\section{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{equation}
  W_{ijk} = g_i(t) g_j^*(t) b_{ijk}(t) V_{ij}(u_k(t),v_k(t)) + noise term
\end{equation}
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.

\begin{itemize}
\item {\em 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.
\item {\em 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 $100\%$ 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.
\item{\em 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.
\item {\em 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.
\end{itemize}

Amplitude calibration and IF passband calibration have now been moved into
the acquisition software. They are however described in section \ref{QRTC}.
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.

\section {The Commands}

\clic\ commands may be separated into several groups, as shown in the
following table: \vspace{1cm}
\par\begin{tabular}{|l|l|p{8cm}|}
\hline
{\em Group}             &{\em Section}          &{\em Commands}\\
{\em of commands}       &{\em of this manual}   &\\
\hline
General purpose         &all
        & {\tt SAVE, SET, SHOW } \\
Data retrieval          &\ref{DSR}
        & {\tt COMPRESS, DUMP, DROP, FILE, FIND, FLAG, MARK, MASK,
        HEADER, IGNORE, LIST, MASK,  NEW\_DATA, TABLE, WRITE}\\
Data display            &\ref{DD}
        & {\tt CURSOR, PLOT, RESIDUALS }\\
Calibration             &\ref{QRTC}, \ref{IPC}
        & {\tt SOLVE, ATMOSPHERE, STORE}\\
\hline
\end{tabular}

\section{Data Access}

\subsection{Data Storage and Retrieval}\label{DSR}

Visibility data and header information are stored on files in a format
which is a simple extension of the one used by the single dish \class\ 
program.  The file organisation is exactly the same as in \class; except
for some additional index entries, specific to interferometric data.
Observation headers are also organised in sections, most of which are
interferometer-specific.  However, the length of the data section may vary,
depending on the number of data records written sequentially. Each data
record contains a short data header with the values of some fast varying
parameters such as time, U and V coordinates, source direction vector,
\ldots

The Input and Output files are selected by the {\tt FILE} command (syntax:
as in \class).

Most commands deal with all observations in the Current Index. The Current
Index is filled by means of the command \com{FIND}. Selection criteria are
mostly identical to the ones in \class, with some additions such as Hour
Angle or Calibration Status. Commands \com{DROP} and \com{IGNORE} have the
same meaning as in \class. Commands such as \com{GET} and \com{WRITE} are
provided for test purposes only.

%Commands which output data will either modify data in place, if possible, or
%write new versions of the data observations, depending of the last issued
%\comm{SET}{WRITE} command. {\tt SET WRITE NEW} will force the opening of 
%new versions, while {\tt SET WRITE UPDATE} will enable modifying observations
%in place (if possible, that is if the input and output files are identical, and
%if the length of the observation is unchanged).

Depending on whether the input and output files are identical or not
commands which output data will either modify data in place, or write new
versions of the data observations.  Command \com{COMPRESS}, is normally
used to time-compress data. To simply transfer data from one file to
another, the command \com{COPY} should be preferred.

Finally command \com{TABLE} outputs a UV Data Table suitable for use in
mapping programs (either \gildas\ tasks, or \aips\ tasks after format
conversion by \gfits).

\subsection{Data Display}\label{DD}

The \clic\ program includes the one dimensional part of \greg\ (SIC
language GREG1).  All graphic display is done internally via \greg\ 
commands or subroutines. So we refer to the \greg\ documentation for
commands like \com{DEVICE}, \com{HARDCOPY}, ... which must be issued to
obtain a graphic display.  \footnote {Note that since {\tt SET} and {\tt
    SHOW} are commands in both \clic\ and \greg, we have predefined a few
  symbols in \clic\ so that, e.g. {\tt SET} will execute {\ 
    CLIC$\backslash$SET} . You will have to type {\tt GREG1$\backslash$SET}
  to execute a \greg\ {\tt SET} command. But {\tt HELP SET} will be
  ambiguous; type {\tt HELP CLIC$\backslash$SET} to get information about
  the \clic\ {\tt SET} command.}

The \clic\ display may do two kinds of plotting: time-like or
spectral-like.  By default, the time-like mode is selected ({\tt SET MODE
  TIME}\index{SET!MODE}). In this mode, two quantities are plotted one
against each other, in one or several \greg\ boxe Both quantities are time
functions, such as time itself, or hour angle, or amplitude (or phase) in a
selected channel.

In spectral-like mode ({\tt SET MODE SPEC}\index{SET!MODE}), the quantity
in abs is a spectral coordinate, such as channel number, frequency or
velocity. The ordinate is then amplitude or phase. Switching between time
mode and spectral mode is done automatically by the \comm{SET}{X} and
\comm{SET}{Y} commands.

\subsubsection{Time-like plots}

The defaults plot settings are the following:{\tt \\
  SET MODE TIME\index{SET!MODE}\\
  SET X TIME\index{SET!X}\\
  SET Y AMPLITUDE PHASE\index{SET!Y} /LIMITS 0 * -180 180\\
  SET ASPECT\_RATIO 2.0 AUTO\index{SET!ASPECT\_RATIO}\\
  SET SUBBANDS C01 TO C06\index{SET!SUBBANDS}\\
  SET BANDS LOWER\index{SET!BANDS}\\
  SET BASELINES 12 13 23 14 24 34\index{SET!BASELINES}\\
  SET AVERAGING NONE\index{SET!AVERAGING}\\
  SET PHASES DEGREES JUMPY\index{SET!PHASES}\\
  SET PLOT POINTS}\\

Thus the \com{PLOT} command will produce six plots in six \greg\ boxes; one
for amplitude versus time and one for phase versus time, for the three
baselines linking the three first antennas. The data plotted will be the
average of the 6 continuum channels (C01 to C06) for the upper side band.
No time averaging will be done (one point plotted for each data point
recorded), the phase will be in degrees with no 360 degrees jumps.

