Glossary for ALMA Data Processing

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  • Amplitude: flux density associated to a visibility (modulus of the visibility); usually expressed in Jy


  • Beam:
    • Primary beam: the angular sensitivity pattern on the sky of one antenna. It defines the field of view for a given pointing in the sky. The primary beam varies as ~1.2(\lambda/D), where \lambda is the observed wavelength (sky frequency) and D is the antenna diameter.
    • Synthesized beam (beam): the angular sensitivity pattern on the sky of the interferometric array. It defines the effective angular resolution (~psf) of the 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 the 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, calibration files, measurement sets (raw and calibrated), images, log files, plots for quality assessment, ....


  • 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 of a scheduling block. The data collected during one execution block is gathered in a measurement set (labeled as 'uid___XXX.ms'), where 'uid___XXX' is a unique identifier for the execution block


  • Gain Calibration: Creation and application of temporal gain corrections (for phase and/or amplitude)


  • Image, Image cube: dataset consisting of flux density maps (in sky coordinates) for either 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 to define the region where real emission from the source could be found


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


  • Phase: angle associated to a visibility (argument of a visibility). Phase information relates to the sky brightness distribution characteristics (position, shape and extent or the source).


  • Scan: temporal subsets in a measurement set. Each scan includes subscans (integration units) of a single source.


  • Scheduling block: set of scripted commands defined to perform a specific observation. A scheduling block is written for a set of targets (if necessary with the corresponding pointing or mosaic patterns), a spectral setup and an array (12-m, 7-m or Total Power array). A scheduling block can be executed multiple times, and each execution of a scheduling block is an execution block.


  • Spectral window: subrange of frequencies defined in the spectral setup of a scheduling block. In general, every spectral window in a measurement set is handled separately during calibration. 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\lambda). 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.