Note-Polarization-0.1-20090522.pdf
Some considerations for including Polarization in the Characterisation Data Model. Apologies if I ahve already broken the nam
Polarization is essentially a set of Observables which are forms or derivative of Flux Density. If no polarization is specified this usually implies total intensity. There are a number of conventions for the order of polarization products etc. but unfortunately these differ between observatories. The examples etc. concentrate on the radio regime, other examples needed if data are available to the VO.
This could simply specify:
A single data set can have one or more polarizations which could be represented as a polarization axis.
A useful level of generalisation could include the following terms. Note that these should be available even for total intensity data, since its fidelity may be affected by polarization calibration.
See talk at 2008 May interop (Trieste) for some illustrations and use cases.
Term | Common symbol | Description |
---|---|---|
Stokes parameters | I Q U V | total intensity, linear, linear, circular |
Linear polarization angle | usually chi or PA or PolA | 1/2 Arctan(U,Q) (anticlockwise from North) |
Linearly polarized intensity | P | Sqrt(Q^2 + U^2) |
Elliptically polarized intensity | M | Sqrt(Q^2 + U^2 + V^2) |
Circularly polarized intensity | abs(V) | Absolute value of V |
Fractional Q | q | Q/I |
Fractional U | u | U/I |
Fractional V | v | V/I |
Fractional linear polarization | p | Sqrt(Q^2 + U^2) / I |
Fractional elliptical polarization | m | Sqrt(Q^2 + U^2 + V^2) / I |
Fractional circular polarization | v | abs(V) / I |
Circular feeds | LCP, RCP (or L, R or LL, RR or LHC, RHC) and LR, RL | Left, right circular polarization, cross hands |
Linear feeds | X, Y or XX, YY and XY, YX | Linear polarization NB relation to Q and U is instrument specific! |
FITS code numbering
The numerical values will appear as the CRVAL etc. for the appropriate axis in the FITS header, for example, an NVSS IQU cube will have
NAXIS3 = 3 / length of data axis 3
CTYPE3 = 'STOKES '
CRPIX3 = 1.000000E+00
CRVAL3 = 1.000000000000E+00
The top line in the table below indicates whether the code corresponds to total intensity (tot), Linear (lin) or circular (cir) polarization e.g. for the purpose of ucd construction.
tot | lin | lin | cir | cir | cir | lin | lin | lin | lin | cir | cir | lin | lin | |||||||||||||||||
I | Q | U | V | RR | LL | RL | LR | XX | YY | XY | YX | UNDEF | POLI | POLA | ||||||||||||||||
1 | 2 | 3 | 4 | -1 | -2 | -3 | -4 | -5 | -6 | -7 | -8 | --- | 5 | 7 |
-- AnitaRichards - 16 Jun 2008
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