The \comm{SET}{X} and \comm{SET}{Y} commands are needed to choose the
variables to be plotted. In time-like mode the variables may be the
components of the visibilities (amplitude, phase, real part or imaginary
part), or the baselines coordinates {\tt U}, {\tt V}, {\tt RADIUS} or {\tt
  ANGLE} (the rectangular or polar coordinates of the baseline vector in
the plane perpendicular to the source direction). Other variables are time
(in hours of Universal Time), scan number, source direction
(\verb!HOUR_ANGLE! and {\tt DECLINATION}), or delays (see the internal help
or section \ref{CLIH} for a complete description).

The \comm{SET}{BANDS} command will select the side bands plotted. Several
side bands may be plotted simultaneously. Side bands may obviously be {\tt
  UPPER} or {\tt LOWER}, or {\tt AVERAGE}, where the weighted average of
both side bands will be used to compute the components of the visibilities
if any are to be plotted, or {\tt RATIO}, in which case the (unweighted)
ratio USB/LSB will be used. The weights for averaging the side bands are
entered by the \comm{SET}{AVERAGING} command.

The \comm{SET}{SUBBANDS} command is used to specify the sub-bands to be
used.  Sub-bands are referred to by their mnemonics {\tt C01}, \ldots, {\tt
  C06}, for the continuum sub-bands, {\tt L01}, \ldots, {\tt L06} for the
line sub-bands.  In \clic, L01 refers to the first available line subband,
L02 to the second one, and so on. They correspond to the bands selected in
the {\tt SPECTRAL} command in \obs.  Several continuum sub-bands may be
averaged together by specifying a range (e.g. {\tt C01 TO C05}, or {\tt C01
  AND C04}, or {\tt C01 AND C03 TO C06}, the latter just excluding {\tt
  C02}).

For line sub-bands all the channels are averaged together, except if the
{\tt /WINDOW} option is used. For instance:\\ {\tt SET SUBBAND L01 /WINDOW
  30 45}\\ will average visibilities in channels 30 to 45 of the first line
sub-band.  However the edge channels (with lower signal--to--noise ratio,
due to anti--aliasing filters in the correlators) may be left out, by using
e.g. {\tt SET DROP 0.1} to leave out $10 \%$ of the bandwidth at each edge
of each spectral subband (the default is only $5\%$).  The central
channels, possibly affected by the Gibbs phenomenon in case of scontinuum
signal in the image subband, are left out by using command
\comm{SET}{GIBBS} {\tt number}, {\tt number} is the number of channels left
out

The \comm{SET}{BASELINES} command obviously selects the baselines for which
data is to be plotted. Baselines are named 12, 13, 23, (14, 24, 34,
\ldots).  Note that the numbers here refers the ordinal numbers of the
antennas that where connected at the time of the observations. If only two
antennas were connected, only one baseline will be in the data, and it will
be referred to as ``12'', even though the antennas you used are labelled
``2'' and ``3'' in black paint, and were at this time connected to
correlator inputs ``1'' and ``3''! However this hardware information is
recorded in the header, for possible needs.

A \comm{SET}{ANTENNA} command is also available. It is used to plot
antenna-related parameters (such as {\tt TSYS}, Elevation, ...) or with
autocorrelation data ({\tt SET Y AUTO}). It may also be used (in time-like
mode only) with correlation data: in that case,
\begin{itemize}
\item amplitude $A_i$ for each antenna is computed from ratios of baseline
  amplitudes $A_{jk}$ in the following
  way:\\
  $A_{i} = \frac{A_{ij} A_{ik}}{A_{jk}}$\\
  (averaged over possible antennas j and k, different of i and of each
  other).  This actually is a measurement of the gain of antenna i, if a
  point source is observed. It is a convenient way of displaying pointing
  and focussing data, but three antennas at least are needed.
\item antenna phases are computed by using the closure relations (the
  phases of all antennas are computed by a least square fit, assuming that
  antenna 1 has a phase of zero).
\end{itemize}

The \comm{SET}{AVERAGING} command deals with the way data is averaged. Data
may be time-averaged by the \com{PLOT} command; {\tt SET AVERAGING TIME t}
will specify the averaging time, while {\tt SET AVERAGING SCAN} will force
averaging observation by observation, and {\tt SET AVERAGING NONE} will
turn off time averaging. The normal method is to average complex
visibilities ({\tt SET AVERAGING METHOD VECTOR}, the default), while an
alternate method is to average separately amplitude and phases ({\tt SET
  AVERAGING METHOD SCALAR}).

Side band averaging, and sub-band averaging are always done in vector
method.  The weights for side band averaging are specified by the commands
{\tt SET AVERAGING UPPER baseline amplitude phase} and {\tt SET AVERAGING
  LOWER baseline amplitude phase}.  These weights should be determined from
the data, using plots of both amplitude and phase of the side band ratios.

The plotting may be done either with \greg\ symbols ({\tt SET PLOT POINTS},
the default), or with lines connecting the points ({\tt SET PLOT LINES}),
with histograms ({\tt SET PLOT HISTOGRAM}), or with error bars (in the case
of visibilities only; type {\tt SET PLOT BARS}).

Finally the command {\tt SET ASPECT\_RATIO}\index{SET!ASPECT\_RATIO}
selects a default x/y aspect ratio in the cases when several boxes will be
plotted. The boxes will be positioned on the screen to approach this ratio
as much as possible ({\tt SET ASPECT\_RATIO value AUTO}), or will exactly
reflect the demanded aspect ratio ({\tt SET ASPECT\_RATIO value EXACT}).

