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Centering the target

PostPosted: Mon Aug 19, 2019 5:07 pm
by bnorfolk

I'm attempting to centre my targets for imaging purposes and I'm having some issues. I've been correcting the RA and DEC with the proper motion of my targets (for targets with recent gaia measurements) and centering on the new RA and DEC with a variety of results. Some targets centre accurately, others are slightly off, and a few are completely off (images attached). I'm not sure if this a systematic problem with ATCA or if it's just a common thing. Any suggestions would be greatly appreciated.


Re: Centering the target

PostPosted: Tue Aug 20, 2019 10:08 am
by Mark.Wieringa
Hi Brodie,

A phase calibrated ATCA observation at 5 GHz or above should have better than 0.5 arcsec position accuracy (usually <0.2"). This is assuming a reasonable beam shape and a config with km baselines.
Your beam sizes seem to be in the 1-2" range, which should give sub arcsecond position accuracy for 5 sigma detections.
For observations where you need to know the position errors you can use two phase calibrators. That way you can calibrate one against the other and check the accuracy.

For imaging you don't really need to centre the target unless it has moved outside the half power beam radius and in that case you really should have pointed the telescope at the actual source position.

If you're finding large offsets, there must be something wrong with either the phase calibration (check secondary) or the computed position. If the phase stability is bad enough to move your source by a few arcseconds it generally also gives a very poor, decorrelated image - i.e., flux is missing and distributed over several beam widths.



Re: Centering the target

PostPosted: Thu Aug 22, 2019 2:55 pm
by bnorfolk
Hi Mark,

Thanks for the reply, I believe I've shifted correctly.

So looking at the image attached, it has the star centred on a two sigma detection (adjusted with gaia proper motions) but there's a 4 sigma detection below. How do I use two phase calibrators to check position errors to see if this 4 sigma detection is actually the target? I have 0454+066 as the phase cal for my observations, but I'm not sure where to go from there?