[daip] [!6804]: AIPS - Calibrating instrumental polarization by using LPCAL

[JongHo Park]

JongHo Park updated #6804
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Calibrating instrumental polarization by using LPCAL
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           Ticket ID: 6804
                 URL: https://help.nrao.edu/staff/index.php?/Tickets/Ticket/View/6804



Hi.

I am a graduate student at Seoul National University in South Korea.

I have some questions about the task 'LPCAL' in AIPS.

According to equations (17) ~ (20) in Roberts, Wardle, & Brown, 1994, ApJ paper and equation (12) of EVN Memo #78 written by Scott E. Aaron, 

the ratio of cross-hand visibilities to parallel hand visibilities will track a circle in the real and imaginary plane due to the D-term of one of the antenna.

In addition, the center of the circle will be offset from the center due to the D-term of the other antenna.

Also, if the source is polarized, the center of the circle will rotate in the complex plane.

If we look at the figure 4 of Roberts+ paper, D-terms are obtained by the radius and the location of the fitted circles.

However, when I tried to obtain the D-terms of KVN (Korean VLBI Network) antennas using the same way, I found that the center of the circular track of 3C84 (almost unpolarized) is always close to zero, even though the obtained D-terms of two antennas are on the similar order (5 ~ 10 %).

In addition, when I used polarized sources like BL Lac, 3C 345, the position of the circles are offset from the center.

Therefore, I suspected that parallactic angle correction I made using the task CLCOR makes the difference between mine and Roberts+ paper.

I think that the equations (17) ~ (20) in that paper are derived from the antenna output voltages without parallactic angle correction (equation (7)~(8)).

1. So, I'd like to know what exactly the task 'CLCOR' does. Is it just multyplying the term exp(i \phi) in the equation (7) and exp(-i \phi) in the equation (8)??

2. If the position of the center of the circle is determined by the source polarization instead of the D-term with the parallactic angle correction, then how the task 'LPCAL' obtain the D-Terms?

I think in this case, fitting is impossible because there must be two circles contributed by two D-terms.


3. In addition, I found that in case of using polarized and resolved sources for estimating the D-terms, the single CC table must be separated into several tables with the task CCEDT.

I want to know the role of this task and how AIPS obtain the D-terms in this case.

What is the difference compared to using unresolved, unpolarized source?


It will be really grateful if I could get the answers.

Thank you!


Best Regards,

Jongho Park

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