After a plot has been obtained, coordinates of special points may be
obtained interactively by command \com{CURSOR}, which calls the interactive
graphic cursor. Striking almost any key will give the user coordinates of
the cursor, while ``E'' is used as usual to exit the cursor mode; ``K''
will delete the selected data point from the current plot (useful to avoid
this point in {\tt SOLVE PHASE}); you may also {\tt IGNORE} or {\tt TAG}
scans in a given range to a given quality: type N to open a new range, and
{\tt I} or {\tt T} to close it (if using {\tt T} you will be prompted for
the quality in the range 0-9). Finally ``F'' may be used in the same manner
as ``T'' to interactively flag bad data points in the data (see section
\ref{CLIFLAG})

After the \com{PLOT} command, the abscissae and ordinates of the plotted
data points are kept in two two-dimensional SIC arrays named {\tt X\_DATA}
and {\tt Y\_DATA}, where the first index is the data point counter (1 to
{\tt N\_DATA}) and the second index is the box number. Thus the data are
available for more specific plots or mathematical operations using the
possibilities of SIC and \greg.

\subsubsection{Spectral-like plots}

In spectral-like mode ({\tt SET MODE SPECTRAL}), the only X variables
allowed are Channel number, IF frequency, RF rest frequency, and Velocity,
while Y variables are the components of the visibilities, or
autocorrelations ({\tt SET Y AUTO}).  {\tt SET ANTENNA} may be used, but
only with autocorrelations.  Some {\tt SET} keywords
have a different meaning:\\
{\tt SET SUBBAND}: here groups linked be the keyword {\tt TO} indicate
concatenation rather than averaging. For instance {\tt SET SUBBAND C01 TO
  C06} will form a single broad band spectrum with the 6 continuum sub-band
visibilities; or {\tt SET SUBBAND L01 TO L02} will concatenate the first
two line sub-bands to form a single spectrum, plotted in a single box.\\
{\tt SET AVERAGING TIME} or {\tt SCAN} or {\tt NONE} has no meaning; all
data in the current index is averaged to form a single spectrum in any
case.

Use of \comm{SET}{DROP} and \comm{SET}{GIBBS} is the same as in time mode,
but their effect here is more obvious on the plots since the channels are
plotted individually.

\section{Data Calibration Steps}

\subsection{Real Time Calibration} \label{QRTC}

Under this denomination we group a certain number of calibration operations
that need to be done on the visibilities, and on autocorrelation data.
These calibration steps are performed by the data acquisition processes
({\tt CORREL} and {\tt RDI}) running on the control computer, {\tt
  BURE01::}.  It is nevertheless useful to briefly describe these
operations here.  They are:
\begin{itemize}
\item {\it Clipping correction of line correlator data} (the clipping
  correction for the continuum correlator is performed in real time on the
  control computer). For this we use the approximation of Kulkarni and
  Heiles ({\it Astron.J.}{\bf 85}, 10, 1413) for correlation data, and a
  numeric integration using cubic splines for auto correlation data.
\item {\it Fourier transform of autocorrelation data}
\item {\it Fourier transform and sideband separation of correlation data}
\item {\it Correction of phases for delay rounding errors}
\item {\it Reduction of atmospheric calibrations}, using an atmospheric
  model and recorded atmospheric parameters. A calibration constant
  ($T_{CAL}$) is determined, as well as the gains in each channel for each
  antenna.
\item {\it Reduction of the IF Passband calibration scans}.
\item {\it Amplitude calibration of correlation data} using the last
  available autocorrelation data, and system temperatures from atmospheric
  calibrations
\item {\it Calculation of velocity scales} for each sub-band and side band.
\end{itemize}

The atmospheric calibration may be re-computed off-line by using the
command {\tt ATMOSPHERE}.  The atmospheric calibration is normally
performed according to parameters given in real time in \obs.  Each antenna
may be set in one of three calibration modes: {\tt FILE}, {\tt AUTO}, or
{\tt TREC}.  {\tt FILE} mode (the default) just means that the mode used
will be the one given in \obs\ for the corresponding antenna.  Any antenna
in {\tt TREC} mode is first used to determine a value of the water content.
If more than one is available, an average is made. Then the resulting water
content is used for the antennas in {\tt AUTO}, for which a value of Trec
is then determined.

Note that the command \com{ATMOSPHERE} may be run several times on the same
data.  The mode for some antennas may be forced in \clic\ with commands
{\tt SET ATMOSPHERE TREC temp /ANTENNA i}, or {\tt SET ATMOSPHERE AUTO
  water /ANTENNA i}. This may be useful if, by chance, the amplitude
calibration parameters were not set to the correct values in \obs\ by the
observer. Incorrect calibration parameters may be updated by command
\comm{MODIFY}{HEADER}.

\subsection{A First Look at the Data}

Before processing to actually calibrate the data, it is useful to evaluate
the data quality. Actually this should be done on the site, as soon as
possible after the data has been taken, to take the appropriate action
(such as re-observing the source in the current interferometer
configuration).  We list here the simplest checks to be done on the data
(these checks do not require knowledge of the astronomic aims of the
observing program):

\begin{description}
\item[Plot the system temperature and water vapor content versus time] (or
  versus elevation).  Use {\tt SET ANTENNA 1 2 3 4}, {\tt SET Y TSYS
    WATER}. {\tt TSYS} should have a reasonable dependence on elevation,
  while {\tt WATER} (the zenith opacity) will reflect any strong variation
  of atmospheric opacity with time.
\item[Plot the total power output versus time.] Use Use {\tt SET ANTENNA 1
    2 3 4}, {\tt SET Y TOTAL}. It should display a reasonable elevation
  dependence.
\item[Plot the calibrator amplitude and phase versus time] The phase
  calibrators are selected from the data set by the command {\tt SET TYPE
    PHASE}.  Both amplitude and phase should ideally be constants.
  
  The amplitude (in kelvins) should be in principle the same as observed in
  single-dish observations. Check for pointing or focussing problems if the
  amplitude suddenly drops in baselines connected to one antenna. Check the
  calibration parameters if the amplitude as a clear dependence on
  elevation.
  
