|How does ALMA work ?|
By Antonio Hales and EPO team.
In theory, the basic notion of interferometry is quite simple. A signal from the sky is captured by two or more antennas which are combined in order to analyze the signal and thus obtain information about its source (whether a star, a planet, or a galaxy). By combining the radio waves collected by several antennas is possible to construct images. Such images are comparable to those that would be obtained with an hypothetical giant telescope or antenna, 14,000 meters in diameter. Since constructing and operating an antenna that size is technically impossible (at least with current technologies), constructing several small antennas and using them combining their output is far more feasible.
Nevertheless, this is not so simple in practice.
To operate properly, ALMA must have its 66 antennas and electronics working in perfect synchrony, with a precision of one millionth of a millionth of a second. In addition, the signals from the different antennas must be combined in a way that the path followed from each antenna until it is combined at the central computer(the correlator) must be known with an accuracy equal to the diameter of a human hair (hundredths of a millimeter). And as if the above were not challenging enough, there is the problem of reducing the possible attenuation and perturbation suffered by the signal from the time it touches each antenna until it is digitalized and transmitted over several kilometers of optic fiber to the central computer. Even earlier, as soon as the signal penetrates the Earth’s atmosphere, it is partially absorbed, deviated and delayed by molecules of CO2, Oxygen and water (even at 5,000 m of altitude and in the dry conditions encountered at he Atacama Desert). Seven weather stations, and specially-built Water Vapor Radiometers to measure the amount of line-of-sight water vapor present in the atmosphere, will be used to correct for these atmospheric effects.
|A schematic view of the path followed by an astronomic signal once it enters an ALMA antenna. The signal is first collected by the antenna, after which it is down-coverted at the cryogenically cooled 4 Kelvin stage of Front End, the Digitized by the Back End and transmitted through optical fiber towards the central building where the Correlator combines the signal from all antennas. ALMA is controlled from the Operations Support Facility (OSF), where the information is received, archived and its quality assessed.|
As its name implies, ALMA is an array or set of antennas. The main technical challenge for ALMA is to be able to aim simultaneously all the antennas at the same region of the sky, to capture the astronomic signal with each antenna, then convert the signal to digital format in order to transmit the signal from each antenna to a central building, where a supercomputer will combine the signals received at the various antennas to create data suitable for performing a scientific analysis of the properties of the source of such signal, all this with unprecedented accuracy and quality. The human hearing system is designed in such a way that each of its components performs a function very similar to those of ALMA. For a sound to reach the brain: an ear is directed to the source of sound, the anatomy channels the sound until it reaches a receiver (the eardrum), capable of capturing it and converting into an electric impulse, and the auditory nerve transmits it to the brain. The brain receives the signals from both ears, combines them and analyzes them so as to discern the nature of the source (e.g. who, how, where).
This chain of channeling, reception, conversion, transmission, combination, and analysis is analogous to the stages that radio waves will undergo once it enters each ALMA antenna, after having travelled through space for millions of years. In this way, the chain defines the Project Engineering architecture of ALMA and its organizational model divided into Integrated Product Teams (IPT). Below we conduct a brief tour of ALMA’s components and show how only perfect joint operation of all of them can make the scientific revolution of ALMA possible.
The function of ALMA’s antennas is to capture and concentrate the radio waves proceeding from the astronomical source at the point known as the focus. The light concentrated at the focus is reflected by a second reflecting surface –called a subreflector- to a point behind the parabolic surface, where there is a receiver geared to capture the signal concentrated by the antenna.
To capture the largest possible number of radio waves, thus optimizing the quality of the signal received, the antennas must aim (point) with unique precision. In addition, if the antenna surface is less than perfect, radio waves will be reflected in other directions and lost. Therefore, it has been established that the surface of the ALMA antennas must not deviate from a perfect parabola in more than 20 microns (50 times less than one millimeter) on average. Antenna IPT comprises North America (responsible for constructing 25 antennas), Europe (another 25 antennas), and East Asia (Japan and Taiwan, 16 antennas).
