5. MERLIN Data Reduction

The data collected by MERLIN form a set of irregularly spaced samples of the Fourier transform of the radio brightness distribution on the sky. Each sample, or visibility, is measured by the correlation within one integration period of the signals from two telescopes which form a baseline with projected co-ordinates $(u,v)$. The whole data set is known as the $uv$ data.

The process of data reduction can be split into calibration: correcting the $uv$ data for the effects of the instrument and the atmosphere; and mapping: producing an image from these data by Fourier transformation followed by deconvolution of the instrumental sampling pattern. These stages are often interleaved; the calibration of the phase-cal may involve using its map as a model, and whenever possible the maps of the target are very significantly refined by self-calibration.

This chapter gives a brief step-by-step guide to the reduction of MERLIN data. §5.1 deals with the initial calibration using local software. §5.2 outlines further $uv$ data calibration using AIPS and §5.3 describes self-calibration and how to refine the mapping of your target source. These sections describe processing a simple continuum data set; §5.4 and §5.5 describe additional procedures for wide-field or spectral line data. §5.6 outlines processing single baseline data and the final section summarises other techniques.

Simple data processing can be carried out using a pipeline or a dedicated AIPS procedure, which automatically run the AIPS tasks required for calibration and initial mapping. Further information can be found in Appendix E and §G.2.5.