[daip] Plotting GMRT baseline position angles

[Andy Biggs]

Hi. In trying to diagnose a feature in my GMRT data, I've become a bit confused about how various AIPS tasks define the baseline position angle. This is partly because the help and explain files for VPLOT are inconsistent:

HELP - uv p.a. (deg N->E)
EXPL - uv p.a. (CCW from u-axis)

UVPLT is consistent, both saying N->E. However, the values plotted don't seem correct. For example, if I take a N-S baseline and plot the baseline position angle versus time, at transit the baseline claims to have a PA of -90 degrees, instead of the zero degrees that I would expect.

Regarding the feature in my data, I'm pretty sure that this is due to bandwidth smearing. I've removed all sources apart from one that is far from the phase centre and, when I average my data in frequency, the amplitude clearly decorrelates at all apart from a particular baseline position angle. The amplitude peak in VPLOT (and UVPLT) occurs at -55 degrees and my source lies at an angle of 55 degrees relative to the phase centre (as reported by IMDIST). If the bandwidth smearing explanation is correct, then I think that these two directions should be orthogonal to each other. I'm confused though:

* If the N->E explanation is correct, then the orthogonality is out by 20 degrees.
* If "CCW from u-axis" is correct, then they are exactly parallel.

As the GMRT has a different phase convention, could this be partly to blame?

I should also mention that the N-S baseline mentioned above will plot using BPARM=11,4 in UVPLT, but the equivalent plot in VPLOT fails with the following error:

localh> VPLOT1: Task VPLOT  (release of 31DEC12) begins
localh> VPLOT1: Multiple IFs not meaningful here
localh> VPLOT1: Doing no flagging this time
localh> VPLOT1: XYSC: AXIS 1 DEGENERATE
localh> VPLOT1: SCALVB: XYSCL ERROR  1

VPLOT will show amplitude, phase, weight, etc., but u, v, w and pa do not work.

Thanks,

Andy