System Specifications
ALMA in a nutshell
| Band | frequency range (GHz) | wavelength range (mm) | angular resolution bmax=200m ... 18km (arcsec) | line sensitivity (mJy) | continuum sensitivity (mJy) | primary beam (arcsec) | largest scale (arcsec) | 3 | 84-116 | 2.6-3.6 | 3.0 ... 0.034 | 8.9 | 0.060 | 56 | 37 | 4 | 125-169 | 1.8-2.4 | 2.1 ... 0.023 | 9.1 | 0.070 | 48 | 32 | 5 | 163-211 | 1.4-1.8 | 1.6 ... 0.018 | 150 | 1.3 | 35 | 23 | 6 | 211-275 | 1.1-1.4 | 1.3 ... 0.014 | 13 | 0.14 | 27 | 18 | 7 | 275-373 | 0.8-1.1 | 1.0 ... 0.011 | 21 | 0.25 | 18 | 12 | 8 | 385-500 | 0.6-0.8 | 0.7 ... 0.008 | 63 | 0.86 | 12 | 9 | 9 | 602-720 | 0.4-0.5 | 0.5 ... 0.005 | 80 | 1.3 | 9 | 6 |
ALMA frequency Bands & Atmospheric transmissionObserving frequencies
The frequency range available to ALMA is divided into different receiver bands. Data can only be taken in one band at a time. These bands range from band 3, starting at 84 GHz, to band 9, ending at 720 GHz. For comparison, a frequency of 300 GHz translates to a wavelength of approximately 1mm. 3 more bands (band 1 around 40 GHz, band 2 around 80 GHz, band 10 around 920 GHz) might be added in the future. Initially, only six ALMA antennas will be equipped with band 5 receivers.
Field of view
The field of view is determined by the antenna size and the observing frequency. It is independent of the array configuration. The field of view is expressed in terms of the primary beam, which describes the antenna response (sensitivity) as function of the angle away from the main axis. The FWHM of the primary beam is usually taken as the diameter of the field of view of an interferometer; however, note that the sensitivity is not uniform over this field having a maximum at the centre and tapering off towards the edges.
The FWHM of the ALMA primary beam is 17" at 300 GHz, and scales linearly with wavelength (diffraction limit of a single 12-m antenna, as opposed to that of the whole array). To achieve uniform sensitivity over a field larger than about a few arcsec, or to image larger regions than the primary beam, mosaicking is required, which is a standard observing mode for ALMA. If you plan to use mosaicking, individual pointings should be separated by 1/2 the primary beam FWHM to achieve Nyquist sampling.
Spatial resolution
The spatial resolution of ALMA depends on the observing frequency and the maximum baseline of the array, following the lambda/D scaling. In the most compact configurations (200 m), resolutions range from 0.4" at 675 GHz to 2.8" at 110 GHz. In the most extended configuration, the resolutions range from 6 mas at 675 GHz to 38 mas at 110 GHz. These numbers refer to the FWHM of the synthesized beam (point spread function), which is the inverse Fourier transform of a (weighted) u-v sampling distribution. The resolution in arcsec can be approximated as: FWHM (") = 62 / max_baseline (km) / frequency (GHz).
Array configurations
Unlike other interferometers, ALMA will not have a fixed set of configurations. From its most compact configuration, with maximum baselines of ~200 m, the ALMA main array will continously 'roll out' to its most extended configuration, with maximum baselines of ~18 km, and back. The Atacama Compact Array (ACA) will have essentially only one configuration, possibly with a north-south extension to provide a better beam shape for far-north/far-south targets.
Imaging of large structures
Large structures (corresponding to short spacings in the u-v plane) are not well imaged by an interferometer. Source structures larger than about lambda/bmin, where bmin is the shortest baseline in the interferometer, are not well reproduced in reconstructed images. These missing short spacing data must be measured with the ACA, either as a compact interferometer, or using the four 12-m antennas as single dishes.
To image regions larger than the primary beam, or to achieve uniform sensitivity over a field larger than about a few arcsec, mosaicking is required.
Spectral resolution
ALMA can deliver data cubes with up to 8192 frequency channels (spectral resolution elements). The width of these channels can range between 3.8 kHz and 2 GHz, but the total bandwidth cannot exceed 8 GHz. At an observing frequency of 110 GHz, the highest spectral resolution implies a velocity resolution of 0.01 km/s, or R=30,000,000. At 110 GHz, a velocity resolution of 1 km/s requires channel widths of 0.37 MHz.
Sensitivity
For an interferometer, the noise level in the resulting data cubes (expressed in mJy) scales roughly as S=(N*(Δν * Δτ)1/2)-1, where N is the number of antennas, Δν is the bandwidth and Δτ is the observing time. For continuum observations, Δν=8GHz, for spectral line observations, Δν is the channel width. The online ETC can be used to estimate noise levels or required integration times to reach a desired noise level. For extended sources that 'fill the beam' the ETC calculates the sensitivity as a brightness temperature with unit K. If you are uncomfortable with calculating sensitivities in K, just use the point source sensitivity and realize that this is the sensitivity per synthesized beam area.
The sensitivity is also a strong function of the atmospheric conditions. The troposphere has an effect on the optical depth, the atmospheric emission, and on the demands for calibration. The amount of water in the atmosphere is measured as the precipitable water vapour (pwv). A value of pwv=1 mm is typical for the ALMA site. For low frequencies (ν<300 GHz), even larger values are fine for many purposes. For the higher frequencies (ν>500 GHz), pwv<0.5 mm is recommended. The table above gives typical line and continuum sensitivities for different frequency ranges available to ALMA. Remember that 1 mJy is equal to a flux density of 10-29Wm-2Hz-1.
Note that initially only six ALMA antennas will be equipped with band 5 receivers, and that these band 5 receivers can only record data in one polarization. All other bands have full polarization capabilities. This explains the much lower sensitivity in this band.