stokes V imaging

[Rami zainal]

Hi all,

I want CABB data to produce V images for some sources. my question is: does the process (atlod, mfcal, gpcal, and imaging) of stokes V the same as stokes I or different?

If it is different, then what is the difference?

Thank you to advance,
Rami

[Mark.Wieringa]

Hi Rami,

yes, the calibration and imaging works the same for Stokes I and V.
If you need high accuracy in circular polarisation there are some extra steps to take, they are described in the document https://www.atnf.csiro.au/observers/memos/circpolguide.pdf

Cheers,

Mark

[Rami zainal]

Thank you Mark for quick respond. However, the guide you mentioned is for pre-CABB data and some options don't work for CABB data such as the option reweight in atlod task, its not working any more and also, nfbin in gpcal didn't exists in pre-CABB but you have specify it in CABB.

Cheers,
Rami

[Mark.Wieringa]

Hi Rami,

yes, you will need to update those parameters to the ones correct for CABB. The section on offsetting the pointing is also no longer relevant, since CABB doesn't have any artefacts at the field centre.
That leaves transfix and the gpcal procedure. You can set nfbin for CABB data.

The transfix program requires the pointing parameter file in use during the observation. I'm not sure what the best way is to get that these days - you might contact Jamie Stevens.

Note again that you may not need any of this 'special processing' unless you need high accuracy measurements of Stokes V.

Cheers,

Mark

[Rami zainal]

Thank you very much Mark!

Cheers,
Rami

[Rami zainal]

Hi Mark,

I have another question regarding circular polarization. How do you know that the stokes V emission is leakage or real? is there a way to check that?

Cheers,
Rami

[Mark.Wieringa]

Hi Rami,

I think the best way is to compare different sources.
Circular polarisation is rare, so if all sources have a small but similar percentage V it is likely to be leakage, but if one source stands out it is probably real.

Cheers,

Mark

[ste616]

Hi Rami,

I would also suggest you read Thompson, Moran and Swenson 3rd edition section 4.7 about polarimetry.

https://link.springer.com/chapter/10.10 ... -44431-4_4

Towards the end it gives some useful advice regarding this situation (in 4.7.5):

With linearly polarized antennas, errors in calibration are likely to cause I v to corrupt the linear parameters Q v and U v , so for measurements of linear polarization, circularly polarized antennas offer an advantage. Similarly, with circularly polarized antennas, calibration errors are likely to cause I v to corrupt V v , so for measurements of circular polarization, linearly polarized antennas may be preferred.

That is, it is really hard for linearly polarised antennas like ATCA to erroneously create sources with significant levels of Stokes V.

[Rami zainal]

Thank you very much Mark and Jamie!

Cheers,
Rami