Time-like plots

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

Thus the 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 SET X and 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 U, V, RADIUS or 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 (HOUR_ANGLE and DECLINATION), or delays (see the internal help or section 8 for a complete description).

The SET BANDS command will select the side bands plotted. Several side bands may be plotted simultaneously. Side bands may obviously be UPPER or LOWER, or 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 RATIO, in which case the (unweighted) ratio USB/LSB will be used. The weights for averaging the side bands are entered by the SET AVERAGING command.

The SET SUBBANDS command is used to specify the sub-bands to be used. Sub-bands are referred to by their mnemonics C01, ..., C06, for the continuum sub-bands, L01, ..., 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 SPECTRAL command in OBS. Several continuum sub-bands may be averaged together by specifying a range (e.g. C01 TO C05, or C01 AND C04, or C01 AND C03 TO C06, the latter just excluding C02).

For line sub-bands all the channels are averaged together, except if the /WINDOW option is used. For instance:
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. 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 SET GIBBS number, number is the number of channels left out

The SET BASELINES command obviously selects the baselines for which data is to be plotted. Baselines are named 12, 13, 23, (14, 24, 34, ...). 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 SET ANTENNA command is also available. It is used to plot antenna-related parameters (such as TSYS, Elevation, ...) or with autocorrelation data (SET Y AUTO). It may also be used (in time-like mode only) with correlation data: in that case,

• 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.
• 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).

The SET AVERAGING command deals with the way data is averaged. Data may be time-averaged by the PLOT command; SET AVERAGING TIME t will specify the averaging time, while SET AVERAGING SCAN will force averaging observation by observation, and SET AVERAGING NONE will turn off time averaging. The normal method is to average complex visibilities (SET AVERAGING METHOD VECTOR, the default), while an alternate method is to average separately amplitude and phases (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 SET AVERAGING UPPER baseline amplitude phase and 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 (SET PLOT POINTS, the default), or with lines connecting the points (SET PLOT LINES), with histograms (SET PLOT HISTOGRAM), or with error bars (in the case of visibilities only; type SET PLOT BARS).

Finally the command 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 (SET ASPECT_RATIO value AUTO), or will exactly reflect the demanded aspect ratio (SET ASPECT_RATIO value EXACT).

After a plot has been obtained, coordinates of special points may be obtained interactively by command 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 SOLVE PHASE); you may also IGNORE or TAG scans in a given range to a given quality: type N to open a new range, and I or T to close it (if using 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 4.6.2)

After the PLOT command, the abscissae and ordinates of the plotted data points are kept in two two-dimensional SIC arrays named X_DATA and Y_DATA, where the first index is the data point counter (1 to 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.