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Ghost Source at Phase centre changing sign every channel

PostPosted: Fri Nov 28, 2014 8:31 pm
by PKamphuis
Hi Guys,

I actually don't know whether this is a calibration issue or an imaging issue. Nobody here in Sydney really knows what it is either. So feel free to adjust the title or move the post to its correct location.

Here is the problem, I am trying to reduce some pre-CABB data from 2004 (Source: ESO92-21, Project C1238, PI P.Serra). The project has data in the 750A configuration as well as in EW367. Data reduction seems to go smooth in a normal fashion. I do perform gpcal with the options noxy,nopol. However, what I end up with in the end is a cube that appears to have a continuum source in the pointing centre but the source is negative one channel positive the next.

Below I show two consecutive channel maps showing the problem. I concentrate here on the EW367 data as the effect is clearest in this set but the 750A dataset has the same problems.

I have tried to clean the positive channels but that did not work. I also created a cube from data without the continuum subtraction and it has the same problem. It is harder to spot in that dirty cube due to the sidelobes of real sources. However a quick clean removes those and the problem is clear. CABB data shows no bright continuum source anywhere near the centre although there is a bright continuum source around the FWHM of the primary beam causing some problems. P. Serra has provided what old data he could find and that has exactly the same problem although it is not clear whether this set was his final data. For the combined set it is at the noise level so he might have ignored it, I do not know. He certainly does not remember the problem.

As I would like to combine it with new data where it manifests itself above the noise level I'd like to get rid off it but I don't know how.

I have uploaded the data for the EW367 configuration to ... 367.tar.gz (523MB)

The files have straightforward names and there is a script with the data reduction steps.

Re: Ghost Source at Phase centre changing sign every channel

PostPosted: Mon Dec 01, 2014 10:17 am
by PKamphuis
Ok solved it by Hanning smoothing the data.