Overview of the data reduction and analysis

Once the data has been acquired by an interferometer such as the NOrthern Extended Millimter Array (NOEMA) or ALMA, two different approaches may be used for its reduction and analysis:

• The first possibility is to clearly separate 1) the calibration, 2) the imaging and deconvolution and 3) the analysis.
• The second possibility is to merge in a single step calibration and imaging. This possibility is known as self-calibration.

While CASA uses the second paradigm, GILDAS mainly implements the first approach, as the program and the data format used for each step is different, but still allows convenient self-calibration. The basic instrumental calibration of NOEMA data is done inside CLIC on the NOEMA raw data format and the outcome is a uv table, which contains only calibrated visibilities of the astronomical source. The imaging and deconvolution are done inside IMAGER on the calibrated uv table and deliver mainly an lmv spectral cube (2 axes of coordinates and 1 axis of velocity/frequency). IMAGER includes VIEWER, a superset of GREG, for the visualization, and provides additional image analysis functionalities, which are not specific to an interferometric use (e.g. they can be used with the IRAM 30 m spectral cubes as well).

The choice of clearly separating calibration and imaging+deconvolution was taken at start of the Plateau de Bure Interferometer, when the limiting number of antennas prevented the use of self-calibration. While many points of the calibration algorithms inside CLIC are specific to NOEMA data (in particular its range of Signal-to-Noise ratio), the algorithms of imaging+deconvolution can be used in many different contexts and the visualization and analysis of spectra cubes is mainly independent of the instrument that delivered the data. This last point implies that users can import data from ALMA (mainly through FITS format) in IMAGER for imaging and deconvolution, and in VIEWER for visualization. But the reverse is also true. While calibration of NOEMA data should be done inside CLIC, imaging+deconvolution and visualization+analysis can be done in other softwares (e.g. MIRIAD, AIPS, CASA for the imaging and deconvolution and KARMA for the visualization and analysis).

With the improvement of NOEMA (increase of the number of antennas and better receiver sensitivities) and with the advent of a new generation of interferometer (ALMA), the additional step of self-calibration may improve the consistency of the final results by imposing additional consistent constraints on the calibration. This self-calibration step is further presented in Section9.