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The Atacama Large Millimeter/submillimeter Array
Alwyn Wootten, A. Richard Thompson
TL;DR
ALMA addresses the need for sensitive, high-resolution observations of major millimeter and submillimeter radiation that cannot be adequately imaged from space with small apertures. The paper outlines its international facility, dual-array design, atmospheric site, receiving and correlator systems, and construction progress; based on specifications, millimeter and submillimeter flux sensitivity and resolution are expected to increase by almost three orders of magnitude.
Problem
Satellite observations identify a strong radiation peak near 200 µm, but current spacecraft apertures are too small to provide good images of its sources.
Method
The paper outlines ALMA’s dual-array architecture, high-altitude observing site, precision antennas, receiving system, digital correlator, and atmospheric phase correction system.
Results
Based on specifications, ALMA’s millimeter and submillimeter flux sensitivity and resolution are expected to increase by almost three orders of magnitude compared with current capabilities.
Takeaways & Limitations
ALMA will provide interferometric and total-power information on atomic, molecular, and ionized gas and dust from the solar system to the high-redshift universe.
Abstract
from arXiv · showhide
The Atacama Large Millimeter/submillimeter Array (ALMA) is an international radio telescope under construction in the Atacama Desert of northern Chile. ALMA is situated on a dry site at 5000 m elevation, allowing excellent atmospheric transmission over the instrument wavelength range of 0.3 to 10 mm. ALMA will consist of two arrays of high-precision antennas. One, of up to 64 12-m diameter antennas, is reconfigurable in multiple patterns ranging in size from 150 meters up to ~15 km. A second array is comprised of a set of four 12-m and twelve 7-m antennas operating in one of two closely packed configurations ~50 m in diameter. The instrument will provide both interferometric and total-power astronomical information on atomic, molecular and ionized gas and dust in the solar system, our Galaxy, and the nearby to high-redshift universe. In this paper we outline the scientific drivers, technical challenges and planned progress of ALMA.
I. INTRODUCTION
ALMA is designed to image major millimeter and submillimeter radiation from the Universe with high sensitivity and resolution. Its dual-array architecture and science goals set requirements for collecting area, receivers, and observing capabilities.
- I. INTRODUCTION: ALMA will image wavelengths from 1 cm to 300 µm, covering regimes of the Universe’s two strongest radiation peaks as atmospheric transmission permits.The instrument’s 31–950 GHz range corresponds approximately to 1 cm–300 µm.
- I. INTRODUCTION: The 12-m array requires 64 antennas, with contracts providing at least 50, and baselines extending from 15 m to approximately 15 km.The array is paired with the Alma Compact Array, or ACA, consisting of four 12 m and twelve 7 m antennas.
- I. INTRODUCTION: ALMA’s first science goal is detecting CO or C+ emission from a Milky Way-luminosity galaxy at redshift 3 in less than 24 hours.This sensitivity requirement sets the collecting area and receiver specifications.
- I. INTRODUCTION: ALMA is an international facility partnered by Europe, Japan, North America, and the Republic of Chile.Construction and operations are led by ESO, NAOJ, and NRAO on behalf of the participating regions.
- I. INTRODUCTION: The project’s second top-level goal is imaging gas kinematics in protostars and protoplanetary disks around young Sun-like stars at 150 pc.The goal includes studying physical, chemical, and magnetic structures and detecting gaps created by forming planets.
II. THE CHAJNANTOR SITE AND THE ARRAY CONFIGURATION
The Chajnantor site combines high altitude and exceptionally low atmospheric water vapor with a reconfigurable array layout. These conditions support observations across high-frequency atmospheric windows while allowing baselines up to roughly 15 km.
- II. THE CHAJNANTOR SITE AND THE ARRAY CONFIGURATION: The Chajnantor site has a 50th-percentile zenith optical depth of 0.061 at 225 GHz, corresponding to slightly more than 1 mm of precipitable water.The site’s low water column enables observations in high-frequency atmospheric windows.
- II. THE CHAJNANTOR SITE AND THE ARRAY CONFIGURATION: Atmospheric windows covered by ALMA’s two highest bands, 602–950 GHz, are observable from Chajnantor for roughly half the time.Transmission up to 30% has also been measured in super-THz windows at 1.035 and 1.3–1.5 THz.
- II. THE CHAJNANTOR SITE AND THE ARRAY CONFIGURATION: The array’s transporters move antennas between nearly 200 foundations at the 2900 m Operations Support Facility and 5000 m Array Operations Site.A complete change from the most compact to the most extended configuration can take several months.
- II. THE CHAJNANTOR SITE AND THE ARRAY CONFIGURATION: ALMA configurations provide maximum baselines on the order of 15 km, with inner foundation positions arranged along a tightly wound spiral.Topography heavily constrains the design of the largest configuration.
III. THE ANTENNAS
ALMA’s antenna systems combine compact-array total-power capability with precision structures engineered for demanding millimeter and submillimeter observations. The designs address extended-source measurements, thermal stability, and high-frequency surface accuracy.
- III. THE ANTENNAS: The ACA targets more extended objects, while its four 12 m antennas provide total-power measurements and can operate with the 12-m array or independently.Total-power measurements provide zero-baseline fringe-visibility values through single-antenna observations.
- III. THE ANTENNAS: ALMA antennas use carbon-fiber-reinforced plastic to maintain stable parabolic shapes across the site’s harsh thermal and wind environment.The stated temperature range is -20°C to +20°C.
