Instrumental polarisations
In hyperdrive (and mwalib and
hyperbeam), the X
polarisation refers to the East-West dipoles and the Y refers to North-South.
Note that this contrasts with the IAU definition of X and Y, which is opposite
to this. However, this is consistent within the MWA.
MWA visibilities in raw data products are ordered XX, XY, YX, YY where X is
East-West and Y is North-South. Birli and cotter also write pre-processed
visibilities this way.
wsclean expects its input measurement sets to be in the IAU order, meaning
that, currently, hyperdrive outputs are (somewhat) inappropriate for usage
with wsclean. We are discussing how to move forward given the history of MWA
data processing and expectations in the community.
We expect that any input data contains 4 cross-correlation polarisations (XX XY
YX YY), but hyperdrive is able to read the following combinations out of the
supported input data types:
- XX
- YY
- XX YY
- XX XY YY
In addition, uvfits files need not have a weight associated with each polarisation.
Stokes polarisations
In hyperdrive:
- \( \text{XX} = \text{I} - \text{Q} \)
- \( \text{XY} = \text{U} - i\text{V} \)
- \( \text{YX} = \text{U} + i\text{V} \)
- \( \text{YY} = \text{I} + \text{Q} \)
where \( \text{I} \), \( \text{Q} \), \( \text{U} \), \( \text{V} \) are Stokes polarisations and \( i \) is the imaginary unit.