6cm band bandpass ripple

[and460]

Hi guys - quick question. I have reduced some ATCA 6cm data using all of the standard procedures. After MFCAL and GPCAL on 1934, then GPCAL & GPBOOT on the secondary, I ran a quick UVSPEC on the secondary and was slightly alarmed to find a sinusoidal oscillation running through the band in the XX and YY products. I had a poke around on the ATCA forum and found this post on a possibly related issue:

viewtopic.php?f=11&t=184

Plots 2--4 in that post clearly show a very nasty problem, but then in plot 5 the problem is said to be fixed. However, there still appears to be a sinusoidal ripple. What piques my interest here is that the frequencies of the peaks and troughs in this spectrum match up almost precisely with what I see in my calibrator (relevant plots appended below).

The problem manifests itself on almost all baselines to at least some extent, and the period of the oscillation does not seem to be noticeably dependent on the baseline. I went ahead and imaged the cal every 32 MHz through the band, and while the images look beautiful, the flux density shows the same oscillations when extracted from the images. Copying the calibration solutions to my other sources and imaging them reveals that they then suffer from the effect too.

Have you any idea what causes this? I'm not having a great deal of luck figuring it out or getting rid of it!

Cheers,
Craig.

My cal:



The forum plot:

[ste616]

Hi Craig,

I haven't seen a ripple like that due to a systematic issue for quite a while, and even when it is a problem with the system it will only affect the antenna with the problem. If you're seeing it all the baselines, then it won't be an antenna problem.

What we have found is that such ripple can be generated by calibrating a confused field as though it was a point source. That is, if your phase calibrator has a reasonably bright confusing source in the field, then by running gpcal on it you will produce a ripple in the spectrum just like you see. I would try imaging the phase calibrator field and looking for confusing sources.

[and460]

Follow-up note: This error turned out to be introduced by accidental inclusion of a small amount of array setup data (i.e. pre-acal, dcal, pcal) in the 1934 calibration process.