Spectral-line case

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Spectral-line case

Examine the data in ngc5921.ms using msplot. First, plot calibrator data in FIELD_ID 1 (1328+307 = 3c286 - the flux calibrator) In the msplot control panel:
Choose the plot type PLOT X VS Y.
Select X AXIS=uvdist.
Select Y AXIS=amplitude.
Limit the spectral selection to look at a single channel, say one in the middle: channel 30.
(click on the SPECTRAL SELECTION button in the ms control panel to choose channels).
Click on the Go button to make the plot in Fig. 1.8.
Use the locate button to identify low points.
The observed data plotted are the RR & LL correlation. Cross correlation (RL & LR) are not in this dataset. Plot data in an XY plot with time vs. amplitude next. Hey! The data look pretty good (Fig. 1.8)! No obvious flagging needed based on the msplot plots. It is noisier than the continuum data set example but that is because the RMS is higher because the band width of a single channel is relatively narrow.

**Figure 1.8:** `msplot` of the calibrators in the VLA spectral-line dataset example. **Left:** 3c286 (1331+305), the flux calibrator. **Right:** 1445+099 - the gain calibrator.
$\begin{figure}\centering\leavevmode \epsfxsize =.45\columnwidth \epsfbox{cookb... ...columnwidth \epsfbox{cookbook.dir/vla.msplotS.gcal.ps} \hrulefill \end{figure}$

Now examine all the channels in a raster plot (Fig. 1.9):
Choose the plot type DISPLAY DATA AS AN IMAGE.
Choose all 63 channels starting with 1 and incrementing by 1 (click on the
SPECTRAL SELECTION button in the ms control panel to see this).
Use the buttons on the right of the raster display to see a movie of the channel data. The data look great here too but beware - the autocorrelations were filled with this data set. We will use the flagger tool to flag autocorrelations in the next section (or they will corrupt the final image).

**Figure 1.9:** `msplot` of the calibrator 3c286 as a raster image. Note that antenna 17 is gone for the first integration.
$\begin{figure}\epsfig{file=cookbook.dir/vla.msplotS.3c286raster.ps, height=3.in} \hrulefill \end{figure}$

It is best not to edit or flag source data until after it is lcalibrated.

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