  If the phase has a sinusoidal dependence on hour-angle, the baseline
  parameters may be incorrect.
\item[Plot the calibrator amplitude and phase versus IF-frequency.]  These
  curves should be relatively flat, since the IF pass-band has been removed
  during real time calibration.  No systematic slope should appear in the
  phase of the calibrator, if the delay offsets have been measured with
  sufficient accuracy. If some strange effect is seen in the correlator
  data (very low amplitude, very strong slope in the phases), you should
  check the IF passband calibrations.
\item [Plot the IF-passband calibrations] (if anything seems wrong with the
  correlator). These calibrations consist of two observations, an
  autocorrelation and a correlation.
  
  With {\tt SET Y AMPLITUDE PHASE, SET X I\_F} and {\tt SET BASELINE 12 13
    23 14 24 34}, you will get the correlation (normalised by the
  autocorrelation). Amplitudes should be very close to 1., except for
  possibly the edge channels of each subband. Phases show a systematic
  slope, varying from one correlator unit to another.
  
  With {\tt SET Y AUTOCORR, SET X I\_F} and {\tt SET ANTENNA 1 2 3 4}, you
  will get the autocorrelation spectrum for each correlator entry. The
  passbands should be rather smooth.
  
  Bad channels should show up as strong oscillations in either plot.
\end{description}

\subsection{Phase Calibration}\label{IPC}

\subsubsection{Determining the calibration curve}

The first operation is to select in the current index the calibrator
observations performed during the time period when calibration is needed,
and to plot the visibilities after having selected the baselines, side
bands and/or subbands needed. This is done via \comm{SET}{BASELINES},
\comm{SET}{BANDS}, \comm{SET}{SUBBANDS} commands. Time should be specified
as X axis and Phase as Y axis (in {\tt SET MODE TIME}, of course). {\tt SET
  PHASE CONTINUOUS} should also be specified, otherwise fitting a smooth
curve will be difficult.

Then a calibration curve is determined by command \comm{SOLVE}{PHASE}.  By
default this command will fit a cubic spline into each measured phase or
amplitude. With option {\tt /PLOT}, the curve will be plotted with the data
points. The calibration curve is then in memory.

The time interval between knots of the cubic splines is available as a
parameter through the \comm{SET}{STEP} command. Its default value is 3
hours. Additional knots may be introduced with the option {\tt /BREAK}, to
match discontinuities or angular points.

Alternatively, with option {\tt /POLYNOMIAL}, a polynomial fit will be
made.  The degree of the polynomial is automatically chosen, using the
statistical errors bars for each data point.

\subsubsection{Storing the Phase Calibration Curves}

The next step is to store this curve (if satisfactory) in the headers of
the observations to be calibrated. This is done after loading those
observations into the current index, by the \comm{STORE}{PHASE} command.
Before doing this, you must have selected an output file.  This may either
be the same as the input file (the headers will then be modified in place),
or a new file.

\clic\ will not apply the calibration to the data itself before it is
processed Calibrated data may be examined however with the \com{PLOT}
command; to do this you have to enter the command {\tt SET PHASE RELATIVE}
\index{SET!PHASE}. This will be switched back to {\tt ABSOLUTE} if you try
to use \comm{SOLVE}{PHASE} on calibrated data.

\subsection{Amplitude Calibration}\label{AIPC}

Phases being angles are naturally defined in radians (or degrees). For the
amplitude two units might be of interest : flux densities (in Jansky) or
brightness temperatures (in Kelvin). In principle, after atmospheric
amplitude calibration have been applied, the amplitude naturally comes out
in brightness temperature scale, which can be converted just knowing the
primary beam size into flux density. However, unavoidable effects such as
short term atmospheric phase fluctuations add extra degradations to the
amplitude scale.

Accordingly, it is often necessary to refer the amplitude scale to a
primary (or secondary) calibrator of known flux density. To summarize,
amplitude may be displayed in 3 different modes and 2 units, with the
following factor :
\begin{center}
\begin{tabular}{|c|c|c|c|}
\hline
{\em Unit $\backslash$ Mode}    &{\tt ABSOLUTE} &{\tt RELATIVE} &{\tt SCALED}\\
\hline
{\tt KELVIN} & 1 & $A_{ij}(t)$   & $1/S$\\
{\tt JANSKY} & $\sqrt{F_i*F_j}$ & $A_{ij}(t)/  \sqrt{F_i*F_j}$  &$1/S$\\
\hline
\end{tabular}
\end{center}
where the $F_i$ are the efficiencies of antenna in Jy/K, $A_{ij}$ is the
amplitude calibration factor (in K/Jy) and S the source flux in Jy.

Calibrating the amplitude usually implies determining both the $F_i$ and
$A_{ij}$ functions.

\subsubsection{Solving for fluxes}

The first step is usually to determine the efficiencies of antenna from
sources of known flux. To do that, the \comm{SOLVE}{FLUX} command forces
{\tt SET AMPLITUDE SCALED}. It uses all sources of known flux from the
current index to compute the (time averaged) values of the efficiencies
$F_i$. The fluxes of other sources in the current index can then be
determined using from the efficiencies.  Sources of known flux are
specified using the \comm{SET}{FLUX} command.  The sources should have been
observed sufficiently closely in time to avoid any variation of the
$A_{ij}$ values. Also, good flux determination requires pointing errors to
be similar on those sources.

The fluxes (measured or manually entered by \comm{SET}{FLUX}) can be
written to the output file using the \comm{STORE}{FLUX} command.

\subsubsection{Determining the calibration curve}

The first operation is to select in the current index the calibrator
observations performed during the time period when calibration is needed,
and to plot the visibilities after having selected the baselines, side
bands and/or subbands needed. This is done via \comm{SET}{BASELINES},
\comm{SET}{BANDS}, \comm{SET}{SUBBANDS} commands. Time should be specified
as X axis and amplitude as Y axis (in {\tt SET MODE TIME}, of course).

Then a calibration curve is determined by command \comm{SOLVE}{AMPLITUDE}.
It forces {\tt SET AMPLITUDE ABSOLUTE KELVIN} to determine the calibration
curve $A_{ij}(t)$ which is then in memory.

