Having no distinction between single- and multi-source files has the
advantage that all visibility tasks can manipulate any visibility
dataset. However manipulating a dataset with, say, several sources can
require more care on the part of the user. For example, it rarely makes
sense to make an image using data from several sources. Generally the
user is given reasonable flexibility to perform whatever s/he deems
appropriate - but this has some disadvantages. For example, consider
determining calibration for an observation where the calibrator (a point
source) and the program source are within the one dataset. In deriving
the antenna gains, you would want to select only the data from the
calibrator, whereas when imaging you would select only the data from the
program source. The visibility selection mechanism (see
Section 5.5) handles this situation. But to forget to select
the appropriate data (calibrating with data including the program
source, or imaging including the calibrator) would result in a mess.
Thus, for example, if
the calibrator was 0823-500, and the program source was vela,
one would use
to select the appropriate source.
The selection criteria most appropriate for datasets containing multiple
sources, frequencies and mosaiced pointing centres are source, frequency,
window, ra, dec, dra and ddec.
Other examples of using the
select keyword are given below:
||Select only data from the 2nd IF
||Select data where the frequency of the first
||channel is 4.74 GHz (%)
||Select visibilities with RA between 5:50 and 6:00
||Select data from only the central pointing (delta
||RA and DEC of 0) of a mosaiced observation.
||Take data for source vela only
||Select data from the 2nd IF where its frequency
||is 4.74 GHz.