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Polarimetry data can be handled in three different ways.
- In the first mode, a single visibility can have only one
polarization state, but different visibilities may have different
states. The polarization state for each visibility is defined by
h%gil%column_pointer(code_uvt_stok). Possible values
are given by the code_stokes_... parameters specified in
the previous section, while antenna polarizations are specified by
the code_polar_... parameters. This mode is indicated
by h%gil%nstokes = 1 and h%gil%order = 0. It
will also have h%gil%nfreq = 0 (see below). Note that
there is no provision for mixed (circular / linear) polarization
states.
- In the second mode, a single visibility can handle several stokes
parameters, but Stokes and Channels are regularly ordered. In this mode,
channels are regularly spaced in Frequency, like in single Stokes mode,
so no h%gil%freqs array is specified. Stokes parameters are
also ordered. For every channel, the ordering of Stokes parameters must
be the same. Stokes parameters may vary first, and Frequencies next (the
so-called h%gil%order = code_stok_chan ordering) or
Frequencies may vary first, and Stokes next (the so-called
h%gil%order = code_chan_stok ordering). Ordering is stored
in variable h%gil%order. Arrays
h%gil%stokes(h%gil%nstokes) indicate the Stokes parameter
for each frequency ``channel''. This mode is indicated by
h%gil%nstokes
1 and h%gil%order
0, but
still has h%gil%nfreq = 0.
- In the third, most complex mode, the Stokes parameter can vary
arbitrarily within one visibility. Then each complex visibility
element must have a Stokes parameter and a frequency indication, which
are given in the arrays h%gil%stokes and
h%gil%freqs respectively, of size h%gil%nfreq =
h%gil%nchan * h%gil%nstokes. As each complex visibility
element is given a specific frequency, this mode can also allow to
store irregularly spaced frequency channels. Such a mode can be
convenient for continuum bandwidth synthesis, for example. Proper
weighting is ensured even though the channel width is not specified, as
the weights are carried along with the Real and Imaginary parts. This
mode is indicated by h%gil%nfreq
0. h%gil%order
is irrelevant in this mode, but should be set to 0.
Next: Changes for end-users
Up: The new layout for
Previous: Nomenclature
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Gildas manager
2023-06-01