For the type of curve (splines, breaks, polynomials), {\tt SOLVE AMPLITUDE}
behaves as the the {\tt SOLVE PHASE} command applies.

\subsubsection{Storing the Amplitude Calibration Curves}

The next step is to store this curve (if satisfactory) in the headers of
the observations to be calibrated. This is done after loading those
observations into the current index, by the \comm{STORE}{AMPLITUDE}
command.

\clic\ will not apply the calibration to the data itself before it is
processed for mapping. Calibrated data may be examined however with the
\com{PLOT} command; using the {\tt SET AMPLITUDE RELATIVE [JANSKY|KELVIN]}
command \index{SET!AMPLITUDE}

\subsection{RF Passband Calibration}\label{RFPC}

The IF part of the passband shape should have been calibrated out either
on-line or by the {\tt SPECTRUM} command. However the RF part remains and
must be calibrated using radio sources.

\subsubsection{Determining the passband curves}

The first operation is to select in the current index the calibrator
observations performed during the time period when calibration is needed,
and to plot the visibilities after having selected the baselines, side
bands and/or subbands needed. This is done via \comm{SET}{BASELINES},
\comm{SET}{BANDS}, \comm{SET}{SUBBANDS} commands. Channel number should be
specified as X axis and Phase and/or amplitude as Y axis, {\tt SET MODE
  SPEC}. You should preferrably use {\tt SET PHASE CONTINUOUS}, since $2
\pi$ jumps may occur in the passband shape. Then a calibration curve is
determined by command {\tt SOLVE RF\_PASSBAND}\index{SOLVE!RF\_PASSBAND}.
This command will fit a polynomial into each measured phase or amplitude,
for each line subband. For the line subbands the polynomial is by default
of degree 0 for amplitude and 1 for phase data.  Higher degrees may be
specified as arguments to the {\tt SOLVE
  RF\_PASSBAND}\index{SOLVE!RF\_PASSBAND} command.  For the continuum, the
ten values are simply stored and will be used as calibration data.

With option {\tt /PLOT}, the curve will be plotted with the data points.
The calibration curve is then in memory. These {\tt PLOT} and {\tt SOLVE}
operations will have to be done in several steps, if many subbands are
used. The curve for each subband and sideband is kept in memory, so that a
single {\tt STORE} command is needed.

\subsubsection{Storing the passband curves}

The next step is to store this curve (if satisfactory) in the headers of
the observations to be calibrated. This is done after loading those
observations into the current index, by the {\tt STORE
  RF\_PASSBAND}\index{STORE!RF\_PASSBAND} command. Before doing this, you
must have selected an output file. This may either be the same as the input
file (the headers will then be modified in place), or a new file.

As for instrumental calibration, \clic\ will not actually apply the
calibration the data before it is processed for mapping. Calibrated data
may be examined however with the \com{PLOT} command; to do this you have to
enter the command {\tt SET RF\_PASSBAND ON} \index{SET!RF\_PASSBAND}. This
will be switched back to OFF if you try to use {\tt SOLVE
  RF\_PASSBAND}\index{SOLVE!RF\_PASSBAND} on calibrated data.

\subsection{Editing the Data}

In some (rare) cases the calibration will reveal unpleasant results: the
weather was very bad indeed, or something did not work properly.  Thus you
may consider modifying some header parameters, or even delete some data
points. The parameters in the headers (and even the data itself) are all
accessible as {\bf SIC} variables. Selection of data records for further
use is done by a set of flags in the header of each data record.

\subsubsection{Variables}

A {\bf SIC} variable may be associated with nearly every header parameter.
Variables are normally not enabled; a whole group of variables may be
enabled,
by means of command \com{VARIABLES}. For instance:\\
{\tt VARIABLES ATMOS ON}\\
will turn on all variables concerning atmospheric parameters. Use {\tt HELP
  VARIABLES} to consult the list of variables and groups of variables.  To
examine the content of variables, the observation, and one of the
associated may be read in with command \comm{GET}{/RECORD}.

The edition of the data headers is done is a more powerful way by command
\comm{MODIFY}{HEADER}. This command will perform a loop on the current
index, loading each header in turn; for each it will execute a procedure
{\tt MODIFY.CLIC}, that you should provide (you may give it another name,
and enter it as a second argument to command \com{MODIFY}). This procedure
may contain {\bf SIC} commands (such as {\tt EXAMINE, LET, SAY ...}), or
some \clic\ commands like \com{HEADER}, \com{DUMP} ... Its execution {\tt
  must} end with the special command \com{GO}, otherwise the loop will be
aborted. The command \comm{GO}{WRITE} will ensure that the modified header
is rewritten to disk.

For instance, assume the flux for 3C84 was entered incorrectly as 100 Jy,
during an observing session (you do not recall when exactly). The correct
flux was 12 Jy. Your {\tt MODIFY.CLIC} might look like:
\begin{verbatim}
IF FLUX.NE.12 THEN
    LET FLUX = 12
    SAY "Flux updated, scan " 'SCAN'
    GO WRITE
ELSE
    GO
ENDIF
\end{verbatim}
You will have to execute the following commands:
\begin{verbatim}
FIND
VARIABLE POSITION ON            ! this is the group for FLUX 
VARIABLE GENERAL ON             ! this is the group for SCAN
MODIFY HEADER
\end{verbatim}

Note that in the above example, the same result could be obtained more
simply
by:\\
{\tt SET FLUX 3C84 12.}\\
{\tt STORE FLUX}\\

Command \comm{MODIFY}{DATA} works in the same way, but it will successively
load all the data records of each observation, and execute (by default) the
procedure {\tt DATA.CLIC}. The range of this command may however be
restricted with options {\tt /AFTER date time} and {\tt /BEFORE date time},
where `date' is a usual date chain ({\tt jj-mmm-yyyy}), and `time' the time
in hours (sexagesimal notation in understood).

