Glossary for ALMA Data Processing

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  • Amplitude: modulus of a visibility point, measured on a flux density scale (usually in Jy)

  • Beam:
    • Primary beam: the angular sensitivity pattern on the sky of each antenna in the array. It defines the field of view for a given pointing in the sky. The primary beam varies as ~1.2([math]\displaystyle{ \lambda }[/math]/D), where [math]\displaystyle{ \lambda }[/math] is the observational wavelength and D is the antenna diameter.
    • Synthesized beam (beam): the effective angular resolution (~psf) of an interferometric array, for a given frequency, sky declination and antennae distribution.

  • Calibrated data: visibilities to which all the necessary calibration tables have been applied (see calibration). The calibrated data can be directly used for analysis in the Fourier plane, or imaged to derive a map of sky brightness distribution in the sky plane.

  • Calibration: process during the data reduction in which different corrections tables are derived and subsequently applied to the data. This includes scaling by system temperature, bandpass response correction, temporal gain correction (for phase and amplitude), absolute flux scaling. Water Vapor Radiometer corrections (small-scale phase corrections) may be applied as well. Data can also be flagged during calibration.

  • Cleaning: processes of deconvolving an image from the secondary lobes of the synthesized beam .

  • Continuum: emission varying very smoothly with frequency (for example, black body emission).

  • Data Package: directory downloaded from the ALMA archive, containing scripts, measurement sets, log files, plots.

  • Doppler tracking: the action of adapting the observed frequency to compensate the Doppler-shift of the emitted light due to the relative motion of the source with respect to the observer.

  • Execution block: Instance of an observation. The data collected during one execution is label with a single uid identifier (' 'uid___XXX') and collected in a measurement set (labeled as '')

  • Image, Image cube: dataset consisting of flux density maps (in sky coordinates) for a single or multiple frequencies

  • Imaging: process consisting in transforming calibrated data (Fourier plane data) into image cubes, through data gridding, inverse Fourier transform and deconvolution

  • Mask, clean mask: user defined region used during imaging

  • Measurement set: directory containing data and calibration tables. A measurement set can be issued from a single execution, or combined from different executions of the same scheduling block.

  • Phase argument of a visibility,

  • Scan

  • Scheduling block

  • Spectral window' : subrange of frequencies, defined by the spectral setup of a project. In ALMA Cycle 0 observations, each execution block contains four spectral windows.

  • u,v distance: distance of a point in the Fourier plane (coordinate Ux, Vx) to the phase center in the Fourier plane (coordinates 0,0). The (u,v) distance of a visibility corresponds to the projected length of the corresponding baseline. It is s expressed either in meters or wavelength number (usually k[math]\displaystyle{ \lambda }[/math]). Generally speaking, visibilities with a short (u,v) distance contain information on large angular scales on the sky.

  • Visibility: the cross-correlation product of signals from two antennas. The data collected from the correlator in an interferometer consists of visibilities. They are complex numbers, whose argument is the phase and modulus is the amplitude. Visibilities coordinates are expressed in the Fourier Plane.