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Pgflag: All channels dim every 30 seconds?

PostPosted: Thu Aug 25, 2016 2:56 pm
by Anita
See attached screen shot

Info on me: I'm a MRes student, not new to miriad but new to pgflag and data reduction
Data info: ATCA, 1420MHz (HI) of a point source (G334.72-0.65 in the screen shot but also seen in G336.49-1.48). The pattern is seen at all baselines. Only seen for observations on 2 May 2014 if that matters... near midnight.

I'm seeing some curious patterns in my data, all channels seem to dim every 30 seconds? This seems to last for the duration of a single pointing as it's not visible at all in the previous pointing... so about 4 minutes here.

I've been advised that it probably isn't a big issue, but now we're curious what might have caused this! I'm only seeing it in two of my sources on one night which were viewed one after the other, so whatever the effect is may have lasted less than 10 minutes.

Any ideas?

Screenshot from 2016-08-25 13-34-44.png
Screenshot from 2016-08-25 13-34-44.png (151.44 KiB) Viewed 2777 times

Re: Pgflag: All channels dim every 30 seconds?

PostPosted: Thu Aug 25, 2016 3:09 pm
by Anita
Correction! Every 60 seconds

Re: Pgflag: All channels dim every 30 seconds?

PostPosted: Thu Aug 25, 2016 3:22 pm
by ste616
Hi Anita,

My first thought is RFI. If there is strong RFI, then the Tsys of the system might increase, which would cause the correlator to reduce the amplitude of the data, since it assumes that the Tsys is not affected by narrow-band emission. If this RFI occurred every 60 seconds, but lasted for less than 10 seconds each time (and there's doubtless some RFI emissions like that), you would see this same effect.

The easiest way to test for this possibility is to use "options=notsys" in atlod when you load your data. Provided you are not using a mosaic scan, this effect should disappear if its cause is RFI. If you are mosaicking, you can't use this option. If you are mosaicking and you have used this option, the problem you're seeing is probably because you've used this option and you're switching between mosaic fields every minute. If this is the case, let us know and we can explain why.

If your problem persists, let us know.

Re: Pgflag: All channels dim every 30 seconds?

PostPosted: Fri Aug 26, 2016 11:25 am
by Anita
Hi Jamie,

That worked! Thanks for the tip!

Now I'm curious what the RFI was...