- III. THE ANTENNAS: The antenna surface accuracy must be better than 25 microns for efficient observations at the highest frequency.This requirement is part of the demanding specifications derived from ALMA’s scientific goals.
- III. THE ANTENNAS: ALMA uses Cassegrain optics with a tilted 0.75 m subreflector to focus signals from feeds positioned slightly off-axis.A central 60 mm cone in the subreflector suppresses power reflected from the central area.
IV. THE ALMA RECEIVING SYSTEM
ALMA’s receiving system covers 31–950 GHz through ten receiver bands, using cooled front ends and sideband-separating designs to support broad, low-noise observations.
- Receiver architecture: 31–950 GHz coverage is provided by ten receiver bands on each 12 m ALMA antenna.The receivers are packaged as cartridges mounted in a roughly 1 m-radius Dewar.
- System overview: The receiving system combines high-frequency coverage with cryogenic low-noise components and digitization-ready intermediate-frequency processing.The supplied passages describe the receiver bands, thermal stages, sideband separation, and downstream bandwidth allocation.
- Receiver architecture: ∼100, 15, or 4 K cooling supports different receiver components within the front-end Dewar.The Dewar contains the receiver cartridges and admits radiation through microwave windows aligned with the antenna subreflector.
- Sideband separation: Bands 3–8 use dual-sideband 2SB mixers to separate upper and lower sidebands without input filtering losses.The RF signal is divided into two components with a 90° relative phase, then recombined through a quadrature hybrid.
- Intermediate frequency processing: 16 GHz total IF bandwidth is available, with 4 GHz per sideband and polarization for bands having separate sideband outputs.Band 6 instead processes 8 GHz per polarization in one sideband.
V. DIGITAL BACK-END AND CORRELATOR
ALMA’s digital back end compensates antenna-dependent delays and correlates signals across the 12 m and ACA arrays, with configurable spectral resolution and bandwidth.
- Delay compensation: Geometrical and transmission-path delays must be compensated before cross-correlation, using precisely known antenna locations.Antenna positions are determined through observations of radio sources with accurately known positions.
- 12 m array correlator: Up to 64 antennas and 16 GHz per antenna are supported by the 12 m array correlator.Its XF architecture cross-multiplies time lags and then Fourier transforms them into frequency spectra.
- 12 m array correlator: 32 digital sub-bands of 62.5 MHz increase spectral resolution by a factor of 32 over the time-lag cross-correlation.A further factor-of-two resolution increase is available with 31.25 MHz filters, at half the total bandwidth.
- ACA correlator: The ACA uses an FX correlator whose 524,288-point input FFT yields 3.8 kHz spectral bandwidth across a 2 GHz IF channel.ACA modes are processed to match those of the 12 m array correlator.
VI. CORRECTION FOR THE ATMOSPHERE
ALMA corrects short-wavelength phase errors caused by atmospheric path-delay variations by monitoring atmospheric water vapor with the 183.31 GHz line.
- Atmospheric correction: 183.31 GHz water-line measurements monitor atmospheric water-vapor variations that induce phase errors in ALMA observations.Corrections are essential at ALMA’s very short operating wavelengths; dry-atmosphere density variations can also affect path delay.
VII. LOCAL OSCILLATOR (LO) SYSTEM
ALMA distributes phase-stable local-oscillator signals from a central facility to each antenna and applies frequency and phase controls needed for interferometric processing. The atmospheric-correction system was still under testing when the paper was written.
- LO distribution: 27–142 GHz first-LO signals, or subharmonics, are distributed from the AOS Technical Building to antennas by optical fiber.Higher-frequency receiver bands use final frequency multiplication at the antenna.
- System status: At writing, the atmospheric-correction system remained in testing, and its techniques and parameters were not yet final.This is an explicit scope and maturity caveat for the correction system.
- LO stability: <12 fsec time stability and <38 fsec phase noise are required for the LO signal.These requirements constrain the phase stability of the local-oscillator system.
- Fiber compensation: A 25 MHz acousto-optic frequency offset and Faraday-mirror reflection allow correction of changing optical-fiber electrical length.The returned signal is compared with the outgoing master-laser signal at the central facility.
- Fringe and phase control: Antenna-specific frequency offsets remove Earth-rotation Doppler fringe frequencies, which reach ∼3 kHz in Band 10 on the longest baselines.Dual phase switching is also introduced through the first LO; 90° offsets remain available after cross-correlation for sideband separation or improved 2SB separation.
VIII. THE ALMA SOFTWARE
ALMA software supports calibration, imaging, and pipelined data reduction for increasingly large and accurate visibility datasets. CASA provides reduction routines, while observers receive raw data, processed images, and reproducibility scripts.
- Higher-frequency ALMA observations often require multiple pointings because the antennas have narrow beams.
- Calibration and imaging routines process ALMA’s cross-correlation visibility data into radio images.
- A prototype test array operated through December 2008 to test software elements, including data reduction.
- CASA supplies the reduction routines, and observers receive raw data, pipeline-reduced images, and image-production scripts.
IX. SUMMARY
ALMA was still under construction, with commissioning and Early Science planned before inauguration. Its specifications projected nearly three orders of magnitude greater millimeter and submillimeter flux sensitivity and resolution than current capabilities.
- Nearly three orders of magnitude: ALMA’s projected flux sensitivity and resolution will exceed current millimeter and submillimeter capabilities.
- Commissioning and astronomical validation were planned near the end of 2009, followed by Early Science in 2011 and inauguration in 2012.