapor radiometers – and integrating these into the complete ALMA Interferometer system. Working in the background however, is another group: the Department of Technical Services (DTS). The Department of Technical Services is building up the staff, equipment and expertise that we will need to operate all of the ALMA equipment after it has passed through the commissioning process. Although comparatively small at the moment, with about 50 staff, in full operation, DTS will employ ~150 people, and thus will be the largest single group within the Joint ALMA Observatory. DTS covers a wide range of activities coordinated by 5 main groups: the Antenna, the Computing, the Electronics, the IT and the Maintenance Groups. The detailed activities for each of these groups are as follows: Antenna Group: This group is responsible for all aspects of operation of the ALMA antennas, including operation of the two ALMA transporters, Otto and Lore. As well as performing all of the preventive and corrective maintenance, the Antenna Group is responsible for the antenna relocations – moving antennas from the close packed “central cluster”, to the outermost reaches of the array, and back – which allows the telescope to reach several possible configurations and thus operate like a “zoom lens”. Computing Group: The computing group will be responsible for all local support and development of the ALMA computing systems over the life-time of the observatory, supported by and integrated with the “Operations Software IPT” which will be the continuation of the Computing IPT once in operations. This will include support for the whole end-to-end software system, from the real-time computers whichcontrol the antennas, to the high-level user interfaces and everything in between. One of the key responsibilities of the computing group is the support of the “master archive”, which will be located in Chile, distributed between two sites (OSF and Santiago). With a peak data-rate of 64 Mbytes/sec, and an expectedaverage data volume of 200 TBytes a year, this is a huge undertaking. By comparison, the Hubble Space Telescope data archive contains 0.8 TBytes, once again illustrating the massive scale of ALMA.
Electronics Group: This group will be responsible for all of the equipment which is installed into the antennas, as well as the Central Local Oscillator (CLO) which provides an extremely precise frequency and timing reference, the Correlators, and other equipment in the Array Operations Site (AOS) Technical Building (TB). Both in the amount of electronics and its technical complexity, ALMA truly represents the state of the art in astronomical telescopes. Thus the Electronics Group will be kept busy with the world’s largest set of superconducting receiver systems, which are cryogenically cooled to -269ºC, the two ALMA correlators - extremely fast, but highly specialized supercomputers - and all of the equipment in between, some 10,000 items in all (counting spares).
IT Group: This group is responsible for support of all personal computing equipment, and the IT infrastructure which is so vital to any modern organization. The IT infrastructure includes electronic mail, web servers, internet links, IP phones and so on. Given the number of meetings ALMA requires, support of the videoconferencing equipment is another task which keeps the IT group permanently busy! Maintenance Group: This group will be responsible for maintaining all of the ALMA technical facilities not explicitly supported by the antenna and electronics groups. This will include maintenance of the power and fiber networks at the High Site, the air-conditioning and fire suppression systems in the AOS-TB and Operations Support Facility (OSF) Technical Facility (TF), and so on. Although perhaps not as high-tech as the other groups, proper maintenance of these facilities is equally vital to achieving the high availability specification that ALMA requires. The maintenance group will also be responsible for the implementation of a Computerized Maintenance Management System (CMMS).
DTS Support Offices include the System Engineering Office and the Spectrum Management Office. The former will be responsible for providing system-level diagnostic and support capabilities, as well as maintaining the ALMA engineering standards, performance budgets and so on into Operations. The Spectrum Management Office will be responsible for monitoring the stringent Radio Frequency Interference (RFI) standards necessary to ensure that ALMA observations are not contaminated by locally-generated interference. Last, but of course by no means least, DTS is supported by outstanding Administrative Assistants, shared with the Department of Science Operations of the JAO and assisted when necessary by AIV. In full operations DTS will also have Documentation Specialists and other support positions. ALMA has a very systematic acceptance process. Equipment first passes through Assembly, Integration and Verification (AIV), and is then handed over for Commissioning and Science Verification (CSV) before being delivered to Operations. Since we have only recently started the CSV process, you would be forgiven if you might think DTS staff currently have an easy life. In fact nothing could be further from the truth! We are currently busy with a wide range of activities. The more prominent of these are:
The Joint ALMA Observatory (JAO) is seeking: Assistant Scientist/A Secondary Title: ALMA Science Operations Astronomers ESO Workshop: The Origin and Fate of the Sun: Evolution of Solar-mass Stars Observed with High Angular Resolution The goal of this workshop is to review recent results on solar-mass stars obtained with infrared and millimeter interferometers, and to discuss their importance for our understanding of stellar evolution from star formation to stellar end products. The workshop will concentrate on the mass range from approximately 0.5 to 2 solar masses, will discuss what new results for one stage of stellar evolution mean for the next stage, and will bring interferometric results into context with our knowledge based on other observational techniques and with theory. It will also include prospects with 2nd generation instruments at the VLTI and with ALMA. Interferometry experts and non-interferometrists alike are welcome to attend the workshop and to bring together their different perspectives. Computational Star Formation Symposium Computational Astronomy is a relatively new branch of research that spans a wide range of skills, including the theory of gaseous and stellar dynamics, computational and algorithmic science, and visualization. It is also usually accompanied by serious comparisons with observations. Simulations of star formation and young cluster evolution have now reached a level of sophistication where they can reproduce the initial stellar mass function, the binary distribution as a function of stellar mass and period, the spatial distribution of stars in young clusters, the evolution of clusters, and the structure and evolution of galaxies. At the same time, there are large differences in techniques, algorithms, and computer hardware, and equally large differences in the assumptions about initial and boundary conditions and what physical processes to include. This Symposium will be the forum to discuss simulations and observations of star formation in 2010. The result of this Symposium will be a better understanding of the similarities and differences between computational techniques, and a recognition of the successes and shortcomings in matching the simulation results to detailed observations of star formation. Dates: 31 May to 4 June 2010. Location: Barcelona, Spain’ If you wish to receive email announcements when new editions become available, please send an email to Esta dirección electrónica esta protegida contra spambots. Es necesario activar Javascript para visualizarla , with "subscribe ALMA newsletter" in the body. To find out if you are already on the email list, send an email to Esta dirección electrónica esta protegida contra spambots. Es necesario activar Javascript para visualizarla , with "which" in the body. This newsletter is also available in PDF here. Please send comments on the newsletter or suggestions for articles and announcements to the editors at Esta dirección electrónica esta protegida contra spambots. Es necesario activar Javascript para visualizarla & Esta dirección electrónica esta protegida contra spambots. Es necesario activar Javascript para visualizarla . More information on ALMA and contact details can be found on the ALMA homepage.
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