|Two different ALMA Front Ends. Band 4 operates from 125 to 163 GHz and is designed and built by NAOJ in Japan. Band 8, also built by NAOJ, is designed to operate at much higher frequencies (385 to 500 GHz) which reflects in the different design.|
Front End (FE) is the name given to the beginning of a complex chain comprising detection, amplification, conversion and digitization of the signal picked up by each antenna. The FE is the first electronic element through which the signal from the sky has to pass, therefore when amplifying and converting the signal it must attempt to maximize the detection of the original signal while introducing the least possible amount of perturbation and unwanted signals (noise). For this reason, in the most critical stage FE is kept at a temperature of 4 Kelvin (-269°C). Leading-edge FE technology has been developed at laboratories in the USA, Canada, the UK, the Netherlands, France, and Japan, with unique world-level instrumentation.
First, the signal from the antenna subreflector is tunneled through a series of small mirrors and waveguides into the cold components were the detectors reside. At the centre of the FE, al 4 Kelvin, the signal from the sky is mixed with a reference frequency aiming to lower the signal from its original frequency (between 30 and 900 Ghz) to an intermediate frequency (or IF, less than 15 Ghz). The IF, being lower, is more easily amplified and processed by the digitalization and transmission system (called the Back End) that will send the data to the correlator over optic fibers as much as 15 km long.
ALMA will be used in 10 frequency bands, thus the FE contains 10 cartridges, one per band. The wide frequency range has required the implementation of different technologies for the receivers of the more extreme frequencies.
The Back End (BE) plays the role of the ALMA’s nervous system. The main task of the BE is to transmit the signal received at each of the FE receivers in each antenna to the central computer (correlator). First, the signal from the FE is again converted to an even lower frequency, between 2 and 4 GHz (or even longer wavelength – 7.5 to 15 cm). Then a digitalization system processes the signal and subsequently transmits it by underground optic fiber to the central building.
However, in addition to transmitting the signal to the correlator, the BE acts as overall orchestra conductor, sending out a reference bar to all ALMA components so as to ensure that antenna movement, electronics, and highly accurate hydrogen atomic clock located in the central building at the Array Operations Site (AOS) generates a reference pulse, which the BE sends out to all ALMA components.
And even further, BE also send a round-trip laser signal to all antennas in order to continuously monitor the length of the optical fibers, as they can expand or contract daily due to temperature variations in the ALMA site. Any changes in the length of routes travelled by the signals coming from the various antennas to the correlator are compensated by real-time, computer controlled stretching or contracting of the fiber. This Line Length Correction system ensures that the path-length of the signal from any antenna is stabilized to about 1 micron.
The correlator is ALMA’s brain. It is basically a supercomputer designed especially for ALMA whose only purpose is to take the signals coming from its manifold antennas and to combine them, in order to generate astronomic data for subsequent analysis. The correlator multiplies the signals from the various antennas and saves the data to files known as Visibilities, which contain all the information necessary to form a 2-D image of the region of the sky observed (in addition to providing spectral information, that is, the capacity to form simultaneous images at thousands of consecutive frequencies). The process of passing from the Visibilities to a scientific image requires a number of calibration and reduction stages for which a specialized data reduction program has been devised.
|When completed, this special-purpose digital processor will outperform the world's most powerful supercomputer.|
Computing IPT (CIPT) is in charge of all ALMA-related information processing. CIPT handles the important software for antenna control, instrumentation, and data filling. CIPT thus supervises that all ALMA parameters and components remain stable and under control throughout the observation, together with design and operation of the correlator data-gathering system in real time. Furthermore, CIPT must provide the scientific community with software for preparing observations (ALMA Observing Tool or ALMA OT), and also a specialized program for data processing and reduction known as Common Astronomy Software Application (CASA)
Science IPT is in charge of performing the scientific verification of the data obtained by ALMA, that is, of interpreting the Visibilities (data) that ALMA will measure in a particular direction in the sky. Science IPT is composed of astronomers and is directly involved in the commissioning of ALMA, verifying proper operation of each of its components as well as the scientific validity of data. Together engineers from the Assembly, Integration, and Verification (AIV) team, Science IPT carries out all calibrations and verifications to check that all items of equipment are operating in accordance with ALMA technical and scientific requirements. Once ALMA is operational, astronomers will continue to coordinate the observations and to monitor.