\subsubsection{Flags}
\label{CLIFLAG}

Each record contains one flag word by antenna, and one flag word by
baseline.  There may ultimately 32 flags in each word. As for now, both
antenna and baseline flag words may contain one flag by subband (named {\tt
  CO1} to {\tt C06}, and {\tt L01} to {\tt L06}; in addition, the antenna
flag words may contain flags {\tt LOCK}, {\tt TSYS}, {\tt POINTING}, {\tt
  SHADOW}, and {\tt DATA}, while the baseline flagwords may contain flag
{\tt DATA}.

For instance, if subband flag {\tt C02} is present in baseline 23, subband
{\tt C02} will be ignored for further processing of baseline 23 (in
particular, it will be absent from plots). If present in antenna 1,
baselines 12 and 13 will be affected. Other flags such as {\tt LOCK}, {\tt
  TSYS}, {\tt POINTING}, {\tt SHADOW}, and {\tt DATA} will affect all
subbands of the relevant baselines.  Flags should normally be set by the
real time system, or the real time calibration step in \clic.

You may wish to use that data anyway, by masking them (command \com{MASK}).
For instance use {\tt MASK C02 /BASELINE 23} if you feel that this subband
was usable, but do not be surprised by the results !

You may also want to suppress temporarily some data channels from plots or
from output tables, without modifying the data file itself. This is done by
command \com{MARK}. Marked data channels will be ignored just as if they
were flagged in the data.

On the opposite, you might have to flag some data yourself. There are
several ways of doing this:
\begin{itemize}
\item using the command \comm{STORE}{FLAG} which will flag all the records
  of all the scans in the current index. For instance
\begin{verbatim}
FIND /SCAN 5436
STORE FLAG DATA /BASELINE 12
\end{verbatim}
  will flag baseline 12 for the whole duration of scan 5436. This is the
  best way to flag one or more complete scans.
\item using command \comm{MODIFY}{DATA}, with a procedure {\tt DATA.CLIC}
  containing the command \com{FLAG}.  For instance, assume that you want to
  discard all data where the total power on antenna 2 is lower than some
  threshold, say 10.  You will create a procedure {\tt DATA.CLIC}:
\begin{verbatim}
IF DH_TOTAL[2].LT.10. THEN
    FLAG DATA /ANTENNA  2 
    GO WRITE
ELSE
    GO
ENDIF
\end{verbatim}
  Do not forget to enable variables of group {\tt DATA\_HEADER}, before
  typing {\tt MODIFY DATA}.
\item interactively, after a \comm{GET}{/RECORD}, use the \com{FLAG}
  command to set or reset a flag for this specific record. The \com{UPDATE}
  command will write back the flagged data to the file.
\item interactively, with command \com{CURSOR}. Assume you have plotted
  amplitude versus time (with no averaging), you should have one point for
  each record. Then you can select a range of records in X coordinate:
  point the cursor to the first point you want to flag, type N (for New
  range), then go to the last point, type F (for Flag). You will be
  prompted for the name of the flag to set. This flag will be set for the
  baseline, or antenna, corresponding to the box you are pointing to with
  the cursor.
\end{itemize}

You may get a status of the flags in the data og the current index by
issuing command \comm{LIST}{/FLAG}.

\section{Calibrating the Interferometer}

New instrumental constants have to be measured each time the antennas have
been moved: delays, antenna coordinates, pointing constants. We describe
here the reduction procedures to be used.

\subsection{Delay determination}

The delay parameters are determined by taking a correlation on a strong
source (the strongest available in the sky), and by measuring the variation
of phase as
{\tt SOLVE DELAY /PLOT}\\
is available to measure the delays. The phase should be plotted as a
function of IF frequency, over the broadest band available (the correlator
should be set up to analyse the 100-600 MHz band by juxtaposition of
several 160~MHz subbands).  {\tt SOLVE DELAY /PLOT}, applied to baseline
12, will output the delay to be applied to antenna 1. If the delay is too
large, increase the range of search by using the option {\tt /SEARCH range}
(by default the search range is between -100 and +100 nanoseconds).

\subsection{Baseline Calibration}\label{BC}

We examine here the way to determine the baseline vectors from calibration
observations done for instance after an antenna move. Assume the relevant
observations are gathered into the current index. One should make a plot of
phase versus both Hour Angle and Declination ({\tt SET X HOUR\_ANGLE
  DECLINATION} and {\tt SET Y PHASE}, for each sideband separately.

The command \comm{SOLVE}{BASELINE} will then do a least square fit and
output the baseline vectors. A linear method is used which works only if
the starting values (that is the baselines values in the header) are within
half a wavelength in any direction of the true values. Different starting
values may be given with the {\tt /OFFSET baseline dx dy dz} option. In
that case the phases are first corrected for the given baseline offsets
before the fit.

A blind search within a given range may be done. For this enter the option
{\tt /SEARCH range} where range is in meters. All offsets within the given
range (in three directions, stepping by half a wavelength) are tried, and
the best rms value is kept. A blind search in a 0.01 meter range will take
about 4 seconds on the microVAX 3400. This the default if only {\tt SOLVE
  BASELINE /SEARCH} is given. Rms values on the range 10-20 degrees are
expected, if the data was scan averaged, and observing conditions were
acceptable.

The program gives for each baseline two series of offsets, and the fitted
baseline for record. The first offsets given ({\tt dx, dy, dy}) are
relative to the baseline coordinates used while observing, while the second
set of offsets ({\tt DX, DY, DZ}) are relative to the ``standard values''
of the antenna positions. After all available baselines have been fitted,
the command \comm{PRINT}{BASELINE} will produce a command procedure {\tt
  CLIC-BASELINE.OBS}.  This command procedure may be used in \obs, in the
{\tt CONFIGURE} mode, to update the antenna position offsets.

