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Introduction

This document gives a description of the CLIC software, developed to calibrate the data taken with the IRAM millimeter-wave interferometer on Plateau de Bure. CLIC stands for ``Continuum and Line Interferometer Calibration''. The way the data must be calibrated is very much dependent on the acquisition procedures and the backend hardware. For that reason it was felt easier to develop a dedicated program that to try to use an existing package such as AIPS. The situation is different for mapping software, for which our needs are certainly covered by the capabilities of AIPS. Thus a gateway from CLIC calibrated visibility files into the AIPS world is provided.

We outline now the different steps in data calibration. The (complex) visibility W measured on the baseline from antenna $i$ to antenna $j$ at frequency channel $k$ is related to the true object visibility $V_{ij}$ by

\begin{displaymath}
W_{ijk} = g_i(t) g_j^*(t) b_{ijk}(t) V_{ij}(u_k(t),v_k(t)) + noise term
\end{displaymath} (1)

where $u_k(t)$ and $v_k(t)$ are the spatial frequencies corresponding to baseline $ij$ at time $t$ and frequency $k$, and we assume the object has a flat spectrum. Calibrating the data is computing the complex ``calibration curves'' $g_i(t)$ and $b_{ijk}(t)$. For the relatively narrow bandpass of millimeter astronomy, $u_k(t)$ and $v_k(t)$ are almost independent of the frequency channel $k$ (even the sideband separation is only 3 % of the observing frequency).

$b_{ijk}(t)$ is the bandpass of the detection system, and is usually almost constant with time. It can be formally decomposed in a product of RF bandpass, caused by receivers and cables and usually with weak dependence on frequency, and IF bandpass, caused by the backend (spectral and continuum correlators at Bure).

For the $g_i(t)$, we must separate the calibration of amplitude and phases since amplitude and phase errors have very different origins. The amplitude corrections is related to several effects: atmospheric absorption, receiver gain, antenna gain (affected by pointing errors, defocussing, surface status and systematic elevation effects), and correlation losses due to phase noise. Phase errors may come from delay errors, baseline errors, or a slow drift in atmospheric or receiver phases.

Amplitude calibration and IF passband calibration have now been moved into the acquisition software. They are however described in section 4.1. Instrumental phase and RF passband calibrations might need more user intervention, depending on the data quality, and are described later, after a section dedicated to the data display capabilities of CLIC. Finally some specific operations, such as baseline calibration, are explained.


next up previous contents index
Next: The Commands Up: CLIC Continuum and Line Previous: Contents   Contents   Index
Gildas manager 2018-10-15