The ATCA Forum

Hi,

I was trawling through the Miriad and ATCA user guides trying to find the definition of Chi (the ATCA feed orientation on the sky). I found that it is related to the parallactic angle by Chi = ParAng + 45 degrees. My question is this - what does Chi = 0 correspond to physically? I.e. is the zero-point defined as being aligned with one of the orthogonal linear feed axes, or something else? Also, for an observation at ParAng = 0, does this mean the object is positioned at +45degrees (clockwise) in relation to the feeds?

Cheers,

Craig.

Hi Craig,
You are right, if you take A-pol and add 45 degrees you are at ParAng=0. Or if you take ParAng=0 and go +45degrees you hit the "B-Pol Axis".
So that answers your first question: CHI=0=B-pol.
That is valid for all but one of ATCA's RX systems : 7mm.... ... there it is the other way round.
Hope this helps,
Cheers,
Christoph

Thanks very much Christoph!