If {\tt SET ANTENNA 2 3} has been used instead of {\tt SET BASELINE}, then
the antenna position offsets of antennas 2 and 3 are directly determined
(the position offset of antenna 1 are by definition zero, like antenna
phases for this antenna in the {\tt SET ANTENNA} mode. This mode is
preferable since baseline closured is ensured.

Residuals may be examined by entering \comm{RESIDUALS}{BASELINE}; phases
should be independent of hour angle and declination.

Phase data can later modified in previous data to reflect this baseline
change (if a previous value proved to be wrong) with {\tt MODIFY BASELINE
  FITTED}.

If {\tt SET X HOUR DECL TIME} is specified, then plots of phase as a
function of time are obtained; \comm{RESIDUALS}{BASELINE} will also display
the variation with time of the residuals. This is useful to detect a time
drift of the phases, especially during long observations (to be avoided in
any case!).  These drifts may be fitted simultaneously with the option {\tt
  /POLYNOMIAL degree}. In that case a time polynomial (degree up to 3) is
added to the fitted phase function.

\subsection{Reduction of Pointing and Focussing Scans}\label{RPF}

This is done by \comm{SOLVE}{POINTING} and \comm{SOLVE}{FOCUS} commands.

These commands will scan the current index for the specific data, and fit
the data for each antenna.  The results may be kept in a data file (Text
format): use the option {\tt /OUTPUT filename NEW} or {\tt /OUTPUT filename
  APPEND} for this.
\begin{itemize}
\item for {\tt SOLVE POINTING}, the measured pointing errors in the file
  may be used to replace the collimations in \obs\ (command {\tt
    CORRECTIONS}, first two arguments), or the data file be used as a whole
  to solve for pointing coefficients with the program {\bf POINT} (see the
  specific documentation on pointing).
\item for {\tt SOLVE FOCUS}, the measured error is to be {\em added} to the
  foc correction in \obs\ (command {\tt CORRECTIONS}, third argument).
\end{itemize}
In both cases, option {\tt /PLOT} will produce a graphic display of each
fitted curve.

\subsection{Reduction of Skydip Scans}\label{RSS}

This is done by \comm{SOLVE}{SKYDIP}.

These commands will scan the current index for the specific data, and fit
the data for each antenna.
%The results may be kept in a data file (Text format):
%use the option {\tt /OUTPUT filename NEW} or  {\tt /OUTPUT filename APPEND} for
%this. 
The data may be used to solve for the forward efficiency and the water
vapor content (default), or to solve for the receiver temperatures and the
water vapor contents. Note that at the frequencies currently used, the
results for the water content are not very reliable, since the dominant
absorption around 90 GHz is oxygen in good weather conditions.

\section{A Few Receipes}\label{RCP}

We give here a few commonly used sequence of commands, to be used as
examples.

\subsection{Finding the best delay offsets}

\begin{verbatim}
FILE IN INTER_DATA:01-dec-1992 ! load the current data file
SET SUBBANDS L01 to L04        ! use all subbands, which
                               ! should be have close to equal phase offsets.
                               ! delay is more accurately determined. 
SET BAND LOWER                 ! select lower side band for instance
SET Y PHASE                    ! 
SET X I_FREQ                   ! Intermediate frequency  as X axis
SET SCAN 1234                  ! or SET NUMBER n 
FIND                           ! 
SOLVE DELAY /PLOT              
\end{verbatim}
The optimum delay is actually the differential delay between first and last
antennas of the baseline in question. It should be entered in the data file
for the first antenna, or its opposite entered in the data file for the
second antenna (or possibly shared between them along these lines). Be
careful to determine the delay with enough precision, in order not to
introduce a slope in the phases (1 nanosecond produces 0.5 turn in the 500
MHz passband).

\subsection{Baseline determination}

\subsubsection{a rough, simple way}

Here we just plot the data of lower sideband.

\begin{verbatim}
!
FILE I INTER_DATA:01-dec-1992
SET SCAN 2393 2440               ! select data by scan number
SET PROCEDURE CORR               ! discard AUTOs and possible other scans
FIND                
PLOT                             ! giving it a glance does not hurt ...
SET ANTENNA 2 3                  ! use antenna mode
SET SUBBANDS C01 TO C06          ! should be OK
SET BANDS UPPER LOWER            ! side bands separately 
SET PHASE JUMPY                  ! best for baseline solving
SET Y PHASE                      ! amplitude not needed
SET X HOUR_ANGLE DECLINATION
PLOT                             ! phases should be displayed
SOLVE BAS/SEARCH                 ! should normally be enough.
                                 ! ...wait a few seconds ...
PAUSE                            ! keep a log of the results !
                                 ! (both side bands should agree)
PRINT BASELINE                   ! to have the results in a OBS procedure
RESIDUALS BASELINE               ! phases should be flat.
\end{verbatim}


