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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 to antenna at
frequency channel is related to the true object visibility
where and are the spatial frequencies corresponding to
baseline at time and frequency , and we assume the object has a
flat spectrum. Calibrating the data is computing the complex ``calibration
curves'' and . For the relatively narrow bandpass of
millimeter astronomy, and are almost independent of the
frequency channel (even the sideband separation is only 3 % of the
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 , 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: Atmospheric absorption and receiver gain
are derived in the same way as for single dish data, the correction
factors being determined from ``chopper wheel calibrations'' being
performed at regular intervals (typically 10-15 minutes). This in the
same time corrects for the amplitude passband (except for antenna
chromatism effects). The atmospheric model calculations are done on-line
to help monitoring data quality, and applied to the data. However, the
calibration parameters are stored in the headers as well as the overall
amplitude factor applied. In this way, the model calculations can always
be repeated at any stage in the data reduction process, with the
possibility of correcting wrong atmospheric parameters.
- IF Passband Calibration: Phase errors introduced in the backend
are measured by connecting all correlator inputs to the same source of
white noise (a noise generator in the IF). Ideally all outputs should
give correlated signals. The phases then are the
channel-to-channel phase errors. Normally this operation should be done
every time the spectral correlator setup is changed. It is actually done
as often as the amplitude calibration, since it can be done during
antenna motion from one source to the next, and since it provides a good
means to trace down hardware problems in the backend.
- Phase Calibration is necessary to correct the raw visibilities
for instrumental and atmospheric short-term phase fluctuations. This is
done by repeatedly observing a nearby point source for which the measured
phases should be zero if delays are correctly tracked. The phase closure
relations may also be used. If the calibrator is very close to the
source, this will also correct to first order baseline errors.
The visibilities amplitudes measured on the calibration source, if strong
enough, give an estimate of the additional amplitude corrections
introduced by pointing and focussing inaccuracies and atmospheric phase
jitter. They are commonly used to calibrate the source amplitude
relatively to the flux of the phase calibrator, thus eliminating to first
order the decorrelation effect due to the atmospheric phase fluctuations.
- RF Passband Calibration: Ideally the phase calibration should
be done separately for each receiver channel (but calibrators are not
strong enough). However if one assumes that phase fluctuations are not
frequency-dependent one may calibrate the relative phases of the
channels on a strong source, before or after the observations. Actually
only the RF passband needs observing a source in the sky for this, since
the passband of the correlators may be calibrated in autocorrelation mode
on a noise source (see above). RF passband calibration may be necessary
only for broad band spectra or objects where a high channel to channel
dynamic range is needed.
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.
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