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We introduce the following letters in order to identify the various
polarization states which can be encountered:
- X,Y for linear polarization in the sky frame,
- H,V for linear polarization in the antenna frame,
- R,L for circular polarization in the sky frame,
- D,S3 for circular polarization in the antenna frame.
We distiguish here the polarizations in the sky frame or in the
antenna frame4. The conversion between the sky and antenna
frames is possible knowing the parallactic angle. However, some
observatories deliver natively polarizations (or Stokes products) in
one frame while other observatories deliver them in the other
frame. As we can not ensure the parallactic angle is always available
for the conversion, the polarization descriptors listed above allow to
store the polarizations in their native frame.
Note that the circular polarizations are also defined in a reference
frame, as their Stokes cross-products differ depending on the frame
(namely:
,
, but
,
).
At this stage, we can define the polarization codes and Stokes product
codes5 (see
Tables 3 and 4). Note that the values for
the Stokes products I, Q, U, V, RR, LL, RL, LR, XX, YY, XY, YX
match the Conventional Stokes Values from the FITS
standard. The code 0 for NONE can be used in the data format to
indicate that no polarimetry data is present in the UV table (see next
section).
Table 3:
Polarization code names and values.
Name |
Value |
code_polar_h |
1 |
code_polar_v |
2 |
code_polar_x |
3 |
code_polar_y |
4 |
code_polar_r |
5 |
code_polar_l |
6 |
code_polar_d |
7 |
code_polar_s |
8 |
Table 4:
Stokes products code names and values.
Name |
Value |
Name |
Value |
code_stokes_none |
0 |
|
|
code_stokes_i |
1 |
|
|
code_stokes_q |
2 |
|
|
code_stokes_u |
3 |
|
|
code_stokes_v |
4 |
|
|
code_stokes_rr |
-1 |
code_stokes_dd |
-9 |
code_stokes_ll |
-2 |
code_stokes_ss |
-10 |
code_stokes_rl |
-3 |
code_stokes_ds |
-11 |
code_stokes_lr |
-4 |
code_stokes_sd |
-12 |
code_stokes_xx |
-5 |
code_stokes_hh |
-13 |
code_stokes_yy |
-6 |
code_stokes_vv |
-14 |
code_stokes_xy |
-7 |
code_stokes_hv |
-15 |
code_stokes_yx |
-8 |
code_stokes_vh |
-16 |
Then the following rules apply:
- any polarization (linear or circular) described in any frame
(sky or antenna) can be found in a UV table,
- there shall be a minimum of self-consistency, e.g. providing
XX + VV is probably non-sense,
- the presence or absence of a parallactic angle column ( code_uvt_para) does not modify the interpretation of the DAPS
provided in the table,
- one can convert between sky and antenna frame by using the
parallactic angle column, if available.
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Gildas manager
2023-06-01