\subsubsection{more sophisticated}

In this we first process the data into a secondary file, perform a RF
passband calibration to be able to average continuum subbands and sidebands
more accurately, and solve for the baseline with a search loop.
\begin{verbatim}
!
FILE I INTER_DATA:01-dec-1992         !  Load the raw data
SET SCAN 2393 2440                    ! select data by scan number
SET PROCEDURE CORR                    ! only correlations
FIND
LIST                                  ! as a check
FILE O W00-N15-E23 NEW                ! open a secondary data file
COMPRESS 60                           ! compress to 1 minute, 
                                      ! (this saves plotting time)
!
! Select the compressed data file and do passband calibration on 3c273
!
FILE BOTH W00-N15-E23                 ! reopen for both input and output
FIND                
PLOT                                  ! giving it a glance does not hurt ...
SET SOURCE 3C273                      ! the strongest souce
SET MODE SPEC                         ! actually not needed
SET X I_F                             ! 
SET PHASE CONTINUOUS                  ! 
SET RF FREQUENCY                      ! use frequency dependent mode
SET Y AMPLITUDE  PHASE
SET SUBBANDS L01 TO L06
FIND 
LIST
SET BANDS UPPER                       ! do upper side band fist
SOLVE RF_PASSBAND 5 10 /PLOT          ! degrees as actually needed ...
SET BANDS LOWER                       ! Then lower side band
SOLVE RF_PASSBAND 5 10 /PLOT
SET SOURCE *                          ! come back to all sources
FIND
STORE RF_PASSBAND                     ! in place, in the data headers
!
! Now solve for baseline, using average of subbands 1 to 6
!
SET RF_PASSBAND ON  FR                ! from now on, plot calibrated data
SET MODE TIME                         ! switch back to time mode
SET SUBBANDS C01 TO C06               ! now subbands may be averaged
SET PHASE JUMPY                       ! best for baseline solving
SET Y PHASE                           ! amplitude not needed
SET X HOUR_ANGLE DECLINATION
SET BANDS AVERAGE                     ! double side band 
PLOT                                  ! phases should be displayed
SOLVE BAS/SEARCH                      ! should normally be enough.
                                      ! ...wait a few seconds ...
PRINT BASELINE
PAUSE                                 ! keep a log of the results !
RESIDUALS BASELINE                    ! phases should be flat.
\end{verbatim}

\section{Producing Images}\label{PI}

Images may be produced either by \aips\ or \gildas. The first step in any
direction is to produce a UV Data Table from calibrated data. This is done
with \clic\ command \com{TABLE}.

\subsection {Creating a UV Table}

You will have first to select which band and subbands are to be used, with
command \comm{SET}{SELECTION}. The format is
\begin{verbatim}
     SET SELECTION key band subbands
\end{verbatim}
`{\tt key}' may be {\tt LINE} or {\tt CONTINUUM}; use {\tt CONTINUUM} to
obtain a table with a single channel, and {\tt LINE} to obtain a table with
multi-channel data. `{\tt band}' may be {\tt USB, LSB,} or {\tt DSB} (the
latter is allowed for continuum mode only, and produces two records: one
for each sideband). Finally `{\tt SUBBANDS}' is a group of subbands, like
{\tt C01 to C06} or {\tt L01 to L02}, to be averaged in continuum mode, and
concatenated in line mode (overlapping spectral bands will cause a
resampling to occur).

The table itself will be a file in the Gildas Data Format, of dimensions
[$3 N_C + 7 , N_V$], for $N_C$ channels and $N_V$ visibilities.  The $3 N_C
$ lines contain:
\begin{enumerate}
\item U in meters
\item V in meters
\item W in meters
\item Observation date (integer \class\ day number)
\item Time in seconds since above date
\item Number of start antenna
\item First frequency point (real part)
\item First frequency point (imaginary part)
\item First frequency point (weight)
\item Same for second frequency point, and so on
\end {enumerate}

The data weights are controlled by command \comm{SET}{WEIGHTS}.  {\tt SET
  WEIGHT TSYS ON} will turn on weighting by $1/T_{SYS}^2$, and {\tt SET
  WEIGHT CALIBRATION ON} will turn on weighting by $1/F_{CAL}^2$, where
$F_{CAL}$ is the amplitude instrumental calibration factor (if amplitudes
are relative).  Both weighting factors may be applied simultaneously. If
both factors are {\tt OFF}, data is weighted proportionally to observing
time.

Do not forget to {\tt SET PHASE RELATIVE}, {\tt SET RF ON}, and {\tt SET
  AMPLITUDE RELATIVE} before you invoke command {\tt TABLE}, otherwise you
will tabulate uncalibrated data.
The command \com{TABLE} itself should contain the name of the table:\\
{\tt TABLE Name Status [/RESAMPLE nc ref val res code] }\\
where `Status' is {\tt NEW} to create a new table or {\tt OLD} to extend an
existing one (default). The default and recommended file extension for the
UV Table is {\tt .UVT}.

Option {\tt /RESAMPLE} resamples the spectrum to a different velocity or
frequency resolution, or to select a part of the spectrum only; see
internal help for details.

\subsection {Going \aips}

You should first convert your table into {\tt UVFITS} format. This is done
by the program \gfits. To invoke in, type {\tt GFITS} under VMS.  The
commands are:
\begin{verbatim}
SET STYLE UVFITS        ! Select type of FITS format
IMAGE name.uvt          ! Define input table
FILE OUT name.uvfits    ! Define output FITS file
WRITE                   ! Write FITS file
EXIT
\end{verbatim}
The output file `{\tt name.uvfits}' will be accepted by \aips\ task {\tt
  UVLOD} (use {\tt INFILE name.uvfits}).  You may also produce a {\tt
  UVFITS} file on tape for further reduction at any site running \aips.

Another possibly is the task {\tt GILDAS\_FITS} available in \gildas\ 
(using the programs VECTOR, GRAPHIC, or OVERLAY), with command {\tt RUN
  GILDAS\_FITS/EDIT}. The initialisation file will be
\begin{verbatim}
! GILDAS_FITS: Gildas to Disk FITS translator
!          (items)                             (user answers)
TASK\FILE "Input Gildas File" IN$              name.uvt
TASK\FILE "Output FITS file" FITS$             name.uvfits
TASK\INTEGER "Number of bits" NBIT$            16 ! or 32 or -16 or -32
TASK\CHARACTER "Style of FITS file" STYLE$     UVFITS
TASK\GO
\end{verbatim}

\subsection {Mapping facilities in \gildas}

\begin{itemize}
\item The UV Table should first be sorted by task {\tt UVSORT}, to obtain a
  UV Sorted Table.
\item From the UV Sorted Table, task {\tt DFT} produces images by Direct
  Fourier Transform, a simple way, but slow and only practical for
  continuum images.
\item Task {\tt UVMAP} produces images by gridding and Fast Fourier
  Transform of a UV Sorted Table.
\item Task {\tt CLEAN} is available for deconvolution with several variants
  of the CLEAN algorithm.
\end{itemize}
See the \gildas\ documentation for further details.

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