33GHz flux discrepancy

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33GHz flux discrepancy

Postby bnorfolk » Wed Aug 14, 2019 1:02 pm

Hi,

I've reduced 7mm observations (6A config) and have attempted to image the result. The obs had nice weather with an average seemon rms of 150. Before I uvcat the 33GHz and 35GHz vis files into a 34GHz file I image each separately. For natural weighting, the 35GHz vis data produces an expected image with an rms of 1.74348E-05 and maximum value of 8.898955E-05 (using histo on restors output) however, the 33GHz vis data has an rms of 9.18948E-03 and a maximum value of 2.978523E-02. This is discrepancy is similar with each respective image, the 35GHz shows an expected image with an elongated beam (attached) whereas, the 33GHz image displays diagonal bands (attached). Obviously then once I uvcat the vis files into 34GHz the 33GHz values dominate over the 35GHz readings and my image is full of diagonal bands.

My reduction steps are as follows:
(1) mfcal on the bpcal with int=0.1
(2) gpcopy (1)'s sol onto the phase cal and flux cal
(3) mfcal on the flux cal with int=0.1 and options=nopassol
(4) mfcal on the phase cal with int=0.1 and options=nopassol
(5) gpboot phasecal with flux cal
(6) gpcopy (5)'s sol onto the targets

My images steps are as follows:
(1) invert with imsize=3,3,beam cell=5,5,res sup=$sup options=mfs,double
(2) clean with cutoff=5*rms
(3) restor
and cgdisp to view

This target has been previously imaged "nicely" at 34GHz (attached) and I'm unsure why I'm getting this large readings for the 33GHz vis files. Any suggestions?

Thanks,
Brodie
Attachments
SR21_25Apr16_34GHz_wholfld_wCA06_natcln copy.pdf
"nice" image
(130.73 KiB) Downloaded 8 times
Screen Shot 2019-08-14 at 11.56.00 am.png
33GHz
Screen Shot 2019-08-14 at 11.56.00 am.png (32.28 KiB) Viewed 167 times
Screen Shot 2019-08-14 at 11.54.58 am.png
35GHz
Screen Shot 2019-08-14 at 11.54.58 am.png (37.77 KiB) Viewed 167 times
bnorfolk
 
Posts: 31
Joined: Fri May 17, 2019 3:56 pm

Re: 33GHz flux discrepancy

Postby Mark.Wieringa » Wed Aug 14, 2019 5:04 pm

Hi Brodie,

the most likely cause for stripes is a feature in the visibilities that is short in time - this could be RFI in target data or a bad calibration solution on the calibrator.
So I would suggest a closer look at both the data and the calibration solutions for the 33 GHZ data, followed by some flagging.

Cheers,

Mark
Mark.Wieringa
ATCA Expert
 
Posts: 273
Joined: Mon Feb 08, 2010 1:37 pm

Re: 33GHz flux discrepancy

Postby bnorfolk » Thu Aug 15, 2019 11:30 am

Hi Mark,

Thanks for the reply.

A few questions,

(1) From previous forum posts I discovered it's unnecessary to RFI flag on 7mm data, but for cases like this I would need too? And in that case I'd want to use: pgflag vis=visfile stokes=xx,yy device=/xs and command=<b for the RFI automatic flagging?

(2) I should only RFI flag when needed? Or would you suggest I use pgflag for all my calibrators with every target?

(3) I'm not sure what to look for specifically in the calibration solutions? I have the pre and post mfcal/gpcopy plots for all my calibrators as a function of amp/time, amp/chan, and phase/chan. During my reduction I look at these plots and search for outliers in the the amp/time plots and any unusual channel readings in the amp/chan plots and flag accordingly with blflag/uvflag. Is this adequate? Or am I missing a crucial step/s? (I must be because the 33GHz band looks shocking)

Cheers,
Brodie
bnorfolk
 
Posts: 31
Joined: Fri May 17, 2019 3:56 pm

Re: 33GHz flux discrepancy

Postby Mark.Wieringa » Thu Aug 15, 2019 4:20 pm

Hi Brodie,

(1,2) yes, generally 7mm data is pretty clean, but you may be unlucky - or there may have been a dropout in the data. I would do manual flagging in this case.
Best to inspect the raw, uncalibrated data. Start with simple amp - vs time plots of all the baselines to see if there is anything obvious and then try to narrow it down to single antenna/pol and check data vs channel. Once you have narrowed things down in time/channel/antenna etc use uvflag to flag the bad data.
You could also make an amp vs channel plot with blflag, it should clearly show any points that are too high and you can flag them straight away - dropouts may be harder to spot.

(3) yes checking through the cal solution plots this way should show most problems - if they all look ok, the problem must just be on the target and not on the calibrator.

Cheers,

Mark
Mark.Wieringa
ATCA Expert
 
Posts: 273
Joined: Mon Feb 08, 2010 1:37 pm


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